Triple-negative Tumors Research Articles

The role of stress in modulating AIPB expression and estradiol synthesis in triple-negative breast tumors.

e13146 Background: In the United States, invasive breast cancer affects 1 in 8 women during their lifetime, with over 3.8 million individuals living as survivors. Breast cancers vary, with some being hormone-responsive and categorized on the presence or absence of estrogen (ER), progesterone (PR), and HER2 receptors. While targeted therapies exist for Estrogen Receptor positive (Er+) and Progesterone Receptor positive (PR+) cancers, triple-negative breast cancer (TNBC), which lacks all three receptors, affects about 15% of patients and is unresponsive to effective molecular treatments. Only a fraction of the cases respond to chemotherapy which emphasizes a need for new prognostic markers and treatments. Aromatase is the key enzyme in converting testosterone to estradiol which plays a crucial role in tumor growth, especially after menopause. Our research lab has identified the “Aromatase Interacting Partner in Breast or AIPB, whose overexpression inhibits aromatase, leading to a reduction in estradiol levels. However, in triple-negative breast cancer, there is an increase in AIPB expression when compared to ER+/PR+/Her2- breast tumors or unaffected women. This devastating prognosis and low survival rates can contribute to a considerable amount of stress in the diagnosed patients, in turn leading to an increase in cortisol production. Our hypothesis suggests that this stress or increase in cortisol could alter AIPB production in the tumorigenic cells which could potentially lead to a decrease in estradiol synthesis. Methods: Stress stimulation was achieved in a lab setting by subjecting engineered triple-negative breast cancer cells, capable of conditional AIPB overexpression upon tetracycline addition, to a 42 °C heat shock for various durations (0,15, 30, 45 minutes). Results: Following the 45 minutes of stress, MDA-MMD-231 cells exhibited a significant reduction in estradiol levels, decreasing from 160ng/ml to 100ng/ml. MDA-MD-231 cells engineered to overexpress AIPB produced only 62ng/ml of estradiol, marking an 18.75% decrease. The introduction of doxycycline led to a further reduction in estradiol levels to 47ng/ml, which is approximately a 29.4% drop. Western blot analysis using a CT antibody revealed heightened AIPB expression in MDA-MD-231-AIPB cells when stress and doxycycline treatments were combined. Conclusions: Given the absence of aromatase in triple-negative breast cancer, our findings suggest that AIPB’s role in estradiol production is independent of aromatase, highlighting its significance in these cancer cells. The data supports the hypothesis that enhancing AIPB expression could potentially improve patient outcomes by reducing estradiol synthesis.

CCNE1 amplification as marker of poor prognosis and novel therapeutic target in advanced breast cancer.

1040 Background: Cyclin E1 (CCNE1) overexpression or gene amplification is linked to dismal outcomes in various tumors, including breast cancer (BC). Recent studies suggest that CCNE1 amplification is a potential target for new synthetic lethality-based treatments, including CDK2-selective and PKMYT1 inhibition. This study explores the clinical and genomic characteristics of CCNE1-amplified BCs. Methods: We analyzed genomic and clinical data from consecutive BCs that underwent clinical tumor-normal targeted panel sequencing (MSK-IMPACT) between April 2014 and December 2021. Allele-specific copy number, including CCNE1 amplification, and fraction genome altered (FGA) were calculated by FACETS. Mutual exclusivity and co-occurrence analyses were performed using CoMEt. Benjamini–Hochberg method for multiple testing correction (q) was applied. Real-world progression-free survival (rwPFS) was assessed by the Kaplan Meier method and Cox models, focusing on patients with available pre-treatment samples. Results: Out of 3,753 BCs, 125 (3.3%) had CCNE1 amplification. A higher occurrence of CCNE1 amplification was noted in post-treatment (n=2,368) compared to treatment-naïve tumors (n=1,385; 4% vs 2.4%, p=0.007). CCNE1 amplification was less common in hormone receptor (HR)+/HER2- BCs (2%) compared to HER2+ (7.6%) and triple-negative (7.2%) tumors (p<0.001). BCs with CCNE1 amplification showed higher median FGA, indicating increased genomic instability. In primary BC, TP53 alterations were more frequent in CCNE1-amplified tumors (q<0.001), while CDH1 alterations were mutually exclusive (q<0.001) suggesting that lobular BCs rarely harbor CCNE1 amplification. Similar trends were seen in post-treatment samples. CCNE1 amplification detected at baseline was linked to shorter median rwPFS in HR+/HER2- metastatic BCs treated with CDK4/6 inhibitors plus endocrine therapy (8.8 vs 15.2 months in CCNE1-amplified [n=9] vs CCNE1-non amplified [n=402]; hazard ratio [HR] 2.82, 95% CI 1.38-5.75), and in HER2+ metastatic BCs treated with first-line THP regimen (7.3 vs 20.8 months in CCNE1-amplified [n=5] vs CCNE1-non amplified [n=106]; HR 3.1, 95% CI 1.24-7.87). In a CCNE1-amplified triple-negative cell model (HCC1569), treatment with the PKMYT1 inhibitor RP-6306 significantly reduced tumor growth. Conclusions: CCNE1 amplification in BCs is associated with greater genomic instability and characterizes a group of metastatic BCs with poor response to standard of care therapies. Novel therapies targeting CCNE1-amplified tumors are under evaluation in preclinical and clinical studies.

Molecular pathways of common breast cancer metastases and the distinguishing features of triple-negative breast cancer

Breast cancer is the most common type of female cancer in Turkey, and metastasis is the most important cause of death, as in other solid organ cancers. Triple-negative tumors constitute 15-20% of breast cancer patients. Within three years after the development of the primary tumor, the tumor spreads to other organs. Breast cancer tends to spread to distant organs, such as bone, liver, brain, lung, and adrenal gland, either through regional lymph nodes or vascular channels. This condition, defined as the tendency to metastasize to specific organs, is called organotropism. Triple-negative breast cancer is a heterogeneous breast cancer subtype showing organotropism for the brain and the lungs. Identifying the molecular changes that may cause tropism for various regions and organs in non-metastatic tumors at the time of diagnosis is vital to developing targeted therapies and achieving longer overall and disease-free survival. In this review, we aimed to summarize the pathogenesis of breast cancer metastasis, the molecular changes involved in the metastatic process, and organotropism, as well as to emphasize the distinguishing features of triple-negative breast cancer in terms of metastatic organotropism.

Genetic trends and common BRCA1/2 pathogenic sequence variants in black African and Indian breast cancer patients presenting at Inkosi Albert Luthuli Central Hospital, KwaZulu-Natal, South Africa.

