Non-canonical Mutations Research Articles

CNSC-03. LINKING CELL FATE DETERMINATION IN THE EMBRYONIC FOREBRAIN TO PEDIATRIC HIGH-GRADE GLIOMA

Abstract BACKGROUND Pediatric high-grade gliomas are treatment resistant, usually fatal cancers encompassing diffuse midline glioma, K27-altered (DMG), and diffuse hemispheric glioma, G34R/V (DHG). Most patients harbour canonical and non-canonical mutations in histone variants H3.1 and H3.3, leading to global epigenetic dysregulation and stalled differentiation states as oligodendroglial progenitor cells (OPC) in DMG and GABAergic interneuron precursors in DHG. DLX2 is a homeobox transcription factor that directly represses OPC cell fate and promotes GABAergic interneuron cell fate and migration during early neurogenesis. Methods & Results Our lab has shown DLX2 promoter occupancy of Gad1/Gad2, and early (Olig2, Nkx2.2) and late (Myt1, Plp1) genes required for OPC differentiation in vivo. Co-expression of DLX2 with these target sequences reduces reporter gene expression in vitro, and the Dlx1/Dlx2 double knockout (DKO) mouse showed increases in Olig2, Nkx2.2, and Plp-1 expression in vivo. Transient over-expression of a Dlx2-GFP construct into murine DMG cells resulted in significant increased expression of Gad1/2 isoforms and decreased Olig2 and Nkx2.2, global restoration of H3K27me3 and loss of H3K27 acetylation, as well as a reduction in migration, invasion, and colony formation in vitro. To validate in human pHGG, we first probed an RNA-seq database containing 62 patient-derived pHGG cell lines which demonstrated an inverse correlation between DLX2 expression and OLIG1/2. Cell fate changes in DHG cell lines were evaluated by knockout of DLX2 using CRISPR/Cas9. Concurrently, OLIG1/2-expressing DMG cell lines underwent stable transfection of a DLX2-mCherry overexpression construct in conjunction with a SOX2-BFP overexpression construct, confirmed by qPCR and immunoblot. Analysis of histone H3 marks, K27M, and OPC signature genes will be compared to OPC genes from Dlx1/Dlx2 DKO mouse ganglionic eminences. CONCLUSIONS Manipulation of DLX2 in pHGG provides a model system for assessing the contribution of a homeobox factor to cell fate decisions regulating differentiation and tumorigenicity.

The somatic mutation profile of estrogen receptor-positive HER2-negative metastatic breast cancer in Brazilian patients.

Breast cancer is the leading cause of cancer death among women worldwide. Studies about the genomic landscape of metastatic breast cancer (MBC) have predominantly originated from developed nations. There are still limited data on the molecular epidemiology of MBC in low- and middle-income countries. This study aims to evaluate the prevalence of mutations in the PI3K-AKT pathway and other actionable drivers in estrogen receptor (ER)+/HER2- MBC among Brazilian patients treated at a large institution representative of the nation's demographic diversity. We conducted a retrospective observational study using laboratory data (OC Precision Medicine). Our study included tumor samples from patients with ER+/HER2- MBC who underwent routine tumor testing from 2020 to 2023 and originated from several Brazilian centers within the Oncoclinicas network. Two distinct next-generation sequencing (NGS) assays were used: GS Focus (23 genes, covering PIK3CA, AKT1, ESR1, ERBB2, BRCA1, BRCA2, PALB2, TP53, but not PTEN) or GS 180 (180 genes, including PTEN, tumor mutation burden [TMB] and microsatellite instability [MSI]). Evaluation of tumor samples from 328 patients was undertaken, mostly (75.6%) with GS Focus. Of these, 69% were primary tumors, while 31% were metastatic lesions. The prevalence of mutations in the PI3K-AKT pathway was 39.3% (95% confidence interval, 33% to 43%), distributed as 37.5% in PIK3CA and 1.8% in AKT1. Stratification by age revealed a higher incidence of mutations in this pathway among patients over 50 (44.5% vs 29.1%, p=0.01). Among the PIK3CA mutations, 78% were canonical (included in the alpelisib companion diagnostic non-NGS test), while the remaining 22% were characterized as non-canonical mutations (identifiable only by NGS test). ESR1 mutations were detected in 6.1%, exhibiting a higher frequency in metastatic samples (15.1% vs 1.3%, p=0.003). Additionally, mutations in BRCA1, BRCA2, or PALB2 were identified in 3.9% of cases, while mutations in ERBB2 were found in 2.1%. No PTEN mutations were detected, nor were TMB high or MSI cases. We describe the genomic landscape of Brazilian patients with ER+/HER2- MBC, in which the somatic mutation profile is comparable to what is described in the literature globally. These data are important for developing precision medicine strategies in this scenario, as well as for health systems management and research initiatives.

