Germline Mutations In BRCA1 Research Articles

Abstract P4-07-29: Olaparib plus Trastuzumab in HER2[+] BRCA-Mutated Advanced Breast Cancer Patients: The OPHELIA Study

Abstract Background: Olaparib (O) is approved for the treatment of HER2[-] patients (pts) with early or metastatic breast cancer and a germline BRCA mutation. Nevertheless, there is no evidence that HER2[+] tumors are resistant to PARPi. Preclinical data support that HER2[+] cells are sensitive to PARPi and strongly suggest that PARP inhibition augments the efficacy of trastuzumab (T). To test whether PARPi is synergistic with anti-HER2 therapy, the OPHELIA study has assessed the efficacy and safety of O in combination with T in pts with HER2[+] germinal BRCA-mutated advanced breast cancer (ABC). Methods: OPHELIA (NCT03931551) is an open-label, multicenter, single-arm, phase II trial. The study enrolled pts aged ≥18 years diagnosed of HER2[+] ABC with germinal deleterious mutations in BRCA1 or BRCA2 who had received at least one prior systemic regimen for advanced disease (including a pertuzumab- or trastuzumab emtansine based regimen). Pts received O (300 mg oral, twice daily) plus T (either loading dose of 8 mg/kg IV infusion, and subsequent 3-weekly doses of 6 mg/kg IV infusion; or 600 mg SC injection, on day 1 of every 21-day cycle) until disease progression, unacceptable toxicity, or consent withdrawal. Primary endpoint was investigator-assessed clinical benefit rate (CBR) for at least 24 weeks as per RECIST v.1.1. Secondary endpoints included overall response rate (ORR), duration of response (DoR), progression-free survival (PFS), overall survival (OS); and safety and tolerability as per NCI-CTCAE v.5.0. The primary analysis evaluated CBR (H0: ≤5%; H1: ≥30%) based on exact binomial test. Sample size was designed to attain a 90% power at 10% one-sided alpha level. Results: From Mar 25, 2019, through Mar 2, 2022, 5 pts (from a total of 42 pts evaluated) were enrolled at 17 sites in Spain. Median age was 37.0 (range 32–54) years, 1 (20.0%) patient was male, 4 (80.0%) pts carried germinal BRCA2 mutations, 4 (80.0%) pts had received ≥ 3 advanced disease treatments lines, and 4 (80.0%) pts presented ≥ 2 metastatic sites. At data cutoff (Mar 2, 2022), with a median follow-up of 18.7 months (min: 11.7; max: 22.1), 40.0% of pts remained on therapy. CBR at 24 weeks was 80.0% meeting the primary endpoint (4 of 5 pts; 95% CI, 28.4% to 99.5%, p< 0.001). ORR (1 complete and 2 partial responses) was 60.0% (95% CI, 17.4% to 94.7%), and median DoR was 3.8 months (95% CI, 2.5 to 8.3 months). Two (40.0%) pts had PFS events due to disease progression at 5.2 and 1.2 months, respectively. Rest of pts were treated for 5.5, 11.2, and 19.0 months. There were 2 (40.0%) deaths at 14.0 and 18.5 months. The most common non-hematological treatment emergent adverse events (TEAEs) of any grade (G) were fatigue (60.0%; 0% G≥3), nausea (60.0%; 0% G≥3), vomiting (40.0%; 0% G≥3), and back pain (40.0%; 0% G≥3). Anemia (40.0%; 20.0% G≥3) and lymphopenia (40.0%; 20.0% G≥3) were the most frequent hematological TEAEs. One (20.0%) patient discontinued treatment because of a drug-related TEAE (leukopenia). A dose reduction of O was reported in 1 (20.0%) patient. No treatment-related deaths were reported. Conclusions: HER2 overexpression in germline BRCA-mutated ABC is infrequent. The activity observed in these 5 pts indicates that O+T combination might be of help in this group of pts. We strongly believe that randomized data are not needed, and RWE studies might help us to understand the real activity of this combination. Toxicity was as expected. Citation Format: José E. Alés-Martínez, Judith Balmaña, Pedro Sánchez-Rovira, Francisco Javier Salvador Bofill, José Ángel García-Sáenz, Isabel Pimentel, Serafin Morales Murillo, Adela Fernández, Ainhara Lahuerta Martínez, Neus Ferrer, Pilar Zamora, Begoña Bermejo, Tamara Díaz-Redondo, María Helena Lopez-Ceballos, María Galán, Andrea Malfettone, Laura Calabuig, Miguel Sampayo-Cordero, José Manuel Pérez-García, Javier Cortés, Antonio Llombart-Cussac. Olaparib plus Trastuzumab in HER2[+] BRCA-Mutated Advanced Breast Cancer Patients: The OPHELIA Study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-07-29.

Abstract P4-01-30: Real-world treatment patterns and survival among adults with metastatic breast cancer with BRCA1/2 mutations