Hereditary breast cancer is characterised by the presence of a pathogenic sequence variant passed from one generation to the next. These cancers are aggressive, develop early, and account for 5 - 10% of all breast cancer cases. In South Africa (SA), the common variants that predispose to hereditary breast cancer have been well documented among white patients and form part of screening panels during targeted testing. For non-white patients, common variants are not well understood, and as such, all populations are offered the same test optimised for white patients. This carries a risk of misdiagnosis, the consequences of which include recurrence and increased mortality. To retrospectively describe genetic trends in the black African and Indian breast cancer patients from KwaZulu-Natal Province, SA. We reviewed clinical and genetic data of breast cancer and high-risk patients who consulted at Inkosi Albert Luthuli Central Hospital between 2011 and 2021. Inclusion criteria were based on clinical and demographic characteristics as defined by SA clinical guidelines. Black African patients were young (mean 37.6 years, standard deviation 11.16) and had the majority of triple-negative tumours (37.5%). Indians represented 50% of bilateral breast cancers and of high-risk individuals. We identified 30 pathogenic BRCA1/2 sequence variants, four large genomic rearrangements and 13 variants of unknown significance. Twenty black patients carried 12, 13 white patients carried 4, 25 Indian patients carried 16, and 3 coloured patients carried 3 pathogenic sequence variants. The most frequent variants were BRCA2 c.5771_5774del, p.Ile1924fs among black patients, BRCA2 c.7934del, p.Arg2645fs among white patients, and BRCA2 c.8754+1G>A among Indian patients. None of the founder mutations common in white patients was reported in either black, Indian or coloured patients, which explains why black, Coloured and Indian SA patients consistently test negative during targeted screening. This study highlights unique genetic trends for SA populations and the need for more inclusive targeted tests that are optimal for these populations.

Exploring the interplay between triple-negative breast cancer stem cells and tumor microenvironment for effective therapeutic strategies.

Triple-negative breast cancer (TNBC)is a highly aggressive and metastatic malignancy with poor treatment outcomes. The interaction between the tumor microenvironment (TME)and breast cancer stem cells (BCSCs)plays an important role in the development of TNBC. Owing to their ability of self-renewal and multidirectional differentiation, BCSCs maintain tumor growth, drive metastatic colonization, and facilitate the development of drug resistance. TME is the main factor regulating the phenotype and metastasis of BCSCs. Immune cells, cancer-related fibroblasts (CAFs),cytokines, mesenchymal cells, endothelial cells, and extracellular matrix within the TME form a complex communication network, exert highly selective pressure on the tumor, and provide a conducive environment for the formation of BCSC niches. Tumor growth and metastasis can be controlled by targeting the TME to eliminate BCSC niches or targeting BCSCs to modify the TME. These approaches may improve the treatment outcomes and possess great application potential in clinical settings. In this review, we summarized the relationship between BCSCs and the progression and drug resistance of TNBC, especially focusing on the interaction between BCSCs and TME. In addition, we discussed therapeutic strategies that target the TME to inhibit or eliminate BCSCs, providing valuable insights into the clinical treatment of TNBC.

PAK6 acts downstream of IQGAP3 to promote contractility in triple negative breast cancer cells

Breast cancer is a heterogeneous disease that remains the most common malignancy among women worldwide. During genomic analysis of breast tumours, mRNA levels of IQGAP3 were found to be upregulated in triple negative tumours. IQGAP3 was subsequently found to be expressed across a panel of triple negative breast cancer (TNBC) cell lines. Depleting expression levels of IQGAP3 delivered elongated cells, disrupted cell migration, and inhibited the ability of cells to form specialised invasive adhesion structures, termed invadopodia. The morphological changes induced by IQGAP3 depletion were found to be dependent on RhoA. Indeed, reduced expression of IQGAP3 disrupted RhoA activity and actomyosin contractility. Interestingly, IQGAP3 was also found to interact with p-21 activated kinase 6 (PAK6); a protein already associated with the regulation of cell morphology. Moreover, PAK6 depletion phenocopied IQGAP3 depletion in these cells. Whereas PAK6 overexpression rescued the IQGAP3 depletion phenotype. Our work points to an important PAK6-IQGAP3-RhoA pathway that drives the cellular contractility of breast cancer cells promoting both cell migration and adhesive invasion of these cells. As this phenotype is relevant to the process of metastasis and re-seeding of metastasis, the pharmacological targeting of PAK6 could lead to clinical benefit in TNBC patients.

Characterization of HER2-low breast cancer in young women with germline BRCA1/2 pathogenetic variants: Results of a large international retrospective cohort study.

Breast cancer (BC) in women aged ≤40 years carrying germline pathogenetic variants (PVs) in BRCA1/2 genes is infrequent but often associated with aggressive features. Human epidermal growth factor receptor 2 (HER2)-low-expressing BC has recently emerged as a novel therapeutic target but has not been characterized in this rare patient subset. Women aged ≤40 years with newly diagnosed early-stage HER2-negative BC (HER2-0 and HER2-low) and germline BRCA1/2 PVs from 78 health care centers worldwide were retrospectively included. Chi-square test and Student t-test were used to describe variable distribution between HER2-0 and HER2-low. Associations with HER2-low status were assessed with logistic regression. Kaplan-Meier method and Cox regression analysis were used to assess disease-free survival (DFS) and overall survival. Statistical significance was considered for p≤.05. Of 3547 included patients, 32.3% had HER2-low BC, representing 46.3% of hormone receptor-positive and 21.3% of triple-negative (TN) tumors. HER2-low vs. HER2-0 BC were more often of grade 1/2 (p<.001), hormone receptor-positive (p<.001), and node-positive (p=.003). BRCA2 PVs were more often associated with HER2-low than BRCA1 PVs (p<.001). HER2-low versus HER2-0 showed better DFS (hazard ratio [HR], 0.86; 95% CI, 0.76-0.97) in the overall population and more favorable DFS (HR, 0.78; 95% CI, 0.64-0.95) and overall survival (HR, 0.65; 95% CI, 0.46-0.93) in the TN subgroup. Luminal A-like tumors in HER2-low (p=.014) and TN and luminal A-like in HER2-0 (p=.019) showed the worst DFS. In young patients with HER2-negative BC and germline BRCA1/2 PVs, HER2-low disease was less frequent than expected and more frequently linked to BRCA2 PVs and associated with luminal-like disease. HER2-low status was associated with a modestly improved prognosis.