Noncanonical mutations in ribosome nascent peptide exit tunnel confer clarithromycin resistance in Mycobacterium abscessus complex

ObjectivesMycobacterium abscessus is a non-tuberculous mycobacterial pathogen that causes pulmonary and skin infections globally. Clarithromycin plays a pivotal role in treating M. abscessus infections, with resistance often leading to treatment failure. While canonical mutations in the 23S rRNA residue 2270/2271 are recognized as the primary mechanism for acquired clarithromycin resistance, resistant isolates lacking these mutations have been widely reported. This study aims to identify new mechanisms of clarithromycin resistance in M. abscessus. MethodsWe selected spontaneous resistant mutants derived from two parental strains characterized by erm(41) T28 and C28 sequevars, respectively. Whole-genome sequencing was performed on mutants lacking the 23S rRNA 2270/2271 mutations. Site-directed mutagenesis was used to confirm the resistance phenotypes of newly identified mutations. Bioinformatic analysis of publicly available genomes was conducted to evaluate the presence of these mutations in clinical isolates. The spatial localization of these mutations in the ribosome was analyzed to investigate potential mechanisms of resistance. ResultsA total of 135 resistant mutants were selected from the parental strains. Sequencing of the 78 mutants lacking the 23S rRNA 2270/2271 mutations identified mutations within the peptidyl-transferase center and hairpin loops 35, 49, and 74 of the 23S rRNA. These noncanonical mutations were identified in 57 of 1875 genomes of clinical isolates. Thirteen representative mutations were introduced into the bacterial genome, and their contributions to macrolide resistance were confirmed. The newly identified mutations all localized at the entrance of the nascent peptide exit tunnel, potentially contributing to resistance by disrupting the macrolide binding pocket. ConclusionSeveral noncanonical 23S rRNA mutations conferring clarithromycin resistance were identified. These mutations enhance our understanding of macrolide resistance in M. abscessus and could serve as important markers for diagnosing clarithromycin resistance.