Abstract Background: Limited information is available about real-world treatment patterns and survival among patients with metastatic breast cancer (mBC) with BRCA1/2 mutations. BRCA1/2 mutations, which are involved in the repair of DNA double-strand breaks, are rare. Since 2018, PARP inhibitors (olaparib and talazoparib) have been approved for the treatment of patients with germline BRCA-mutated HER2-negative locally advanced and/or mBC. Methods: This retrospective chart review study included adults diagnosed with mBC with ≥1 oncogenic somatic BRCA1/2 mutation detected by tumor comprehensive genomic profiling who began a line of therapy on or after July 1, 2014, at 3 oncology centers in the American Association for Cancer Research (AACR) Project Genomics Evidence Neoplasia Information Exchange (GENIE) consortium. The index date was the first initiation of a new therapy for mBC in this period (which could be first-line [1L] or subsequent line of therapy). Data on all available lines of therapy post-index were collected. Treatments received in different lines of therapy for mBC and overall survival (OS) were described separately for patients with HR-positive/HER2-negative (HR+/HER2-) mBC, triple negative mBC (TNBC), and HER2-positive (HER2+) mBC. OS from the start of each line of therapy was assessed using Kaplan-Meier analysis. Results: A total of 48 women with mBC met the inclusion criteria. The mean age at index was 58 years; 73% of women were White. Common sites of metastases included lymph nodes (56%), bone (52%), and lungs (31%). Somatic BRCA1 mutation was detected in 48% and BRCA2 mutation was detected in 54% of women. Assessment of germline BRCA mutations was conducted in 25 (52%) women; 2 (8%) of assessed women tested positive for germline BRCA mutations. 29 (60%) of women had HR+/HER2- mBC, 9 (19%) had TNBC, 9 (19%) had HER2+ mBC, and 1 woman had unknown HER2 status. Treatments received in different lines of therapy for mBC and OS from start of each line of therapy by HR and HER2 status are reported in Table 1. Common treatments for women with HR+/HER2- mBC included endocrine-based therapy, and chemotherapy. Women with TNBC were most often treated with chemotherapy. Women with HER2+ mBC most often received endocrine-based therapy in 1L; chemotherapy or HER2-targeted combination therapy in second-line (2L); and chemotherapy or HER2-targeted monotherapy in third-line (3L). Median OS from the start of 1L therapy was 37.5 months among women with HR+/HER2- disease; 20.0 months for women with TNBC; and 51.7 months for women with HER2+ disease. Conclusions: These findings provide real-world insights about treatment patterns and survival among women with mBC and BRCA1/2 mutations. Further studies with larger sample sizes are needed to confirm these findings. Table 1. Treatment patterns and survival by line of therapy, HR, and HER2 status among patients with BRCA1/2 mutated mBC Citation Format: Priyanka Bobbili, Jasmina Ivanova, David B. Solit, Niharika Mettu, Shannon McCall, Mallika Dhawan, Maral DerSarkissian, Bhakti Arondekar, Jane Chang, Alexander Niyazov, Jocelyn Lee, Risha J. Huq, Michelle Green, Michelle Turski, Aruna Muthukumar, Mianzhao Guo, Mei Sheng Duh, William Oh. Real-world treatment patterns and survival among adults with metastatic breast cancer with BRCA1/2 mutations [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-01-30.

Abstract P5-03-20: Mutations of TP53 and genes related to homologous recombination repair in breast cancer with germline BRCA1/2 mutations

Abstract Background Germline mutations of breast cancer susceptibility gene BRCA1 and BRCA2 (gBRCA1/2) are associated with elevated risk of breast cancer in young women in Asia. BRCA1 and BRCA2 proteins contribute to genomic stability through homologous recombination (HR)-mediated double strand DNA break repair (DDR) in cooperation with other HR-related proteins. In this study, we analyzed the targeted sequencing data of the breast cancer patients with gBRCA1/2 mutations to investigate the landscape of HR-related gene mutations and their clinical implications. Materials and Methods Data of the breast cancer patients with pathogenic gBRCA1/2 mutations and qualified targeted next generation sequencing, SNUH FiRST cancer panel, were analyzed. Single nucleotide polymorphisms, small insertions and deletions were analyzed with functional annotations using ANNOVAR. HR-related genes were defined as ABL1, ATM, ATR, BARD1, BRCA1, BRCA2, CDKN1A, CDKN2A, CHEK1, CHEK2, FANCA, FANCD2, FANCG, FANCI, FANCL, KDR, MUTYH, PALB2, POLE, POLQ, RAD50, RAD51, RAD51D, RAD54L, and TP53. Mismatch-repair genes were MLH1, MSH2, and MSH6. Clinical data were analyzed with cox proportional hazard models and survival analyses. Results Fifty five Korean breast cancer patients with known gBRCA1/2 mutations and qualified targeted NGS data were analyzed. Ethnically distinct mutations in gBRCA1/2 genes were noted, with higher frequencies of Val1833Ser (14.8%), Glu1210Arg (11.1%), and Tyr130Ter (11.1%) in gBRCA1 and Arg2494Ter (25.0%) and Lys467Ter (14.3%) in gBRCA2. Considering subtypes, gBRCA1 mutations were associated with triple-negative breast cancers (TNBC), while gBRCA2 mutations were more likely hormone receptor-positive breast cancers. At least one missense mutation of homologous recombination (HR)-related genes were observed in 44 cases (80.0%). The most frequently co-mutated gene was TP53 (38.1%). In patients with gBRCA1/2 mutations, however, genetic variations of TP53 occurred in locations different from the known hotspots of those with sporadic breast cancers. The patients with both gBRCA1/2 and TP53 mutations were more likely to have TNBC, high Ki-67 values, and increased genetic mutations, especially of HR-related genes. Survival benefit was observed in the TP53 mutants of patients with gBRCA2 mutations, compared to those with TP53 wildtypes. Conclusion Our study showed distinct genetic landscape of breast cancer patients with gBRCA1 and gBRCA2 mutations in the Asian populations. Further studies on precision medicine are needed for tailored treatments of patients with genetic diversity among different ethnic groups. Citation Format: Jinyong Kim, Kyeonghun Jeong, Hyeji Jun, Kwangsoo Kim, Tae-Yong Kim, Dae-Won Lee, Go-un Woo, Songyi Park, Hanbaek Yi, Kyung-Hun Lee, Seock-Ah Im. Mutations of TP53 and genes related to homologous recombination repair in breast cancer with germline BRCA1/2 mutations [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-03-20.

Abstract P4-09-10: Spatial protein and RNA expression in tumor, immune and stromal cells in BRCA1/2 mutated metastatic breast cancer