Metaplastic breast cancer with a unique presentation and complete response to chemotherapy: a case report

BackgroundMetaplastic breast carcinomas are a rare variant group of breast carcinomas. They are usually high-grade and triple-negative tumors. They often present with large primary tumor sizes. However, the involvement of axillary lymph nodes is infrequent at the time of diagnosis. Metaplastic breast carcinomas are associated with a worse prognosis and a poorer response to chemotherapy in comparison with other non-metaplastic triple-negative breast cancers. Up until this point, there are no specific treatment recommendations for metaplastic breast carcinomas beyond those intended for invasive breast cancer in general.Case presentationA 40-year-old woman complained of a palpable mass in her left axilla. On ultrasonography, the mass was solid, spindle-shaped, hypoechoic with regular borders, and exhibited decreased vascularity. At first, the mass appeared to be of a muscular origin. There was not any clinical nor ultrasonic evidence of a primary breast tumor. On magnetic resonance imaging, the axillary mass was a well-defined with regular borders, measuring 24 × 35 mm. Needle biopsy showed a spindle cell tumor with mild to moderate atypia. The subsequent surgical resection revealed a spindle cell neoplasm within a lymph node, favoring a metastatic origin of the tumor. The tumor cells lacked expression of estrogen, progesterone, and HER2 receptors. PET-CT scan indicated pathological uptake in the left breast. Accordingly, the patient was diagnosed with metaplastic breast cancer that had metastasized to the axillary lymph node. She commenced a combined chemotherapy regimen of doxorubicin and cyclophosphamide. After six treatment cycles, she underwent left modified radical mastectomy with axillary lymph node dissection. Pathological examination of the specimens revealed a total burn-out tumor in the breast due to excellent treatment response. There were no residual tumor cells. All dissected lymph nodes were free of tumor. At the one-year follow-up, the patient showed no signs of tumor recurrence.ConclusionThis report sheds light on a distinctive presentation of metaplastic breast carcinoma, emphasizing the need for vigilance in diagnosing this rare and aggressive breast cancer variant. In addition, the patient’s remarkable response to chemotherapy highlights potential treatment avenues for metaplastic breast cancer.

Canine Mammary Tumors as a Potential Model for Human Breast Cancer in Comparative Oncology.

Clinical and molecular similarities between canine mammary tumors (CMTs) and human breast cancer (HBC) propel scientists to further study their application in comparative oncology as a model for human breast cancer. In total, 64 canine mammary tumors were selected to study the most common markers, which are applicable for human breast cancer treatment, including estrogen and progesterone receptors (ER and PR), human epidermal growth factor (HER2/neu), Ki67, and cyclooxygenase 2 (Cox2). Immunohistochemistry (IHC) was used to assess the protein expression. The Veterinary Nottingham Prognostic Index (Vet-NPI) was also computed. Moreover, univariate and multivariable Cox proportional hazard analyses were applied to estimate hazard ratios (HRs). The results demonstrated that Ki67 was strongly expressed in the triple-negative tumors, and Ki67 protein expression continuously increased over the increase of Cox2 protein expression (p < 0.001). Further analysis revealed a significant difference among canine mammary subtypes and Vet-NPI, in which triple-negative tumors displayed the highest mean score compared to other subtypes (p < 0.001). In addition, the multivariable analysis revealed that the regional mastectomy procedure (adjusted HR = 2.78 (1.14-6.8)), the triple-negative tumors (adjusted HR = 48.08 (7.74-298.8)), strong Ki67 protein expression group (adjusted HR = 7.88 (2.02-30.68)), and strong Cox2 protein expression group (adjusted HR = 29.35 (5.18-166.4)) demonstrated significantly lower disease-free survival rates compared to other corresponding groups. Overall, canine mammary tumors showed strong similarities to human breast cancer in terms of clinical and molecular aspects; therefore, they could be suggested as a model for human breast cancer in comparative oncology.

IL21R hypomethylation as a biomarker for distinguishing benign and malignant breast tumours

ABSTRACT Some benign and malignant breast tumours are similar in pathological morphology, which are difficult to be distinguished in clinical diagnosis. In this study, we intended to explore novel biomarkers for differential diagnosis of benign and malignant breast tumours. Methylation EPIC 850K beadchip and RNA-sequencing were used to analyse 29 tissue samples from patients with early-stage breast cancer (BC) and benign breast tumours for differently methylated and expressed genes. The altered methylation of IL21R was semi-quantitatively validated in an independent study with 566 tissue samples (279 BC vs. 287 benign breast tumours) using mass spectrometry. Binary logistic regression analysis was performed to evaluate the association between IL21R methylation and BC. BC-associated IL21R hypomethylation and overexpression were identified in the discovery round. In the validation round, BC patients presented significant IL21R hypomethylation compared to women with benign breast tumours (ORs ≥1.29 per-10% methylation, p-values ≤ 5.69E–14), and this hypomethylation was even enhanced in BC patients with ER-negative and PR-negative tumours as well as with triple-negative tumours. The methylation of IL21R showed efficient discriminatory power to distinguish benign breast tumours from BC (area under curve (AUC) = 0.88), and especially from ER-negative BC (AUC = 0.95), PR-negative BC (AUC = 0.93) and triple-negative BC (AUC = 0.96). We disclosed significant IL21R hypomethylation in patients with BC compared to women with benign breast tumours, and revealed the somatic change of DNA methylation could be a potential biomarker for molecular pathology of BC.

CD163+ macrophages in the triple-negative breast tumor microenvironment are associated with improved survival in the Women’s Circle of Health Study and the Women’s Circle of Health Follow-Up Study

BackgroundTumor-associated macrophages (TAMs) are a prominent immune subpopulation in the tumor microenvironment that could potentially serve as therapeutic targets for breast cancer. Thus, it is important to characterize this cell population across different tumor subtypes including patterns of association with demographic and prognostic factors, and breast cancer outcomes.MethodsWe investigated CD163+ macrophages in relation to clinicopathologic variables and breast cancer outcomes in the Women’s Circle of Health Study and Women’s Circle of Health Follow-up Study populations of predominantly Black women with breast cancer. We evaluated 611 invasive breast tumor samples (507 from Black women, 104 from White women) with immunohistochemical staining of tissue microarray slides followed by digital image analysis. Multivariable Cox proportional hazards models were used to estimate hazard ratios for overall survival (OS) and breast cancer-specific survival (BCSS) for 546 cases with available survival data (median follow-up time 9.68 years (IQR: 7.43–12.33).ResultsWomen with triple-negative breast cancer showed significantly improved OS in relation to increased levels of tumor-infiltrating CD163+ macrophages in age-adjusted (Q3 vs. Q1: HR = 0.36; 95% CI 0.16–0.83) and fully adjusted models (Q3 vs. Q1: HR = 0.30; 95% CI 0.12–0.73). A similar, but non-statistically significant, association was observed for BCSS. Macrophage infiltration in luminal and HER2+ tumors was not associated with OS or BCSS. In a multivariate regression model that adjusted for age, subtype, grade, and tumor size, there was no significant difference in CD163+ macrophage density between Black and White women (RR = 0.88; 95% CI 0.71–1.10).ConclusionsIn contrast to previous studies, we observed that higher densities of CD163+ macrophages are independently associated with improved OS and BCSS in women with invasive triple-negative breast cancer.Trial registrationNot applicable.