Molecular Landscape of c-CBL Mutation in Adults' Myeloid Malignancies

Background: CBL proteins are a family of RING finger (RF) E3 ubiquitin ligases that regulate proliferative signals via ubiquitination and degradation of tyrosine-phosphorylated signaling protein ( e.g., FLT3, JAK2, MPL). Thus, the loss of CBL activity due to hypomorphic or loss of function (LOF) mutations functionally may result in oncogenic effects. However, simple LOF does not explain the high rates of c-CBL (most abundant CBL isoform, located on chromosome 11)missensemutations affecting a canonical position in the RF domain, catalytic region responsible for ubiquitin ligase activity. We hypothesized that studying the molecular and clinical associations of c-CBL mutations will help clarifying the consequences of its mutations and conceptualize innovative treatment strategies. Methods: We analyzed molecular and clinical features of a cohort of 11036 patients with myeloid neoplasia (MN; 63% AML, 29% MDS, 5% MPN/MDS, 3% MPN) from The Cleveland Clinic Foundation and public series. Clonal burden of c-CBL was analyzed to infer clonal hierarchy and to establish founder vs secondary mutations based on our previously described criteria. Results: Overall, c-CBL mutations were detected in 2.4% (270/11036) of the patients. The most common c-CBL mutations were missense (80%) and mainly occurred in the RF domain and linker region (83% of mutations, Fig1). c-CBL mutations were found in 2% of both primary AML (pAML; 3% of CBF) and sAML/HR-MDS and 8% of CMML. Among CMML patients, mutations were associated with low-risk features (7% of CMML2 vs. 13% of CMML1). The most frequent co-associated mutations were TET2 (22%) and RUNX1 (19%) in all AML, ASXL1 (50%), SRSF2 (25%) and TET2 (25%) in MDS, and ASXL1 (50%), SRSF2 (44%) and JAK2 (28%) in MPN and MPN/MDS. In terms of associations with tyrosine kinase receptors ( FLT3, c-KIT, CSF3R, c-MPL), we found that 18% of c-CBL mutants carried at least one mutation in one the four receptors, contrarily to what expected from previously reported results of c- CBL/FLT3 mutational exclusivity. In most of the cases, c-CBL mutations were subclonal (53%), arguing against its primary driver/initiator function. Indeed, we observed c-CBL mutations as the only lesion in 7% of the cases with no specific disease association (66%, AML; 25%, LR-MDS; 8%, CMML). There was no difference in MN phenotypes between carriers of canonical vs non canonical c-CBL mutations; however, we found that non canonical mutations were more often ancestral or the only founder lesion detected compared to canonical mutations (46 vs 31%, P = 0.031). Del(11q) or monosomy 11 were found in 8 patients, without association with c- CBL VAF >50%. Furthermore, c-CBL VAF>50% was not associated with marrow blast infiltrate, WBC or shorter survival. We then analyzed the effect of c-CBL in cooperation with RAS genes ( NRAS, KRAS, PTPN11, NF1) and found that, in AML, the effect of c-CBL with 2 RAS genes (5 patients, 3%) showed some trends toward higher blast count ( P = 0.079), higher WBC ( P = 0.076) and lower rates of normal cytogenetics compared to the effect of the remaining c-CBL mutant (0 vs 48%, P = 0.061). Conclusions: In sum, our study of the molecular landscape of c-CBL mutations highlights that these lesions occur mainly during disease development with majority of the cases carrying subclonal hits with a weaker effect on disease phenotype and prognosis. Such mutations can be present in both primary and secondary AML (in contrast with previous reports) and additive effects can be observed when c- CBL mutationsco-occur with more than one RAS gene in these disease groups. In some cases, especially in non-canonical mutations, c- CBL could also act as ancestral event and as an independent leukemic driver.

Paroxysmal Nocturnal Haemoglobinuria (PNH) Arising from Non-Canonical Mutations

Introduction Acquired somatic mutations in the X-linked PIGA gene are the most commonly described pathological mutations found in the serious haematopoietic disorder paroxysmal nocturnal haemoglobinuria (PNH), though incidental cases of mutations in PIGT, PIGB and PIGV have been reported. Mutations in the family of 20 PIG (phosphatidylinositol glycan) genes can disrupt biosynthesis of glycosylphosphatidylinositol (GPI) anchors, critical for tethering many important proteins, including the vital complement regulators CD55 and CD59, to the cell membrane. Their absence in PNH leads to complement dysregulation and complications including potentially life-threatening haemolysis and thrombosis, which may be mitigated by lifelong anti-complement therapy. From the cohort of 273 PNH patients currently on complement inhibitor therapy at the Leeds National PNH Centre we have identified 37 patients whose diagnostic flow cytometry report or clinical course differs significantly from expectation, particularly with evidence of inflammatory presentations, which may be indicative of an alternative, non- PIGA, aetiology. Methods Patients consented to the Leeds PNH Research Tissue Bank with unusual clinical and laboratory presentations were identified, genomic DNA was extracted from peripheral blood and initially screened by targeted high sensitivity NGS of the PIGA gene. From the first ten patient samples sequenced, three lacked any evidence of PIGA mutations and so whole exome sequencing (WES) was carried out. Results Patient 1:An 80 year old man, diagnosed with MDS with rearrangement of chromosomes 3 and 7 resulting in a dicentric chromosome and loss of chromosome 20q, was diagnosed with PNH following an increase in transfusion requirement. In addition to classical symptoms of PNH he was diagnosed with seronegative rheumatoid arthritis. Flow cytometry showed an unusual PNH phenotype with a large population of partially GPI-anchor deficient cells only (Table 1). Symptoms improved on eculizumab but he remained transfusion dependent and the disease later transformed to acute myeloid leukaemia. WES: exonic frameshift insertion in PIGT (figure1), pathogenic variant rs760395465. Patient 2:A 34 year old woman presented with a 13 year history of intermittent episodes of generalized urticaria associated with nausea, vomiting, general malaise and myalgia, increasing in intensity and latterly being associated with haemoglobinuria, lethargy and jaundice leading to a PNH diagnosis. Episodes were triggered by pregnancy or infections, but she remained asymptomatic between attacks. Flow cytometry revealed a PNH clone with a majority of cells being partially GPI deficient. She responds well to C5 inhibition with no further symptoms, including urticaria. WES: exonic frameshift insertion in PIGT (figure 1), pathogenic variant rs760395465. Patient 3: A 14 year old girl presented with intermittent episodes of urticaria and severe lethargy and was diagnosed with symptomatic significant transfusion-dependent PNH, with completely GPI-deficient cells. Diagnosis followed an episode of massive haemolysis resulting in transient acute renal failure requiring haemodialysis. She commenced anticoagulation initially until eculizumab was approved 2 years later; despite occasional episodes of breakthrough haemolysis she displays no further urticaria. She has also had symptoms of diffuse arthralgia. WES: stopgain mutation in PIGV (figure1), pathogenic variant rs145160045. Conclusions These cases add to the emerging picture of non- PIGA associated PNH cases and suggest that the presence of joint problems and episodic urticaria may be sufficient indication to prompt screening for alternative causative mutations. Similar to the previously reported PIGB mutated case, the patients with PIGT mutations only had partially deficient cells. It is important to fully characterise these emerging non-canonical variants in order to improve our understanding of the pathophysiology of this disorder and to provide more individualised care.