Abstract Background: Spatial transcriptomics and proteomics are cutting-edge techniques that allow for quantification of RNA and protein expression within separate cell populations or regions of interest in a tissue biopsy specimen. We aimed to independently characterize the tumor cells and immune infiltrate in metastatic site biopsy specimens from patients with somatic or germline BRCA1/2 mutations. Methods: We performed spatial transcriptomics and proteomics using the Nanostring GeoMx Digital Spatial Profiler (DSP) on 9 formalin-fixed paraffin-embedded (FFPE) tissue biopsy specimens from 5 patients with metastatic breast cancer and a known somatic or germline BRCA1/2 mutation. Each patient had 1-3 separate metastatic site tissue biopsy specimens. Cell populations of interest were delineated with fluorescently labeled antibodies to pan-cytokeratin on tumor cells, CD45 of immune cells, and a nuclear stain. On each sample, 4-5 regions of interest (ROIs) were selected near the interface of the metastatic tumor deposit and surrounding normal tissue. Each ROI was segmented into 3 areas of interest (AOIs): pan-cytokeratin-positive tumor cells, CD45-positive immune cells, and the remaining stromal cells negative for both pan-cytokeratin and CD45. Data was background corrected and normalized with internal negative and positive internal controls. Relative abundance of immune cell subtypes was derived from CD45+ cell gene expression data using the TIMER algorithm. Results: The expression of 57 proteins and 84 RNAs was quantified in each of the 396 AOIs. Separate analysis of CD45+ cells, pan-cytokeratin cells, and stromal cell populations confirmed expected differential expression of immune, tumor and stromal markers, respectively. Protein expression of the estrogen receptor (ER) and progesterone receptor (PR) on pan-cytokeratin-positive tumor cells using the DSP correlated with pathologist reviewed immunohistochemistry (IHC) for ER and PR. Significant differential expression was found within each cell population between hormone receptor (HR) positive and negative cohorts, including enrichment for S100B in HR negative samples. Comparison will be shown for expression of 14 overlapping genes/proteins. Immune cell compartment gene expression deconvolution showed heterogeneity in the relative proportion of infiltrating immune cell subtypes between samples and also between ROIs within a sample. Changes in gene and protein expression from a patient with 3 serial biopsy specimens can be tracked with substantial heterogeneity across distinct AOIs within individual specimens and across metastatic sites.. Conclusions: Spatial transcriptomics and proteomics is feasible for concurrent, relative quantification of protein and gene expression in tumor, immune, and stromal cells in FFPE tissue biopsies from patients with metastatic breast cancer harboring germline or somatic BRCA1 or BRCA2 mutations. Heterogeneity within individual samples and across serial time points offers opportunity for methods development in the analyses and interpretation of spatial transcriptomic/proteomic data. Citation Format: Katharine A. Collier, Katherine Miller, Zaibo Li, Jesse Westfall, David Tallman, Mark Vater, Gabriel Tinoco, Elaine Mardis, Daniel Stover. Spatial protein and RNA expression in tumor, immune and stromal cells in BRCA1/2 mutated metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-09-10.

Abstract P6-02-11: Implementation of multigene panel testing in triple-negative breast cancer. The PERSONA-breast trial

Abstract Kahler Ribeiro Fontana S1, Bonetti E2, Bernard L3, Calvello M4, Bonanni B4, Bonizzi G 5, Veronesi P1,6, Mazzarella L 2, Galimberti V1 Introduction Triple-negative breast cancer (TNBC) is frequently associated with germline genetic variants associated with cancer predisposition. Approximately 20% of TNBC carry a germline BRCA1 or BRCA2 mutation. Germline mutations in other genes involved in DNA repair, specifically Homologous Recombination (HRR), including ATM, BARD1, BRIP1, CHEK2, PALB2, RAD50, RAD51C, RAD51D may be associated with TNBC however remain imprecise in several populations as in Italy. In recent years, there has been an increase in multigenic panel testing thanks to better technology and the fact that genetic testing is no longer done just for prevention but they have become relevant in the clinical setting and this is especially true for triple negative disease. At the European Institute of Oncology, we conducted a prospective clinical trial, the PERSONA Breast trial, aimed at providing a more comprehensive picture of the mutational landscape and cancer risk in patients with TNBC by multigene germline genetic testing. Methods PERSONA is a prospective observational trial conducted between June 2018 and January 2022 on 313 patients with a diagnosis of TNBC ≤ 60 years and able to undergo surgery (primary or post-neoadjuvant). Peripheral blood DNA was sequenced with the Illumina TruSight Cancer panel (94 cancer predisposition genes). Genes were classified as germline actionable (n. 15) or non-actionable (n. 79) according to their associated relative risk of cancer. Genetic variants were classified according to the American College of Medical Genetics and Genomics (ACMG) guidelines and the databases of genetic variants (ClinVar, LOVD, BRCA-Exchange,). All enrolled patients were followed up six-monthly for 10 years from informed consent or to death or withdrawal of consent. Results We present preliminary germline results from a 94-gene panel testing performed on a cohort of 313 TNBC patients. The clinical data of these patients was considered for a descriptive analysis of the cohort. Data on outcome such as overall survival and disease-free survival were not yet available. Germline multigene testing detected 62 unique (i.e., n. 49 in actionable, n. 13 in non-actionable genes) pathogenic (C5) and likely pathogenic (C4) variants in 25.2% of TNBC patients (79/313). As expected, 53.2% (42/79) of TNBC patients were carriers of a C5/C4 in BRCA1. C4/C5 were identified also in other actionable genes: 13.9% (11/79) in BRCA2, 8.9% (7/79) in MUTYH, 3.8% (3/79) in PALB2, 2.5% (2/79) in MSH2, 1.3% (1/79) in PMS2, and 1.3% (1/79) in TP53. In addition, 12 TNBC patients had C4/C5 variants in non-actionable genes, and 4 were carriers of both C4/C5 variants in actionable and non-actionable genes. Multigene testing resulted in the identification of 655 (i.e., n.82 in actionable, n. 573 in non-actionable genes) variants of uncertain significance (C3 or VUS) in 89.8% (281/313) of patients. Of the 281 C3 carriers, 60 had other variants (C4 and/or C5), of an uncertain result (in whom C3 was the highest class of variant) only in 70.6% (221/313) of TNBC patients. In 13 patients (13/313; 4.1%) only benign (C1) or likely benign (C2) variants were identified. Regarding family history, 67% of BRCA1 carriers versus 30% of BRCA2 carriers were familial. Conclusion Germline multigene testing in TNBC can identify C4/C5 in actionable genes providing information for a more tailored management of TNBC. Our study showed that the rate of VUS remains high using multigene testing. Of note, VUS were mainly identified in non-actionable genes supporting the rationale of the use in the clinical setting of phenotype-specific multigene panels, including a minor, but more appropriate, number of genes. Citation Format: sabrina K. kahler ribeiro fontana, emanuele bonetti, loris bernard, mariarosaria calvello, Bernardo Bonanni, ppina Bonizzi, Paolo Veronesi, Luca Mazzarella, Viviana Galimberti, claudia sangalli. Implementation of multigene panel testing in triple-negative breast cancer. The PERSONA-breast trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-02-11.