Abstract PO4-11-03: Impact of Short Term Fasting (STF) on Fatigue developing in primary breast cancer (BC) patients receiving the Henderson or Sparano Scheme in the prospective multicentre FIT2- cohort

Abstract Background: Experimentally, it has been shown that short-term fasting (STF) induces a “protective mode” in healthy cells against chemotherapeutic toxicity while it enhances the susceptibility of tumor cells to chemotherapy. In healthy cells, fasting triggers distinct protective metabolic and gene expression changes, such as upregulation of DNA repair mechanisms, autophagy, and downshift of metabolic pathways including IGF-1-dependent signaling. This study was conducted to assess feasibility, quality of life (QoL) including chemotherapy-induced fatigue (CTF) comparing STF around each chemotherapy (4xAC or 4xEC) to a plant-based low-sugar diet (PBD). Methods: Newly diagnosed BC-patients treated by the Henderson or Sparano scheme were included in this randomised, controlled, multicenter study. CTF was assessed by the FACIT-F questionnaire before (day 0) and after chemotherapy (day 7) as well as at months 4 (V1) and 6 (V2). Results: Of the included n=106 patients, 52 were randomized to STF and 54 to PBD. n=90 (84.9%) patients obtained AC/EC+Pac and 16 (15.1%) AC/EC+Doc. FACIT-F showed no significant difference at baseline (STF 42.8; PBD 41.3; p=0.708). The differences between the corresponding FACIT-F Trial Outcome Index resp. Total FACIT-F at cycle 4 day 7 were 13.1 (SE 4.7) resp. 15.8 (SE 5.6), both in favor of STF (statistically significant (p &amp;lt; 0.007) and clinically relevant (minimal important difference (MID) &amp;gt; 5). This difference was visible for all breast cancer subtypes, i.e. luminal A &amp; B, as well as HER2 overexpressed and triple negative tumors, with the STF group showing no clinically relevant fatigue after 4 cycles of chemotherapy (FACIT-F &amp;gt; 34), across all subtypes. The opposite was true for the PBD group (FACIT-F &amp;lt; 34) across all subtypes. FACIT-F (22.7) for PBD in HER2 overexpressed and triple negative tumors was significantly worse than those in luminal A and B patients (FACIT-F= 29.1; difference 6.4 &amp;gt; MID=3). STF was well tolerated and there were no serious adverse effects. Conclusion: STF during chemotherapy is well tolerated and appears to improve QoL including fatigue during chemotherapy compared to a plant-based low-sugar diet, independent of intrinsic subtypes. Citation Format: Daniela Koppold, Christian Kessler, Manfred Wischnewsky, Ixquic Alvarado, Nico Steckhan, Cornelia Kempter, Marion Paul, Barbara Brückner, Dorothea Fischer, Aysen Wunschel, Rainer Stange, Etienne Hanslian, Michael Jeitler, Andreas Michalsen. Impact of Short Term Fasting (STF) on Fatigue developing in primary breast cancer (BC) patients receiving the Henderson or Sparano Scheme in the prospective multicentre FIT2- cohort [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-11-03.

Abstract PO5-08-11: Genetic Landscape of Early Breast Cancer in an Ashkenazi Jewish Cohort

Abstract Background: The Ashkenazi Jewish (AJ) population exhibits a distinctive mutation profile in BRCA1 and BRCA2 (BRCA1/2), characterized by 3 common founder mutations: BRCA1 185delAG, BRCA1 5382insC, and BRCA2 6174delT. These 3 mutations are associated with 10% of invasive breast cancer (BC) cases among AJ women. However, the frequency of non-founder pathogenic variants in BRCA1/2 and other BC-related genes in AJ women with BC remains unclear. Methods: This retrospective study included AJ patients diagnosed with stage I-II human epidermal receptor growth factor 2 negative BC between 2004-2022 at the Weill Cornell Breast Center. We collected clinicopathological characteristics and genetic information to determine the frequency of predisposing mutations in BC and non-BC related genes in AJ patients with early BC. We also analyzed the clinicopathological characteristics and outcomes of patients with and without germline mutations in these genes. Results: The study included 232 self-identified AJ patients, with 98% being females. The median age at BC diagnosis was 62 years (IQR: 50-70). Stage I and II cancers accounted for 68% and 32% of cases, respectively. Hormone receptor positivity (HR+) was observed in 71% of patients, while 29% had triple-negative tumors. Genetic testing was performed on 88% (n=203) of the patients, while 5% of patients declined to undergo genetic testing. Among the tested patients, 12% (n=25/203) had pathogenic variants in BRCA1 or BRCA2, with 44% and 16% of these corresponding to the BRCA1 and BRCA2 founder mutations, respectively. Testing beyond BRCA1/2 genes was conducted on 154 patients, and only 6% (n=9/154) were found to have pathogenic variants in CHEK2, while no mutations were detected in other prevalent BC-related genes, including PALB2 and ATM. Fourteen-percent of patients had pathogenic mutations in other genes including APC, CFTR, FH, DIS3L2, FANCC, MUTYH, NF1 and VHL. The frequency of pathogenic BRCA1/2 variants was inversely proportional to the age at BC diagnosis: 50% (n=8/16), 21% (n=8/39), 15% (n=6/40), and 3% (n=3/108) of patients diagnosed at age &amp;lt; 40, 40-49, 50-59, and &amp;gt;60 years, respectively, had a pathogenic variant in either BRCA1 or BRCA2. The frequency of founder mutations varied according to age at diagnosis: 63%, 75%, 17%, and 100% for patients aged &amp;lt; 40, 40-49, 50-59, and &amp;gt;60 years, respectively. Patients with BRCA1/2 mutations exhibited higher-risk clinicopathological features compared to those without mutations, including a higher proportion of high histological grade (88%), and triple-negative tumors (76%). Mastectomy was the primary surgical treatment for 56% and 23% of patients with and without BRCA1/2 mutations, respectively. No difference in disease-free survival was found between patients with and without BRCA1/2 mutations with a median follow up of 3 years (IQR: 1-4). Conclusions: In our cohort, 17% of AJ patients with early BC carried a pathogenic variant in a BC-related gene, with BRCA1/2 being the most commonly affected genes. The proportion of pathogenic variants in BRCA1/2 showed an inverse correlation with the age at BC diagnosis, reaching up to 50% in patients diagnosed under the age of 40. These findings contrast with the lower prevalence of pathogenic variants in BRCA1/2 (38%) reported in the existing literature for this age group. Additionally, 8% of patients had pathogenic variants in other tested cancer genes carrying a genetic predisposition to colorectal cancer (CRC). These findings contribute to our understanding of the genetic landscape of early BC in the AJ population and emphasize the importance of comprehensive genetic testing, particularly among young patients. These results also suggest the need for earlier and more frequent CRC screening in a population of BC patients that may not be adhering to recommended guidelines. Furthermore, our study provides insight into how this genetic landscape may vary when analyzing late-stage BC in women of AJ heritage. Citation Format: Laura Munoz Arcos, Eleonora Nicolò, Maira Pires, Letizia Pontolillo, Leticia Varella, Tessa Cigler, Eleni Andreopolu, Anne Moore, Ashley Schreier, Carlos Munoz-Zuluaga, Lisa Newman, Georgia Syrnioti, Massimo Cristofanilli. Genetic Landscape of Early Breast Cancer in an Ashkenazi Jewish Cohort [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-08-11.