PATH-43. INFRATENTORIAL IDH-MUTANT ASTROCYTOMAS REQUIRE HIGH INDEX OF SUSPICION FOR ACCURATE DIAGNOSIS

Abstract BACKGROUND Banan et al (PMID 2776277) recently identified infratentorial IDH-mutant astrocytomas (Inf-IDHmt) as a distinct subtype in which 80% have non-canonical mutations, precluding the use of IDH1 R132H immunohistochemistry (IHC) for diagnosis. They note “molecular testing is critical for detection of these tumors”. Only 47% have ATRX loss by IHC and only 56% have MGMT promotor methylation. Thus, it is critical that neurosurgeons take this into account during surgical planning due to the requirement for adequate tissue sample volume necessary for diagnosis. We review these diagnostic challenges in our recent single institution experience with these tumors. METHODS Database text word searches were conducted for the years 2007-2023 to generate cases, coupled with chart review. RESULTS 7 cases were identified: 3 male and 4 female, ages 27-44. All cases were small stereotactic biopsies. The histological diagnosis was made exceptionally challenging by paucity of tumor cells (especially in grade 2), coupled with the fact that 5 of 7 had non-canonical IDH-mutation and 5 of 7 showed ATRX retention by IHC, necessitating a very high index of suspicion on the part of the pathologist to send for next-generation sequencing (NGS). Due to the rare co-expression of IDH and histone mutations in this subtype, 6 of 7 had H3 K27M IHC conducted; all were negative. Due to small tissue volumes, MGMT promotor methylation testing could only be conducted on 3 of 7 cases; testing was uninformative on these 3. CONCLUSION Preoperative high index of suspicion is required to obtain maximal safe biopsy volumes of tissue for molecular and MGMT methylation testing, which should be conducted on virtually all brainstem and cerebellar biopsies.

Germline MPL mutations may be a rare cause of “triple-negative” thrombocytosis

Hereditary thrombocytosis (HT) is a rare inherited disorder with clinical features resembling those of sporadic essential thrombocythemia. This study included 933 patients with persistent isolated thrombocytosis for whom secondary reactive causes were excluded. Of 933 patients screened, 567 were JAK2-mutated, 255 CALR-mutated, 41 MPL-mutated, 2 double-mutated, and 68 were triple-negative. Two patients carried germline non-canonical mutations in exon 10: MPL W515* and MPL V501A. One triple-negative patient carried another germline non-canonical MPL mutation outside exon 10: MPL R102P. As germline MPL mutations may be underlying causes of HT, we recommend screening patients with triple-negative isolated thrombocytosis for non-canonical MPL mutations. Although clear evidence concerning HT treatment is still lacking, individuals with HT should probably be excluded from cytoreductive treatment. Thus, an accurate diagnosis is pivotal in avoiding unnecessary treatments.