Abstract PD5-01: PD5-01 Genomic and epigenomic BRCA alterations predict adaptive resistance and response to platinum-based therapy in triple negative breast cancer and ovarian carcinoma

Abstract Background. Homologous recombination deficiency (HRD), induced by germline and somatic BRCA1 or BRCA2 gene mutations (BRCAmut) and by BRCA1 promoter methylation (BRCA1meth), has been associated with better response to platinum agents in both triple negative breast cancer (TNBC) and ovarian carcinoma (OvCa). A major conundrum arising from recent studies is why patients with BRCA1meth cancers do more poorly compared to those with BRCAmut cancers given the biologically equivalent HRD in both states. Here, we address this question by performing detailed genomic analyses of primary TNBC and OvCa cohorts and through patient-derived xenografts (PDXs) and their derivative cell lines. Methods. Both new and publicly available cohorts of primary TNBC and OvCa encompassing 499 individuals treated with a combination of platinum and taxane chemotherapy were analyzed by whole genome and transcriptome sequencing as well as limited epigenetic analyses. A cohort of 43 PDX models of TNBC was genomically characterized and responses to both single agent platinum and docetaxel were evaluated in vivo. PDX longitudinal studies were performed to assess the dynamics of BRCA1 methylation following treatment. Results. Genomic analyses revealed that BRCA1mut and BRCA1meth cancers share the same pattern of BRCA1-linked genomic rearrangements. Nonetheless, in all four primary cancer cohorts examined we found that patients with BRCAmut cancers, but not those with BRCA1meth cancers, had significantly better response outcomes compared to those with BRCA proficient cancers. A separate analysis of PDX TNBCs with BRCA1 promoter methylation showed that PDXs derived from treatment naïve cancers had complete methylation of the BRCA1 promoter, whereas those derived from post-treatment cancers invariably had only partial methylation. Compared to PDXs with complete methylation, those with partial methylation had a lower response rate to in vivo platinum-based therapy, but not to docetaxel. Using single cell clonal expansions from BRCA1meth PDX models, we confirmed that partial methylation was the result of demethylation of one of the BRCA1 promoter alleles and not of the outgrowth of a non-methylated clone. Exposure of TNBC PDXs with complete methylation to a single course of platinum therapy resulted in the emergence of an unmethylated BRCA1 promoter allele, which associated with an increase in BRCA1 expression. We confirmed that platinum treatment results in progressive loss of BRCA1 methylation and restoration of BRCA1 expression in the clinical setting, by studying the BRCA1 status of a longitudinal series of four TNBC PDX models established from the same patient at different stages of her clinical history. Differential gene expression analysis revealed an increased immune transcriptional signal, especially an elevated M1 macrophage signature, associated with enhanced response to platinum therapy only in patients with BRCA proficient cancers, in both TNBC and OvCa. Integrating both the strength of this cancer immune signature and the presence of BRCA mutations resulted in more accurate predictions of response when compared to either HRD or BRCA status alone. Conclusions. These results suggest that unlike BRCAmut cancers, where BRCA deficiency is more genetically stable, BRCA1meth cancers are highly adaptive to genotoxin exposure resulting in demethylation of one allele, recovery of BRCA1 expression and acquired insensitivity to platinum. On the other end, a high immune transcriptional signature identifies patients with BRCA proficient cancers that are more likely to benefit from platinum therapy. Taken together, our study underscores the importance of characterizing molecular heterogeneity to optimize predictive precision in assigning response probabilities in TNBC and OvCa. Citation Format: Francesca Menghi, Kalyan Banda, Pooja Kumar, Robert Straub, Lacey E. Dobrolecki, Isabel Rodriguez, Susan E. Yost, Harshpreet Chandok, Marc Radke, George Somlo, Yuan Yuan, Michael T. Lewis, Elizabeth Swisher, Edison Liu. PD5-01 Genomic and epigenomic BRCA alterations predict adaptive resistance and response to platinum-based therapy in triple negative breast cancer and ovarian carcinoma [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD5-01.

Abstract P5-03-14: Prevalence of germline BRCA mutations in unselected Korean patients with HER2-negative breast cancer: A Prospective cohort study

Abstract Backgrounds Since OlympiAD study, National Comprehensive Cancer Network guideline recommends assessment of germline BRCA1/2 mutation in all patients with recurrent or metastatic breast cancer to identify candidates for PARP inhibitor therapy, which is not always possible in clinical practice due to limited resources for testing. Data on the prevalence of gBRCA mutation is still lacking, especially in patients with non-high risk for hereditary breast and ovarian cancer syndrome. In this study, we investigated prevalence of gBRCA mutation in unselected Korean patients with HER2-negative advanced BC in a prospective cohort and analyzed oncologic outcome. Methods Eligible patients were diagnosed with HER2-negative advanced BC and had initiated palliative systemic treatment. Peripheral blood was prospectively drawn from each patient and gBRCA mutation status was assessed by next generation sequencing using NGeneBio BRCAaccuTest®. In 100 patients, somatic mutations including BRCA1/2 from tumor tissue were investigated using targeted panel sequencing. To estimate the prevalence of gBRCA mutation with margin of error to be no more than ±4% at the 95% confidence interval in a population size of 20,000, 583 patients were to be enrolled. Results A total of 583 patients were enrolled between Oct 2019 and Mar 2022, and the prevalence of gBRCA mutation was analyzed in 570 patients, excluding ineligible patients. Median age was 54 years old (range 26-87) and 567 patients were female. 475 patients had HR+/HER2- BC and 94 patients had triple negative breast cancer (TNBC). The overall prevalence of gBRCA1/2 pathogenic mutation was 7.3% (42/570) in unselected patients. The prevalence of gBRCA1 mutation was 1.6%(9/570) overall, 0.8%(4/475) in HR+/HER2- BC, and 5.3%(5/94) in TNBC. The prevalence of gBRCA2 mutation was 5.8%(33/570) overall, 6.3%(30/475) in HR+/HER2- BC, 3.2%(3/94) in TNBC. Prevalence in low risk TNBC (>60 years at first BC diagnosis, no known family history of relevant cancer and unilateral breast cancer) was 10.5% (2/19, all 2 patients had gBRCA2 mutation). Prevalence in low risk HR+/HER2- (>40 years at first BC diagnosis, no known family history of relevant cancer and unilateral breast cancer) was 5.9% (18/307, 17 patients had gBRCA2 mutation). The overall prevalence of gBRCA1/2 pathogenic mutation in Korean patients with low risk HER2-negative advanced BC was 6.1%. The result of somatic mutation, treatment patterns and clinical outcome according to gBRCA1/2 mutation will be further analyzed. Conclusions The prevalence of gBRCA mutation among Korean patients with HER2-negative advanced BC classified as low risk (6.1%) in this study supports routine testing of gBRCA mutation in this population. Citation Format: Hee Kyung Ahn, Jee Hung Kim, Mirae Kim, Seri Park, Su-Jin Koh, Joo Hyuk Sohn, Myoung Joo Kang, Kyung Hae Jung, Kyoung Eun Lee, Jieun Lee, Sung Ae Koh, Yee Soo Chae, Jae Ho Byun, In Hae Park, Hee-Jun Kim, Jee Hyun Kim, Han Jo Kim, Joo Young Jung, Jung Lim Lee, Yoon Young Cho, Kyong Hwa Park, Ji-Yeon Kim, Seock-Ah Im, Yeon Hee Park. Prevalence of germline BRCA mutations in unselected Korean patients with HER2-negative breast cancer: A Prospective cohort study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-03-14.