Abstract PS18-09: The DARC side of Breast Cancer - DARC, Duffy-null and African ancestry influence in the Triple Negative Breast Cancer tumor microenvironment

Abstract Background. Despite a recent convergence of breast cancer (BC) incidence rates between African American (AA) and European American (EA) women, AA women continue to have ~40% higher mortality rates. BC is a heterogeneous collection of diseases, where African ancestry women are disproportionately burdened with the most aggressive subtype of BC, triple negative BC (TNBC). We have previously reported that the African ancestry-specific Duffy-null allele (FY-) is a risk factor for TNBC diagnoses. FY- is a promoter variant that removes expression of the Duffy Antigen Receptor for Chemokines (DARC/ACKR1), from red blood cells (RBCs). DARC serves as a portal of entry for Plasmodium vivax malaria parasite, and FY- confers immunity, leading to fixation of FY- among Sub-Saharan African populations. DARC is an atypical chemokine receptor that functions to modulate chemokine levels in circulation and tissues to aid immune cell recruitment. DARC is also expressed on breast tumor epithelial cells, however the significance of DARC in the TNBC tumor microenvironment (TME) has not fully been explored. Leveraging our African ancestry enriched International Study for the Center of Breast Cancer Subtypes cohort and in vivo murine models, we employed multi-omics approaches to explore the function of FY- and DARC expression across cell types in the TNBC TME. Genomics. Using transcriptomic data from the TCGA BC cohort, we have previously reported that DARC expression is highly correlated with deconvoluted immune cell abundance using CIBERSORT. To build upon this finding among TNBC patients, we have analyzed RNAseq data from two cohorts of African ancestry women to explore correlation of DARC expression status and immune cell abundance. Among TNBC cases, we report that higher DARC expression was significantly associated with increased immune cell abundance, with increases in B cell, T cell and monocyte populations. Using whole genome sequencing data across 86 African ancestry cases, we are currently investigating germline FY- status with mutational signatures. Circulating biomarkers. Patient plasma was collected prior to surgery, where Luminex multiplex assays were performed to quantify levels of &amp;gt;40 circulating chemokines and cytokines. FY- status has been determined for ~550 patients using single-plex or multiplex PCR genotyping methods from germline saliva DNA. Our univariate analyses show several circulating chemokines, including CCL2, CXCL2, CXCL6 and CXCL11, have significant differences between FY- and non-FY- BC patients. Curation of clinicopathological variables for multivariate analysis is ongoing. Murine models. We crossed C3(1)Tag BC transgenic mice with a DARC knock-out (KO) line to generate mice lacking DARC expression that spontaneously develop tumors in the mammary fat pads. The C3(1)Tag model is reported to have similar gene expression profiles to basal-like TNBC tumors. Tumors were collected at pre-determined time points of 25 and 30 weeks, where DARC KO mice had increased tumor size and tumor burden at 30 weeks. Initial fluorescent staining revealed an increase in immune cell infiltration in the TME of DARC expressing mice compared to DARC KO mice. To further explore this, we employed multiplex staining across a representative cohort of DARC expressing and KO mice at 25 and 30 weeks to capture the specific immune cell populations infiltrating the TME. Conclusions. TNBC tumors have worse prognosis due to lack of targeted therapeutic options. Increased immunogenicity of TNBC tumors, especially among African ancestry women may be driven in part by DARC expression. Characterization of FY- status, in coordination with DARC among other cell types in the TNBC TME could present new opportunities for biomarker and/or therapeutic development to improve prognosis for these underserved populations. Citation Format: Rachel Martini, Stevens Patino, Emma Guyonnet, Brian Stonaker, Isra Elhussein, Julie Sahler, Avery August, Nancy Manley, Rick Kittles, Clayton Yates, Lisa Newman, Melissa Davis. The DARC side of Breast Cancer - DARC, Duffy-null and African ancestry influence in the Triple Negative Breast Cancer tumor microenvironment [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS18-09.