Correlating MRI features with additional genetic markers and patient survival in histological grade 2-3 IDH-mutant astrocytomas

PurposeThe increasing importance of molecular markers for classification and prognostication of diffuse gliomas has prompted the use of imaging features to predict genotype (“radiogenomics”). CDKN2A/B homozygous deletion has only recently been added to the diagnostic paradigm for IDH[isocitrate dehydrogenase]-mutant astrocytomas; thus, associated radiogenomic literature is sparse. There is also little data on whether different IDH mutations are associated with different imaging appearances. Furthermore, given that molecular status is now generally obtained routinely, the additional prognostic value of radiogenomic features is less clear. This study correlated MRI features with CDKN2A/B status, IDH mutation type and survival in histological grade 2-3 IDH-mutant brain astrocytomas.MethodsFifty-eight grade 2–3 IDH-mutant astrocytomas were identified, 50 with CDKN2A/B results. IDH mutations were stratified into IDH1-R132H and non-canonical mutations. Background and survival data were obtained. Two neuroradiologists independently assessed the following MRI features: T2-FLAIR mismatch (<25%, 25–50%, >50%), well-defined tumour margins, contrast-enhancement (absent, wispy, solid) and central necrosis.Results8/50 tumours with CDKN2A/B results demonstrated homozygous deletion; slightly shorter survival was not significant (p=0.571). IDH1-R132H mutations were present in 50/58 (86%). No MRI features correlated with CDKN2A/B status or IDH mutation type. T2-FLAIR mismatch did not predict survival (p=0.977), but well-defined margins predicted longer survival (HR 0.36, p=0.008), while solid enhancement predicted shorter survival (HR 3.86, p=0.004). Both correlations remained significant on multivariate analysis.ConclusionMRI features did not predict CDKN2A/B homozygous deletion, but provided additional positive and negative prognostic information which correlated more strongly with prognosis than CDKN2A/B status in our cohort.

Mutation spectrum of FLT3 and significance of non-canonical FLT3 mutations in haematological malignancy.

Fms-like tyrosine kinase 3 (FLT3) is frequently mutated in haematological malignancies. Although canonical FLT3 mutations including internal tandem duplications (ITDs) and tyrosine kinase domains (TKDs) have been extensively studied, little is known about the clinical significance of non-canonical FLT3 mutations. Here, we first profiled the spectrum of FLT3 mutations in 869 consecutively newly diagnosed acute myeloid leukaemia (AML), myelodysplastic syndrome and acute lymphoblastic leukaemia patients. Our results showed four types of non-canonical FLT3 mutations depending on the affected protein structure: namely non-canonical point mutations (NCPMs) (19.2%), deletion (0.7%), frameshift (0.8%) and ITD outside the juxtamembrane domain (JMD) and TKD1 regions (0.5%). Furthermore, we found that the survival of patients with high-frequency (>1%) FLT3-NCPM in AML was comparable to those with canonical TKD. In vitro studies using seven representative FLT3-deletion or frameshift mutant constructs showed that the deletion mutants of TKD1 and the FLT3-ITD mutant of TKD2 had significantly higher kinase activity than wild-type FLT3, whereas the deletion mutants of JMD had phosphorylation levels comparable with wild-type FLT3. All tested deletion mutations and ITD were sensitive to AC220 and sorafenib. Collectively, these data enrich our understanding of FLT3 non-canonical mutations in haematological malignancies. Our results may also facilitate prognostic stratification and targeted therapy of AML with FLT3 non-canonical mutations.

Identification of putative noncanonical driver mutations in patients with essential thrombocythemia.

Essential thrombocythemia (ET) cases without canonical JAK2, CALR, or MPL mutations, that is, triple-negative (TN) ET, have been found in 10%-20% of ET cases. Owing to the limited number of TN ET cases, its clinical significance remains unclear. This study evaluated TN ET's clinical characteristics and identified novel driver mutations. Among 119 patients with ET, 20 (16.8%) had no canonical JAK2/CALR/MPL mutations. Patients with TN ET tended to be younger and had lower white blood cell counts and lactate dehydrogenase values. We identified putative driver mutations in 7 (35%): MPL S204P, MPL L265F, JAK2 R683G, and JAK2 T875N were previously reported as candidate driver mutations in ET. Moreover, we identified a THPO splicing site mutation, MPL*636Wext*12, and MPL E237K. Four of the seven identified driver mutations were germline. Functional studies on MPL*636Wext*12 and MPL E237K revealed that they are gain-of-function mutants that increase MPL signaling and confer thrombopoietin hypersensitivity with very low efficiency. Patients with TN ET tended to be younger, although this was thought to be due to the inclusion of germline mutations, hereditary thrombocytosis. Accumulating the genetic and clinical characteristics of noncanonical mutations may help future clinical interventions in TN ET and hereditary thrombocytosis.