Homologous Recombination Defects and Mutations in DNA Damage Response (DDR) Genes Besides BRCA1 and BRCA2 as Breast Cancer Biomarkers for PARP Inhibitors and Other DDR Targeting Therapies.

Homologous recombination repair (HRR) is the cellular mechanism for error-free repair of double strand DNA (dsDNA) breaks. Cancer cells with mutations in both alleles of genes encoding for proteins involved in HRR, such as BRCA1 and BRCA2, have defects in the repair process. As a result, these cells repair dsDNA breaks with alternative mechanisms, such as non-homologous end joining. In breast cancers with germline mutations in BRCA1 and BRCA2 genes, HRR defects result in sensitivity to PARP inhibitors, drugs that interfere with the function of PARP enzyme and promote trapping of the enzyme on DNA and stalling of the process of repairing single strand breaks. HRR defects also lead to sensitivity to DNA damaging chemotherapy due to the inability of cells to repair chemotherapy induced DNA lesions. Besides germline mutations in BRCA1 and BRCA2, somatic mutations in these genes or germline and somatic mutations or other genetic and epigenetic alterations of other genes involved in homologous recombination (HR) may produce HRR defects leading to sensitivity to PARP inhibitors. However, studies are less conclusive, a fact that may relate to the common lack of bi-allelic loss of function in these cases, as opposed to cancers with germline BRCA1 or BRCA2 defects that usually acquire bi-allelic loss of function. In addition, there is heterogeneity between the different HRR genes and the severity of the resulting HRR defects, as measured by HR defect assays. This review article examines the landscape of HRR gene mutations in breast cancer and the possible therapeutic implications of HRR defects other than germline BRCA1 and BRCA2 mutations for targeted therapies. Identification of a wider range of breast cancers with HRR defects may expand the subset of patients that derive benefit from PARP inhibitors and other DDR-targeting drugs in the clinic.

Obesity promotes breast epithelium DNA damage in women carrying a germline mutation in BRCA1 or BRCA2.

Obesity, defined as a body mass index (BMI)≥30, is an established risk factor for breast cancer among women in the general population after menopause. Whether elevated BMI is a risk factor for women with a germline mutation in BRCA1 or BRCA2 is less clear because of inconsistent findings from epidemiological studies and a lack of mechanistic studies in this population. Here, we show that DNA damage in normal breast epithelia of women carrying a BRCA mutation is positively correlated with BMI and with biomarkers of metabolic dysfunction. In addition, RNA sequencing showed obesity-associated alterations to the breast adipose microenvironment of BRCA mutation carriers, including activation of estrogen biosynthesis, which affected neighboring breast epithelial cells. In breast tissue explants cultured from women carrying a BRCA mutation, we found that blockade of estrogen biosynthesis or estrogen receptor activity decreased DNA damage. Additional obesity-associated factors, including leptin and insulin, increased DNA damage in human BRCA heterozygous epithelial cells, and inhibiting the signaling of these factors with a leptin-neutralizing antibody or PI3K inhibitor, respectively, decreased DNA damage. Furthermore, we show that increased adiposity was associated with mammary gland DNA damage and increased penetrance of mammary tumors in Brca1+/- mice. Overall, our results provide mechanistic evidence in support of a link between elevated BMI and breast cancer development in BRCA mutation carriers. This suggests that maintaining a lower body weight or pharmacologically targeting estrogen or metabolic dysfunction may reduce the risk of breast cancer in this population.

A systematic review and meta-analysis of germline BRCA mutations in pancreatic cancer patients identifies global and racial disparities in access to genetic testing

Germline BRCA1 and BRCA2 mutations (gBRCAm) can inform pancreatic cancer (PC) risk and treatment but most of the available information is derived from white patients. The ethnic and geographic variability of gBRCAm prevalence and of germline BRCA (gBRCA) testing uptake in PC globally is largely unknown. We carried out a systematic review and prevalence meta-analysis of gBRCA testing and gBRCAm prevalence in PC patients stratified by ethnicity. The main outcome was the distribution of gBRCA testing uptake across diverse populations worldwide. Secondary outcomes included: geographic distribution of gBRCA testing uptake, temporal analysis of gBRCA testing uptake in ethnic groups, and pooled proportion of gBRCAm stratified by ethnicity. The study is listed under PROSPERO registration number #CRD42022311769. A total of 51 studies with 16 621 patients were included. Twelve of the studies (23.5%) enrolled white patients only, 10 Asians only (19.6%), and 29 (56.9%) included mixed populations. The pooled prevalence of white, Asian, African American, and Hispanic patients tested per study was 88.7%, 34.8%, 3.6%, and 5.2%, respectively. The majority of included studies were from high-income countries (HICs) (64; 91.2%). Temporal analysis showed a significant increase only in white and Asians patients tested from 2000 to present (P < 0.001). The pooled prevalence of gBRCAm was: 3.3% in white, 1.7% in Asian, and negligible (<0.3%) in African American and Hispanic patients. Data on gBRCA testing and gBRCAm in PC derive mostly from white patients and from HICs. This limits the interpretation of gBRCAm for treating PC across diverse populations and implies substantial global and racial disparities in access to BRCA testing in PC.

Evaluation of ctDNA in patients with CRPC with pathogenic germline mutations in BRCA2.