Abstract PO2-14-04: Circulating tumor DNA after neoadjuvant chemotherapy is a better prognostic test than Residual Cancer Burden in patients with triple negative breast cancer and residual tumor

Abstract Background The presence of residual tumor at surgery (non-pathological complete response or non-pCR) occurs in about half of TNBCs treated with neoadjuvant chemotherapy (NAC) signals chemoresistance. Although the addition of further adjuvant chemotherapy (Capecitabine/Xeloda) significantly improves RFS in these patients, the majority of patients nevertheless have a good prognosis, and thus may not require adjuvant Capecitabine. One of the best prognostic biomarkers in these patients is Residual Cancer Burden (RCB), which relies on the pathological examination of the residual tumor bed and lymph nodes obtained at surgery. Circulating tumor DNA (ctDNA) is a plasma-based biomarker, which has been shown to accurately detect minimal residual disease in breast cancer. We have previously shown that detection of ctDNA after NAC but before surgery in patients with non-pCR signals very poor prognosis, while non-detectable ctDNA is associated with an excellent prognosis. We now compare ctDNA to RCB score in TNBC patients with non-pCR. Methods In the multicenter TRICIA study, 44 TNBC patients with non-pCR after NAC in 5 hospital centers had consented to serial plasma collection prior to and after surgery. RCB score was calculated from pathology records. Whole exome sequencing (WES) was performed on DNA extracted from primary tumor biopsies or residual tumors at surgery to identify tumor-specific mutations (5 variants/patient). Digital droplet PCR (ddPCR) assays were developed for these mutations and tests were run in an academic hospital setting. Patients with at least one detectable mutation were considered ctDNA positive for a given time point. The detection of ctDNA was correlated with relapse-free survival (RFS). Results: The median follow up was 40 months. 19 of the 44 patients relapsed within 2 years after surgery. Detection of ctDNA at the end of NAC but before surgery (T1) was correlated with very poor RFS (H.R. = 0.15, p&amp;lt; 0.0001 (95% CI = 0.067-0.35)). 14/15 patients (93%) without detectable ctDNA did not recur, while 76% of patients with detectable ctDNA relapsed. The RCB score distribution was as follows: 7 RCB1, 21 RCB2 and 16 RCB3. According to RCB score, 1/7 RCB1, 11/21 RCB2 and 11/16 RCB3 patients relapsed. All but one of the relapsed patients had detectable ctDNA in at least one timepoint. All 11 RCB3 patients that relapsed had detectable ctDNA while none of the 5 RCB3 patients without relapse had detectable ctDNA. The one RCB1 patient with detectable ctDNA also relapsed. In cox regression analysis, the lack of ctDNA detection was significantly associated with excellent RFS in RCB2 (HR=0.16, p=0.003, (95% CI=0.05-.53)) and RCB3 groups (HR=0.10, p=0.0005, 95% CI= 0.03-.36). Conclusion ctDNA testing using ddPCR in an academic hospital-based setting at the post-NAC time point identifies an excellent prognostic group in TNBC patients with non-pCR and can discriminate very good from very poor prognosis in both RCB2 and RCB3 score subgroups. The detection of ctDNA may be superior to RCB scores in assessing prognosis in patients with non-pCR. Citation Format: Mark Basik, Talia Roseshter, Anna Klemantovich, Luca Cavallone, Adriana Aguilar-Mahecha, Josiane Lafleur, Cathy Lan, Oluwadara Elebute, Sarah Jenna, Jean-Francois Boileau, Manuela Pelmus. Circulating tumor DNA after neoadjuvant chemotherapy is a better prognostic test than Residual Cancer Burden in patients with triple negative breast cancer and residual tumor [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-14-04.

Abstract PO4-06-11: Development of fully human anti-progranulin monoclonal antibodies as inhibitors of triple negative breast cancer cell growth and tumor formation

Abstract Triple negative breast cancer (TNBC) is characterized by invasiveness and poor survival. Identifying novel TNBC targeted therapies to potentiate standard of care (SOC) therapy, is an unmet need. In the past 4 years, several targeted therapies for TNBC have been approved by the Food and Drug Administration (FDA). However, the identification of targeted therapies against other biological drivers of TNBC remains important. Progranulin (PGRN/GP88) is a biological driver of tumorigenesis, survival, and drug resistance in breast cancer. PGRN/GP88 tissue expression is an independent prognostic factor of recurrence while elevated serum PGRN/GP88 level is associated with poor outcomes such as progression of disease and shortened survival. PGRN/GP88 expression is elevated in 30% TNBC. The importance of inhibiting PGRN/GP88 effect on the proliferation and tumor growth of triple negative breast cancer cells was previously established. In the current study, in association with Precision Antibody, we developed anti-PGRN/GP88 fully human monoclonal antibodies by immunizing TC transgenic humanized mice with recombinant human progranulin. Within 60 days since the start of immunization, several fully human monoclonal antibodies were successfully developed, characterized by several functional assays including neutralization, inhibition of PGRN/GP88 cell surface binding, proliferation, migration in MDA-MB-231 cells in vitro and in vivo. Results: The inhibition of PGRN/GP88 action by fully human monoclonal antibodies inhibited proliferation and migration in a dose-and time-dependent fashion in TNBC cells. Octet analysis determined Kd in the range of 10−11 M to 10−10 M. Transwell assay showed that anti-PGRN treatment inhibited migration In vivo xenograft studies with MDA-MB-231 cells injected in athymic nude mice showed that several fully human anti-PGRN antibodies inhibited tumor growth when compared to antibody control treated mice. In vivo dose response of AG01 in MDAMB-231 tumor bearing mice will be provided. Conclusion: PGRN/GP88 represents a therapeutic target for TNBC with two companion diagnostics (tissue test and ELISA to measure GP88 circulating levels). The use of the TC transgenic mice allows for the rapid development of fully human high affinity monoclonal antibodies bypassing the need for chimerization or humanization of mouse monoclonal antibodies. The development of fully human anti-progranulin antibodies provides novel targeted therapeutic option for TNBC. This work is supported by a an SBIR grant 5R44 CA 224718 from the National Cancer Institute to GS. Citation Format: Ginette Serrero, Jianping Dong, Jun Hayashi, Chun Dong. Development of fully human anti-progranulin monoclonal antibodies as inhibitors of triple negative breast cancer cell growth and tumor formation [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-06-11.

Abstract PO5-13-10: Determining tumor-driving cell signaling pathways in breast cancer: Support for targeted therapy selection