The Spread of SARS-CoV-2 Omicron Variant in CALABRIA: A Spatio-Temporal Report of Viral Genome Evolution.

We investigated the evolution of SARS-CoV-2 spread in Calabria, Southern Italy, in 2022. A total of 272 RNA isolates from nasopharyngeal swabs of individuals infected with SARS-CoV-2 were sequenced by whole genome sequencing (N = 172) and/or Sanger sequencing (N = 100). Analysis of diffusion of Omicron variants in Calabria revealed the prevalence of 10 different sub-lineages (recombinant BA.1/BA.2, BA.1, BA.1.1, BA.2, BA.2.9, BA.2.10, BA.2.12.1, BA.4, BA.5, BE.1). We observed that Omicron spread in Calabria presented a similar trend as in Italy, with some notable exceptions: BA.1 disappeared in April in Calabria but not in the rest of Italy; recombinant BA.1/BA.2 showed higher frequency in Calabria (13%) than in the rest of Italy (0.02%); BA.2.9, BA.4 and BA.5 emerged in Calabria later than in other Italian regions. In addition, Calabria Omicron presented 16 non-canonical mutations in the S protein and 151 non-canonical mutations in non-structural proteins. Most non-canonical mutations in the S protein occurred mainly in BA.5 whereas non-canonical mutations in non-structural or accessory proteins (ORF1ab, ORF3a, ORF8 and N) were identified in BA.2 and BA.5 sub-lineages. In conclusion, the data reported here underscore the importance of monitoring the entire SARS-CoV-2 genome.

Mutation Spectrum of FLT3 and Significance of Non-Canonical FLT3 Mutations in Hematological Malignancies

Background FLT3 mutations are detected in 15%-35% of newly diagnosed AML patients. The canonical FLT3 mutation types are internal tandem repeats (ITD) in the JMD and TKD1 regions and point mutations at codons D835/I836 (TKD). Conventional sequencing for FLT3 mutations only covers the above-mentioned hotspot regions. With the advent of next generation sequencing which spans the full-length FLT3, a series of non-canonical mutations have also been reported. However, the kinase activities and clinical characteristics of these mutations remain unclear. Methods To profile the spectrum of FLT3 mutations, we retrospectively analyzed 869 consecutive newly diagnosed AML, ALL, and MDS patients with FLT3 mutation by NGS-seq from June 2016 to December 2020 at Jiangsu Institute of Hematology. The clinical data of 316 AML (excluding APL) patients were retrospectively collected and analyzed after informed consent. Since previous studies have elaborated the FLT3 functional characteristics of FLT3-ITD and FLT3 point mutations, we focus on other FLT3 mutation types in this study. Seven representative FLT3 mutations, including four deletion mutations in JMD, one frameshift mutation in JMD, one deletion mutation in TKD1 and one ITD in TKD2, were constructed to study the kinase activities and sensitivities to FLT3 inhibitors in vitro. Results FLT3 mutational spectrums of 869 patients showed that the NGS method could detect numerous novel non-canonical FLT3 mutations, accounting for 21.2% of FLT3 mutation events compared with conventional tests (Figure 1). According to the changes in gene structures resulted by different mutations, we classified the non-canonical mutations into four types: non-canonical point mutations (NCPM) (19.2%), deletion (0.7%), frameshift (0.8%), and ITD outside the JMD and TKD1 regions (0.5%). Given the enormous differences in frequency, NCPM was further divided into FLT3 recurrent point mutations of NPM (FLT3-NCPM-RPM) and FLT3 non-recurrent point mutations (FLT3-NRPM). Kaplan-Meier analyses of the RFS and OS showed that there is no difference between canonical FLT3-TKD and FLT3-NCPM-RPM while statistical differences were observed not only between canonical FLT3-TKD and FLT3-NRPM but also between FLT3-NCPM-RPM and FLT3-NRPM. Due to the similarities in prognosis and kinase activity, classical FLT3-TKD and FLT3-NCPM-RPM were grouped into a class called FLT3 recurrent point mutations (FLT3-RPM). Further prognosis analyses indicated that patients with FLT3-ITD, FLT3-RPM and FLT3-NRPM had significant differences in RFS (P=0.005) and OS (P=0.01). Interestingly, the presence of FLT3-NRPM may confer a favorable prognosis compared with FLT3-ITD and FLT3-RPM. Multivariable analysis using Cox models also confirmed that FLT3-NRPM had a survival benefit over FLT3-ITD and FLT3-RPM. The significant difference in the prognosis of patients with three FLT3 mutation subtypes and Cox models proved the rationality of this strategy. Besides the non-canonical point mutation, a series of FLT3 mutations with small deletions and frameshift alterations were also detected. In addition, we also discovered several completely random insertion mutations in the non-ITD region. According to our previous study, these mutageneses were considered a subtype of FLT3-ITD. Based on the results, FLT3 alterations could be divided into FLT3-ITD, FLT3-RPM, FLT3-NRPM, FLT3-deletion, and FLT3-frameshift groups. In vitro analysis showed that the deletion mutants in TKD1 (p.K614_G617del) and the FLT3-ITD mutant in TKD2 (p.S941_F942insRVS) had significantly higher kinase activity than wild-type FLT3. Whereas the deletion mutants in JMD have comparable phosphorylation levels to wild-type FLT3 (Figure 2). All tested deletion mutations and ITD in the TKD2 region were sensitive to both AC220 and sorafenib. Conclusion Conventional sequencing that only covers hotspot regions omitted part of FLT3 mutation and full-length sequencing of the FLT3 gene may be indispensable. FLT3 alterations detected by NGS-seq could be reclassified into 5 types: FLT3-ITD, FLT3-RPM, FLT3-NRPM, FLT3-deletion, and FLT3-frameshift. In vitro studies indicated patients with FLT3 deletion mutations and FLT3-ITD outside the JMD and TKD1 regions might be ought to be treated with FLT3 inhibitors. Our new classification of FLT3 mutation was expected to promote more precise management of patients with FLT3 non-canonical mutations. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Tropomyosin-Related Kinase Fusions in Gastrointestinal Stromal Tumors.