253 Background: Approximately 3-5% of advanced prostate cancer patients have pathogenic BRCA2 mutations in germline tests. In this study, we examine the relationship between pathogenic germline BRCA2 mutation and somatic changes in ctDNA. Methods: Germline screenings were performed by Invitae multi-cancer gene panel which includes 50-84 genes. ctDNA alterations were detected by Guardant 360 assays which report somatic changes in 70-83 genes. All ctDNA samples were collected in post-abiraterone and/or enzalutamide (in CRPC patients). Any pathogenic/likely pathogenic somatic alterations in the ctDNAs with more than 0.1% of allelic fraction were included in this cohort. The type of mutation detected in ctDNA was also assessed (truncating, point, etc.). Statistical significance for comparison is calculated with Fischer Exact Probablity Test and Chi-Square Test. Results: A total of 11 patients had germline BRCA2 pathogenic mutations and ctDNA assays; 267 patients had no germline DNA pathogenic alterations and ctDNA assays. Compared to germline normal patients, the germline BRCA2 mutations were less likely to have AR alterations on ctDNA (OR=0.2133, 95% C.I. [0.087, 0.525], p-value = 0.0003). BRCA2 germline positive patients were also more likely to have a mutated BRCA1, BRCA2, and TP53 ctDNA (OR=7.899, 95% C.I. [1.2745, 48.9548], p=0.055), (OR=7.899, 95% C.I. [1.7529, 16.059], p=0.008), and (OR=6.442, 95% C.I. [2.449, 16.946], p=0.00001), respectively. All other ctDNA assessed genes were mutated at a similar frequency between germline BRCA2 mutated and “normal” germline patients. BRCA2 germline mutations patients are less likely to have copy number alterations (CNVs) (OR=0.3992, 95% C.I. [0.2168, 0.7352], p=0.0031) and more likely to have frameshift mutations (OR=2.3182, 95% C.I. [1.169, 4.5972], p=0.0183). Conclusions: The ctDNA testing in this CRPC population (after abiraterone and/or enzalutamide) was less likely to find AR alterations and more likely to find pathogenic mutations for BRCA1, BRCA2, and TP53 in BRCA2 germline positive patients. In addition, BRCA2 germline positive CRPC patients were more likely to have frameshift mutations and less likely to have CNVs than those with an intact germline. Characterizing the mutational landscape in BRCA2 germline mutated patients may help to better define underlying disease biology. Those with BRCA mutated germline may be less likely to have AR driven tumors. More study is needed to better understand patients with underlying DNA repair defects.

Inherited mutations in DNA damage gene repair (gDDR) in Italian men with metastatic prostate cancer.

223 Background: The need for genetic prognostic and predictive markers is urgent in order to identify potential target therapies or personalized therapeutic sequences for men with metastatic prostate cancer (mPC). The prevalence of pathogenic germline mutations in DNA damage repair (gDDR) such as germline mutations in BRCA2 (gBRCA) is well established: in the Anglo-American population with mPC was 5.3%, while in the Chinese population was 4.3% and in Spanish population 3.3%. This prospective multicenter cohort study evaluated the prevalence of gDDR alterations in the Italian population affected by mPC. Methods: In an observational prospective trial, 300 consecutive Italian patients affected by mPC were recruited between 2015 and 2019 and were screened for gDDR mutations in 107 genes. The primary aim was to assess the frequency of ATM/BRCA1/BRCA2/PALB2 germline mutations in the Italian population of patients with mPC. Secondary aims included the association of gDDR subgroups with metastatic onset, Gleason Score ≥8, and time to castration resistance. Results: We analyzed 300 patients and we identified 297 valuable patients. 46 patients had a pathogenic/likely pathogenic variants (15.4%): the more frequent is gBRCA2 in ten cases, gATM in five (1.7%), gMUTYH in four cases (1.3%), gCHEK2 in three cases (1%) and one case in PALB2 (0.3%). No mutations in BRCA1 were identified. We also highlighted six patients (2%) with clonal hematopoiesis of indeterminate potential (CHIP) interference in ATR (n=2), ATM (n=2), CHEK1 (n=1) and BRIP1 (n=1). Finally, we identified 117 alterations of variants of unknown significance (VUS) in 98 patients. In a univariate analysis, the median time to castration resistance in gBRCA2 patients was 11.6 months versus 22.3 months in patients with other mutation versus 23.4 months in no mutated patients. There were no associations of gDDR subgroups with metastatic onset and Gleason Score ≥8. Conclusions: The prevalence of gBRCA2 in the Italian population is 3%, which is similar to those of Spanish population, identifying interesting similarities between people of the Western Mediterranean area. The presence of gBRCA2 mutations correlates with lower time to the onset of castration resistant disease, according to a more aggressive phenotype.

A case of unresectable intrahepatic cholangiocarcinoma with a germline BRCA1 mutation.

Intrahepatic cholangiocarcinoma (ICC) is a malignant liver tumor with poor prognosis. Various mutations in cancer-predisposing genes have been reported in ICC, and germline BRCA1/2 mutations, which are the causative genes for hereditary breast and ovarian cancer syndrome (HBOC), have been reported in many patients with ICC. Here, we report a case of unresectable ICC with a germline BRCA1 mutation. A 73-year-old man was found to have a mass in the left lobe of the liver on abdominal ultrasonography during a medical check-up and was referred to our institution. Contrast-enhanced computed tomography revealed a 30-mm mass with a delayed enhancement pattern, tumor invasion into the major blood vessels, and enlarged regional lymph nodes. Ultrasound-guided percutaneous tumor biopsy revealed a well-differentiated adenocarcinoma, and the patient was diagnosed with clinical Stage IIIB ICC. Systemic chemotherapy with gemcitabine and cisplatin was initiated because of the unresectable nature of the disease. Regarding family history, his eldest daughter was diagnosed with HBOC with a germline BRCA1 mutation at the time of breast cancer surgery, and she developed pancreatic cancer seven years later. The patient underwent BRCA1 single-site analysis and was diagnosed with HBOC with a germline BRCA1 mutation.