Abstract Introduction: Success of therapeutic interventions largely rely on the pathological and biological characteristics of the tumor and varies due to the heterogeneous nature of breast cancers. Signal transduction pathways play an important role and are often used as target for specific therapies like hormone treatment, PI3K inhibitors etc. Immunohistochemistry (IHC) is currently the first assessment to divide tumors for subtyping and treatment and, especially in more advanced disease, mutation analyses are often used. Both IHC and mutation analyses only analyze a specific part of a signal transduction pathway and do not correlate well with outcome. Therefore, there is a need for new approaches to look at signaling pathways in cancer. We have developed a methodology that measures and quantifies signal transduction pathways (STPs) activities to reveal potential tumor-driving STPs to identify new options for targeted therapy in breast cancer. Method: Using the mRNA-based OncoSIGNal pathway activity profiling qPCR test (InnoSIGN), STP activities of 7 pathways (AR, ER, PI3K, MAPK, HH, TGFβ and Notch) were measured and quantified on a scale from 0-100. By using a set of non-tumor epithelial breast tissue samples, a reference range for normal physiological pathway activity was defined. A threshold was set on the 95th percentile of this range. A pathway activity score above this threshold was considered to be high (activated). Using this methodology, 201 ER positive (based on IHC) and 64 triple negative FFPE breast tumor samples obtained from different study sites and commercial biobanks were analyzed, including samples from primary and metastatic lesions, to determine for each individual tumor its pathway activity profile and identify activated tumor-driving pathways. Results: In the 195 primary ER positive tumors, the ER pathway was found to be activated in 78% of the samples, even though all samples were strongly ER immunohistochemistry stain positive. In addition to ER pathway activation, occasionally other pathways were activated: PI3K in 37%, HH in 25%, MAPK in 19%, AR in 20%, TGFβ in 6%, and Notch in 1%, of these samples. Analysis of 27 metastatic samples of ER positive tumors revealed in only 63% an active ER pathway. Additionally, more frequently PI3K pathway was observed in 70% of the samples. Other frequencies of pathway activation were: AR in 7% and MAPK in 4%, No Notch and TGFβ activity was found in this cohort. In 76 of the 78 triple negative tumors (TNBC) the ER pathway was inactive as expected. Remarkably, 2 samples appeared to have an activated ER pathway. An activated MAPK pathway was found in 50%, PI3K in 49%, HH in 18%, AR in 14%, TGFβ in 5% and Notch in 3% of these TNBC tumors. Conclusion: Compared to ER positive primary tumors we observed in metastatic ER positive tumors less frequent activation of the ER and more frequent activation of the PI3K pathway, suggesting that the latter pathway is associated with a more aggressive tumor phenotype. The triple negative subtype is characterized predominantly by activation of the PI3K, MAPK, AR and/or HH pathways, which may create new options for personalized targeted therapy of this subtype. OncoSIGNal can be used to determine the tumor driving signaling pathways in breast cancer patients, guiding selection of personalized targeted therapies. Citation Format: Paul van de Wiel, Yvonne Wesseling-Rozendaal, Anke Pierik, Dianne van Strijp, Saskia Vermeer, Fatema Tashrifwala, Josette Hartnett, Robert Babkowski, Suzanne Rose. Determining tumor-driving cell signaling pathways in breast cancer: Support for targeted therapy selection [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-13-10.

Abstract PO1-17-04: Influence of regulatory agency approval of trastuzumab-deruxtecan on the diagnosis of HER2-low status at a reference cancer center in Brazil

Abstract BACKGROUND HER-2 low status is defined by immunohistochemistry (IHC) expression for human epidermal growth factor receptor 2 (HER-2) 1+ or IHC 2+ without gene ERBB2 amplification. HER2-low breast cancers (BC) accounted for approximately 55% of all cases, and among hormone receptor positive (HR+) tumors it represents up to 60% of patients (pts), while 10-20% in triple negative tumors (TNBC). Trastuzumab-deruxtecan (T-dxd) has been FDA approved for HER-2 low breast cancer after Destiny-Breast 04 trial (DB04) demonstrated a robust overall survival benefit either in HR+ or TNBC heavily pre-treated pts. The Brazilian Health Regulatory Agency (ANVISA) has later approved T-dxd for HER-2 low BC. However, due to its dynamic nature and subjectivity, the definition of HER-2 low status still lacks better criteria. This study aims to evaluate whether HER-2 low status diagnosis was impacted according to the time of the diagnosis - before DB04 publication and after ANVISA drug approval - in a reference cancer center in Brazil. METHODS This retrospective study was conducted in a single institution in Sao Paulo, Brazil. Pts diagnosed with BC between October 1st 2021 to June 1st 2023 were classified in 2 groups according to the time of diagnosis. Group 1 (g.01) included those who were diagnosed between October 1st 2021 until DB04 publication in July 7th 2022; and Group 2 (g.02) included pts diagnosed after ANVISA T-dxd approval on October 31st, 2022 until June 1st 2023. All pathological analysis of the primary breast tumor was performed by 2 experienced breast pathologists. HER-2 IHQ Score was defined according to ASCO/CAP guideline 2018. The primary endpoint was to evaluate the proportion of patients who were defined as HER-2 low according to the time of diagnosis. Secondary endpoint consists in evaluate change in pathologists practice to distinguish IHC 1+ results from 0 after the definition of HER-2 low. To compare the categorical variables between the groups, the Chi-square test was used. P values below 0.05 were considered statistically significant. Statistical analyses were carried out with the R program (R Foundation, Vienna, Austria). RESULTS A total of 713 pts with diagnosis BC were included in the study, 401 (56%) in g.01 and 312 (44%) in g.02. Overall, before DB04 publication, the diagnosis HER “Negative-NOS” without subdivision in score 0 or 1 was found in 127 (31.7%), but only in 27 (8.7%) reports in g.02 (p=0.007-13). HER-2 low status was observed in 12.2% in g01 and in 33.3% of the pts in g.02 (p &amp;lt; 0.00001). Specifically, negative score 0 was found in 45.6% of pts in g.01 and 44.6% in g.02 (p= 0.014). A significant difference was also observed among pts with negative Score 1+ between groups (6.0% vs 21.2%, p=0.009-3), as shown in Table 1. No significant difference was seen in the frequency of HER2 score 2+ and score 3+ between g.01 and g.02. CONCLUSION The recent impact of a highly effective treatment derived from definition of HER-2 low BC led to a greater relevance in differentiating IHC 1+ from IHC 0, both considered HER negative by ASCO/CAP protocols. Distinguishing between HER2 0 and 1+ is subject to considerable interobserver variability , and the time of regulatory approval demonstrated a direct influence on the HER-low diagnosis. Table 1 Distribution of HER2 IHQ results in Group 1 and Group 2 Citation Format: Paula Martinez Vianna, Venancio Avancini Ferreira Alves, Pedro Exman. Influence of regulatory agency approval of trastuzumab-deruxtecan on the diagnosis of HER2-low status at a reference cancer center in Brazil [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-17-04.