Simple SummaryThe canonical mutations in gastrointestinal stromal tumors (GISTs) are typically activating mutations in KIT and platelet-derived growth factor receptor alpha (PDGFRA). Imatinib, the treatment of choice for GISTs, shows a lower response in KIT/PDGFRA wild-type GISTs. Neurotrophic tyrosine receptor kinase (NTRK) fusion, which can be treated with an NTRK target agent, has been reported in KIT/PDGFRA wild-type GISTs, and, therefore, the Yonsei Cancer Center analyzed NTRK fusion incidence in KIT/PDGFRA wild-type GISTs. At the Yonsei Cancer Center, NTRK fusion was confirmed in 16% of cases. Confirmation of NTRK fusion in KIT/PDGFRA wild-type GISTs provides important information for improving therapeutic outcomes. NTRK fusion was confirmed in 16% of KIT/PDGFRA wild-type GIST cases at the Yonsei Cancer Center. Confirmation of NTRK fusion in KIT/PDGFRA wild-type GISTs will improve therapeutic outcomes.The canonical mutations in gastrointestinal stromal tumors (GISTs) are typically activating mutations in KIT and platelet-derived growth factor receptor alpha (PDGFRA). GISTs with non-canonical mutations are a heterogeneous group. Here, we examined tropomyosin-related kinase (TRK) fusion in GIST cases without KIT/PDGFRA mutations (KIT/PDGFRA wild-type (WT) GISTs). We retrospectively analyzed patients who were diagnosed with GISTs at the Yonsei Cancer Center, Severance Hospital, between January 1998 and December 2016. Thirty-one patients with KIT/PDGFRA WT GISTs were included in the analysis. TRK expression in tumor samples was assessed by pan-TRK immunohistochemistry (IHC), and the neurotrophic tyrosine receptor kinase (NTRK: the gene encoding TRK) rearrangement was analyzed by fluorescence in situ hybridization (FISH). IHC analyses revealed that five cases in this cohort exhibited a weak to moderate TRK expression. NTRK1 fusions were detected in three tumor samples, and two samples harbored NTRK3 fusions. The remaining 26 samples did not harbor NTRK fusions. Two types of NTRK fusions were detected, and the overall NTRK fusion frequency in KIT/PDGFRA WT GIST cases was 16% (5/31). Our data provide insights into the molecular alterations underpinning KIT/PDGFRA WT GISTs. More effort should be devoted to improve methods to identify this distinct disease subtype within the KIT/PDGFRA WT GIST group.