OlympiAD extended follow-up for overall survival and safety: Olaparib versus chemotherapy treatment of physician's choice in patients with a germline BRCA mutation and HER2-negative metastatic breast cancer

BackgroundIn the Phase III OlympiAD study, olaparib significantly prolonged progression-free survival versus chemotherapy treatment of physician's choice (TPC) in patients with germline BRCA-mutated (gBRCAm), human epidermal growth factor receptor 2-negative metastatic breast cancer (mBC). In the final pre-specified analysis (64% maturity), median overall survival (OS) was 19.3 months for olaparib and 17.1 months for TPC (P = 0.513). Post-hoc extended follow-up, 25.7 months longer than previously reported for OS, is reported. Patients and methodsPatients with gBRCAm, human epidermal growth factor receptor 2-negative mBC, who had received ≤2 lines of chemotherapy for metastatic disease, were randomised 2:1 to olaparib (300 mg bid) or TPC. During extended follow-up, OS was analysed every 6 months using the stratified log-rank test (overall population) and Cox proportional hazards model (pre-specified subgroups). ResultsIn the overall population (302 patients; 76.8% maturity), median OS was 19.3 months for olaparib and 17.1 months for TPC (hazard ratio 0.89, 95% confidence interval 0.67–1.18); median follow-up was 18.9 and 15.5 months, respectively. Three-year survival was 27.9% for olaparib versus 21.2% for TPC. With olaparib, 8.8% of patients received study treatment for ≥3 years versus none with TPC. In first-line mBC, median OS was longer for olaparib than TPC (22.6 versus 14.7 months; hazard ratio 0.55, 95% confidence interval 0.33–0.95) and 3-year survival was 40.8% for olaparib versus 12.8% for TPC. No new serious adverse events related to olaparib were observed. ConclusionsOS was consistent with previous analyses from OlympiAD. These findings support the possibility of meaningful long-term survival benefit with olaparib, particularly in first-line mBC.

BRCA1 haploinsufficiency impairs iron metabolism to promote chrysotile-induced mesothelioma via ferroptosis resistance.

Malignant mesothelioma (MM) is still a social burden associated with asbestos exposure. Local iron accumulation thereby represents the major pathogenesis, followed by oxidative DNA strand breaks and genomic alterations in the mesothelium. BRCA1 is a critical component of homologous recombination repair directed to DNA double-stranded breaks, whereas BRCA1 germline mutation is an established risk for breast/ovarian cancer, its role in MM development remains to be elucidated. Murine Brca1 mutant models so far have not reproduced human phenotypes. However, a rat Brca1 mutant model (Mut; L63X/+ ) recently reproduced them at least partially. Here we describe the differential induction of MM in Brca1 mutant rats by intraperitoneal injection of chrysotile or crocidolite. Only Mut males injected with chrysotile revealed a promotional effect on mesothelial carcinogenesis in comparison with wild-type and/or females, with all the MMs Brca1 haploinsufficient. Array-based comparative genomic hybridization of MMs disclosed a greater extent of chromosomal deletions in Brca1 mutants, including Cdkn2a/2b accompanied by Tfr2 amplification, in comparison with wild-type tumors. Mutant MMs indicated iron metabolism dysregulation, such as an increase in catalytic Fe(II) and Ki67-index as well as a decrease in Fe(III) and ferritin expression. Simultaneously, mutant MMs revealed ferroptosis resistance by upregulation of Slc7A11 and Gpx4. At an early carcinogenic stage of 4 weeks, induced Brca1 expression in mesothelial cells was significantly suppressed in chrysotile/Mut in comparison with crocidolite/Mut, whereas significant preference to iron with a decrease in Fe(III) has been already established. In conclusion, chrysotile exposure can be a higher risk for MM in BRCA1 mutant males, considering the rat results.

Prevalence of Pathogenic Germline Mutations in 13 Hereditary Cancer-Related Genes in Breast Cancer Patients in Narathiwat Province, Thailand.

BRCA1 and BRCA2 genes are known to increase breast cancer's lifetime risk. Early identification of women with this inherited risk can potentially reduce the risk of breast and/or ovarian cancer and, together with early screening, decrease the mortality rate. This study explored the frequency and distribution of genetic variants in consecutive cases of breast cancer in Narathiwat province, one of the three provinces in the southernmost Thai border. A series of 64 consecutive breast cancer patients who underwent treatment in two general hospitals in the province during the period from the year 2021 to 2022. Genotyping studies were performed using a whole exome sequencing platform. Moderate to high penetrance variants recommended by the National Comprehensive Cancer Network (NCCN) guidelines 2022 (ATM, BARD1, BRCA1, BRCA2, CDH1, CHEK2, NF1, PALB2, PTEN, RAD51C, RAD51D, STK11, TP53) were annotated and filtered for pathogenic, likely pathogenic, or high-impact variants. Pathogenic germline variants were found in 8/64 cases (12.5%), namely BRCA1 in 3 (4.7%), BRCA2 in 4 (6.3%), ATM in 1 (1.6%), and PALB2 in 1 (1.6%). One patient had two concomitant germline mutations in BRCA2 and ATM. This is the first study on the frequency of germline mutations in BRCA1/2 and other breast cancer-predisposing genes in the southernmost provinces of Thailand. At least one pathogenic germline mutation was identified in 12.5% of the study patients, which suggests that genetic testing in this population has a high potential to provide benefits.

Is Breast Conserving Surgery Efficacious in Breast Cancer Patients with BRCA1 or BRCA2 Germline Mutation?

The optimal surgical therapy for newly diagnosed breast cancer with germline mutations in susceptibility genes is still uncertain for many physicians. In this study, we aimed to determine the efficacy of breast conserving surgery (BCS) in breast cancer patients with BRCA1 or BRCA2 mutation by assessing its outcomes and locoregional recurrence (LR) rates. Seventy-five patients operated with BCS or mastectomy for breast cancer between 2006 and 2017 and had BRCA1 or BRCA2 mutation were included in the study. Effects of the performed breast surgery and clinicopathological characteristics on surgical outcomes, LR rates and survival were analyzed with showing the distribution of BRCA1 and BRCA2 germline mutations. The median age of the patients was 42 years (20-77). BRCA1 mutations were found in 46 (61.3%) patients and BRCA2 mutations in 29 (38.7%) patients. Compared to BRCA2 carriers, BRCA1 carriers were more likely to have higher tumor grade (84.8% vs 44.8%; p = 0.001) and non-luminal subtype tumors (67.4% vs 13.8%; p = 0.001). A total of 44 (58.7%) patients underwent unilateral mastectomy and 31 (41.3%) patients underwent BCS. At a median follow-up time of 60 (12-240) months, LR was observed in 6 patients equally divided in both BCS and mastectomy groups. LR rates were slightly higher after BCS versus mastectomy (9.7% and 6.8%, respectively). Additionally, there were no statistically significant differences in disease-free survival (DFS) and disease-specific survival (DSS) rates after 10 years in the BCS group versus the mastectomy group (p = 0.117 and 0.109, respectively), but in fact, the rates were better in the BCS group. Our findings indicate that BCS may serve as an efficacious alternative to mastectomy for breast cancer patients with BRCA1 or BRCA2 mutation. Additionally, tumor size, lymph node positivity, and TNM stage should be taken into consideration for a better surgical decision-making.