Abstract PO3-09-04: Identification of germline pathogenic variants to guide systemic therapy: Prevalence and spectrum of PVs among Mexican patients with metastatic breast cancer

Abstract BACKGROUND Currently, international genetic cancer risk assessment (GCRA) guidelines recommend genetic testing for the identification of germline pathogenic variants (PVs) in patients with metastatic breast cancer (MBC) to guide systemic therapy. Patients with PVs in BRCA genes are candidates to receive poly(adenosine diphosphate–ribose) polymerase inhibitors (PARPi). The prevalence of PVs in patients with HER2-negative MBC has been reported to be 10%. The proportion of MBC in low- and middle-income countries (LMIC) is higher. However, access to genetic testing in LMIC is limited and contributes to the gap in genetic epidemiology knowledge. We aimed to describe the frequency of germline PVs in breast cancer associated genes and clinic-pathologic characteristics of Mexican women with MBC. METHODS This retrospective study included Mexican women from 7 centers in Mexico, who were diagnosed with MBC and had genetic testing. Pathogenic and likely pathogenic variants were reported. Demographic, histopathological characteristics, family history and access to treatment with PARPi, were obtained from chart review. RESULTS From August 2017 to May 2023, 114 Mexican women with MBC were included, median age at diagnosis was 47 years (range 24-80 y), 42% had de novo metastatic, and 58% recurrent disease. 86.8% had infiltrating ductal carcinoma, 51.3% had HR+HER2- (n=58), 4.4% HR+HER+ (n=5), 5.3% HR-HER2+ (n=6), and 38.6% triple negative tumors (n=44). Sites of metastatic disease were as follows: 61.4% non-regional nodes (n=70), 57% bone (n=65), 40.3% lung (n=46), 30.7% liver (n=35), 15.8% central nervous system (n=18) and 7% peritoneal carcinomatosis (n=8). The frequency of PVs was 22% (25/114); 84% of the PVs were heterozygous, mainly identified in BRCA genes (BRCA1 n=12, BRCA2 n=9), and in 4 patients in other genes (PALB2 n=2, PTEN n=1, MUTYH n=1). Among PARPi candidates, 40% (10/25) received Olaparib, the majority received it on the second or subsequent lines of treatment (7/10) with a median progression free survival of 6 months (3-13 m). BRCA carriers were younger at BC diagnosis (38.5 vs. 49 years, p=0.00694) and had a significant higher proportion of family history of ovarian cancer (p=0.00039), but not for breast (p=0.06), prostate (p=0.71) or pancreatic cancer (p=0.098), compared to non-carriers. CONCLUSION We found a high frequency of PVs in BRCA genes in Mexican women with MBC. Access to the genetic testing and targeted therapy should be improve in the Mexican population in order to decrease disparities in cancer care in the Mexican Population. In addition, genetic testing allows to identify other family members at risk with cascade testing and to recommend them risk reduction strategies. Citation Format: Jose Luis Rodriguez-Olivares, Maria Fernanda Esparza-Orozco, Jose Alfredo Ramirez-Gonzalez, Roberto Ivan Sanchez-Reyes, Enrique Isay Talamantes-Gamez, Gregorio Quintero-Beulo, Gilberto Lopez-Rosas, Karla Alicia Centelles-López, Brizio Moreno-Jaime, Daniela Vazquez-Juarez, Ana Maria Hernandez-Murillo, Ilana de la Puente Tawil, Jeffrey Weitzel, Cynthia Villareal Garza, Yanin Chavarri-Guerra. Identification of germline pathogenic variants to guide systemic therapy: Prevalence and spectrum of PVs among Mexican patients with metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-09-04.

Abstract PS13-01: Worse Prognosis for Breast Cancer in the Second and Third Trimesters and Shortly Postpartum:An Update of the Dutch Pregnancy-Associated Breast Cancer Cohort

Abstract Rationale: Pregnancy-Associated Breast Cancer (PABC), often defined as breast cancer during pregnancy (PrBC) or in the first year postpartum (PPBC), is known for its aggressive histopathology and higher stage at diagnosis. In a previous study of the nationwide Dutch PABC Cohort, we observed an impaired survival for patients diagnosed in the second and third trimester of pregnancy. However, the etiology of these differences, and how this relates to the patients diagnosed with breast cancer in the postpartum period (PPBC) or after an abortion (AABC), remains to be elucidated. While we are exploring the etiologic hypotheses, we updated and expanded our database to compare these specific PABC subgroups. Methods: All pathology reports of invasive breast carcinoma between January 1st 1988 and July 1st 2022 were screened for pregnancy-related keywords to find all patients diagnosed with PrBC, PPBC or AABC within one year after childbirth or an interrupted pregnancy (induced abortion or miscarriage before a gestational age of 24 weeks). A local patient series provided 22 additional cases. Pregnant patients were subdivided by gestational trimester at time of diagnosis. The different PrBC, PPBC and AABC subgroups were compared by histopathology, clinical characteristics and outcome. Results: 787 patients were included, of whom 60% were pregnant during their diagnosis. Median age at diagnosis was 34 years. In line with our previous findings, a large majority (68%) of patients had a Bloom &amp; Richardson (B&amp;R) grade III tumor and 37% of tumors were triple negative. Moreover, over a third of patients presented with lymph node metastases (38%). We observed two distinct groups based on histopathology and prognosis: group A, consisting of three patient groups: (I) patients diagnosed in the first trimester of pregnancy, (II) patients diagnosed between 6-12 months postpartum, and (III) patients diagnosed after an abortion, and group B, consisting of two patient groups, (I) patients diagnosed in the second or third trimester of pregnancy or (II) within 6 months after childbirth. Compared to group A, patients in group B more often had B&amp;R grade III tumors: (73% vs. 60%, p=0.002) and more frequently had triple negative tumors (43% vs. 28% in group A, p&amp;lt; 0,001). Survival was significantly worse for patients in group B (5-year OS 67,3% versus 88.2%, p&amp;lt; 0.001). In a multivariable analysis, corrected for grade, type of surgery and overall stage, these differences upheld with an HR of 1.515 (95% CI 1,012 – 2,268). Conclusions: With this extensive update of our nationwide Dutch PABC cohort, we show that especially PrBC patients diagnosed in the second and third trimesters and PPBC patients within six months after childbirth exhibit unfavorable tumor characteristics, with an associated worse prognosis. This highlights the need for in-depth analyses in these specific groups of PABC patients to elucidate the etiologic mechanisms involved. Citation Format: Carsten Bakhuis, Carmen Van Dooijeweert, Britt Suelmann, Janneke Verloop, Pieter Westenend, Sabine Linn, Paul Van Diest, Elsken Van der Wall. Worse Prognosis for Breast Cancer in the Second and Third Trimesters and Shortly Postpartum:An Update of the Dutch Pregnancy-Associated Breast Cancer Cohort [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS13-01.