Identification of a subpopulation of patients with gastric cancer responsive to PARP inhibitors.

432 Background: Treatment with PARP inhibitors (PARPi) has been approved in some tumors bearing a deficient Homologous Recombination (HR) system. Despite negative results of clinical trials conducted on the overall population of gastric cancer (GC) patients, we wondered whether a PARPi therapeutic strategy might represent a window of opportunity for a subpopulation of these patients. 7-12% of gastric cancers exhibit a mutational signature associated with HR failure, suggesting that these patients might benefit from PARPi. Methods: To analyze responsiveness to PARPi we exploited a proprietary human Gastro-Esophageal Adenocarcinoma (GEA) annotated platform of Patient-Derived Xenografts (PDXs) and PDX-derived primary cells on which we performed in vivo and in vitro experiments. Results: We selected 10 PDXs with loss-of-function mutations in HR pathway genes. Cell viability assays and preclinical trials showed that treatment with PARPi was effective in PDXs harbouring BRCA2 germline mutations and somatic inactivation of the second allele in a microsatellite stable background. PARPi responsive tumors were sensitive to oxaliplatin as well. Evaluation of HR deficiency and mutational signatures efficiently stratified responder vs non-responder PDXs. A retrospective analysis on 57 GEA patients showed that those carrying BRCA2 inactivating variants had longer PFS upon platinum-based regimens (used as proxy). In 5 out of 7 BRCA2 germline mutated patients we identified the p.K3326* variant, currently classified as “benign”. However, familial history of cancer, the absence of RAD51 foci in the tumor cells and a high HR deficiency score suggest a deleterious effect for this mutation in gastric cancer. Conclusions: PARP inhibition could represent a new therapeutic perspective for BRCA2 mutated and/or high HRD score GEA patients, including familial intestinal gastric cancer patients.

Targeting BRCA-mutant biliary tract cancer: Current evidence and future perspectives.

Biliary tract cancers (BTCs) are aggressive tumors of the biliary system, which are often diagnosed at the advanced stage with a dismal prognosis. Among BTC patients, germline or somatic breast cancer-related gene 1/2 (BRCA1/2) mutation has been reported and the use of poly(ADP-ribose) polymerase inhibitor (PARPi) has achieved a certain effect, with no obvious severe side effects. The frequencies and mutated types of BRCA1/2 in advanced BTCs vary among studies. BRCA1 and BRCA2 play distinct roles in the development of BTC regardless of age or gender difference. Surprisingly, some BTC patients with germline BRCA mutation can achieve better therapeutic effects than those with a somatic mutation, and patients who carry BRCA mutation are more likely to benefit from immunotherapy alone or in combination. Herein, we provide an overview of the targeted therapies in BRCA-mutant BTCs, with a particular focus on the differences between germline and somatic BRCA1/2 mutations, as well as review the current status and perspectives.

Clinical and mutational profiles of microsatellite instability (MSI-H) in intrahepatic cholangiocarcinoma (iCCA).

607 Background: Microsatellite instability (MSI-H) and increased tumor mutation burden (TMB) have been associated with clinical benefit from immunotherapy. A small subset of patients with intrahepatic cholangiocarcinoma (iCCA) have microsatellite instability (MSI-H). Understanding their clinical outcomes and genomic landscape has been limited. These patients may derive clinical benefit from immunotherapy compared to chemotherapy. Methods: 7565 cases of iCCA underwent comprehensive genomic profiling of 324 genes (FoundationOne CDx). Prevalence of genomic alterations in cases with MSI-H were compared with MSI-low and MSS iCCA using Fischer’s exact test with Bonferonni correction. Clinical and pathological data were collected by retrospective chart review for patients with MSI-H/dMMR iCCA. Results: Among 7565 iCCA cases identified, 137 (1.8%) were MSI-H. Compared to MSS/MSI-low cases, MSS-H patients were more likely to be male and have Lynch syndrome. Median TMB was 21.7 (muts/Mb). MSI-H cases were enriched for genomic alterations (GA) in APC (13.9%), TP53 (59.9%), ARID1A (13.9%), and PRBM1 (37.2%). MSI-H cases had lower frequencies of previously identified, actionable iCCA drivers such as FGFR2 fusions (0% v 9%, p=NS), IDH1 (3.7% v 14.5%, p&lt;0.0001) and IDH2 (0% v 4.1%, p=0.007). Compared to MSS/MSI-low, MSI-H cases had higher GA in PIK3CA (16.8% v 6.6%, p=&lt;0.0001), PTEN (10.9% v 2.8%, p=&lt;0.0001), and BRCA2 (10.2% v 2%, p=&lt;0.0001). MSI-H cases had lower GA in CDKN2A (24.1% v 30.9%, p=0.09) and MTAP loss (3.4% v. 15.5%, p&lt;0.0001). Five patients were identified with dMMR/MSI-H iCCA out of 974 patients at a single institution (0.5%). Median age at diagnosis was 53 years (range 46-70) and 83% were male (n=4). Two patients had resectable disease at time of diagnosis. TMB range was 20-29 (muts/Mb). Patients had 4-6 co-mutations on average with the most common GA being TP53, ARID1A and CDKN2A. One patient had Lynch syndrome with germline alteration of MLH1 gene and one patient had a germline BRCA mutation. One patient was noted to have FGFR2 rearrangement. Gemcitabine/Cisplatin was given to 3 of 6 patients as first-line (1L) therapy, with median progression free survival of 4 months. One hundred percent of patients received immunotherapy (IO) as first or second-line (2L) therapies. Patients receiving IO in the 1L had responses ranging from 4 to 9 months. Three patients have ongoing responses. Median overall survival ranged from 12 months to &gt;18 months. Two patients presented with CNS involvement and did not respond to immunotherapy-based regimens. Conclusions: MSI-H represents a small subset of patients with iCCA. High-TMB noted amongst MSI-H iCCA. Currently, MSI testing is not universally performed and may be necessary to identify and best treat these patients. These findings support the development of personalized immunotherapy treatment strategies for this distinct patient population.