Optimization and Validation of Nanopore Based Sequencing Method for Molecular Testing of CNS Tumours

BackgroundThe World Health Organization (WHO) introduced molecular identifiers for the diagnosis and prognosis of CNS tumors including the mutational status of isocitrate dehydrogenase or IDHgenes in glial tumors. Currently used immunohistochemistry (IHC) is not capable of detecting the non‐canonical mutations, and sequencing is often required as a follow‐up. Current next‐generation sequencing (NGS) technologies used in tumor molecular marker detection introduce key challenges including high capital cost, complex infrastructure requirements, and long turnaround times. These challenges considerably limit the ability to perform NGS testing in many pathology laboratories. In this study, we aimed to use third generation nanopore sequencing technology to resolve these limitations. The Oxford Nanopore MinION, a pocket‐sized nanopore sequencing device, has minimal capital costs and infrastructural requirements, and shorter turnaround times. However, the nanopore technology has not been validated in clinical practice and has not been optimized on formalin‐fixed paraffin‐embedded (FFPE) tissue.MethodsDNA extraction of selective tumor areas was performed from the corresponding FFPE tissue blocks from a cohort of gliomas with confirmed IDH1and IDH2gene statuses (n=65). A PCR amplicon‐based approach was used to amplify hot spots of the IDH1and IDH2genes, starting with 30ng DNA material. The amplicon libraries were sequenced for 2 hours in multiplex on the MinION device and IDHSNPs were called with the Nanopolish software. ASIP Abstract Mashiat MimosaResults26 IDH mutant samples were identified: 21 IDH1R132H, 2 IDH1R132G, 2 IDH2R172G, and 1 IDH2D177H. All cases showed concordant IDHmutational status when compared to the reference methods (IHC or NGS) and both analytical sensitivity and specificity were 100%. Precision analysis of variant allele frequency (VAF) showed the coefficient of variation was less than 5% (both inter and intra runs), and the limit of detection for VAF was 3%. The range of read depth obtained was 882X to 43,000x with an average of 20,000x. This assay revealed a $50‐$100 material cost per sample, and the time taken from extracted nucleic acid to final result generation was 1‐2 business days.ConclusionThis project is the first to optimize and validate an approach to detect SNP mutations in FFPE samples using nanopore technology. It has demonstrated the feasibility and efficacy of the nanopore amplicon sequencing method in cancer FFPE tissue with excellent test performance characteristics, significantly shorter turnaround times at considerably lower costs and without any infrastructural needs. Thus, it can be used to circumvent challenges to current NGS testing platforms and can be the milestone that would make cancer NGS testing available for every laboratory.

Non-canonical genomic driver mutations of urethane carcinogenesis.

The carcinogen urethane induces pulmonary tumors in mice initiated by an incredibly specific Q61L/R oncogenic mutation in the proto-oncogene Kras. Previous Whole-Exome Sequencing of urethane-induced tumors revealed a bias towards A➙T/G and G➙A substitutions. Subsequent ultra-sensitive Maximum-Depth Sequencing of Kras shortly after urethane exposure suggest a further refinement to CA➙CT/G substitutions. As C182AA➙C182T/GA substitutions in Kras result in Q61L/R mutations, the extreme bias of urethane towards these genomic driver mutations can be ascribed to the specificity of the carcinogen for CA➙CT/G substitutions. However, we previously found that changing rare codons to common in the Kras gene to increase protein expression shifted mutations in urethane-induced tumors away from Kras, or when detected in Kras, to G12D mutations that are usually rarely detected in such tumors. Moreover, the loss of p53 partially reversed this effect, generating tumors with either Q61L/R or G12D oncogenic Kras mutations, or no Kras mutations, presumably due to other genomic driver mutations. Determining the origin of these G12D and other unknown non-canonical genomic driver mutations would provide critical insight into the extreme bias of carcinogens for specific genomic driver mutations. We thus compared the types of Single Nucleotide Variations detected by previously performed Maximum-Depth Sequencing immediately after urethane exposure to the mutation signatures derived from Whole Exome Sequencing of urethane-induced tumors. This identified two types of non-canonical mutations. First, a V637E oncogenic mutation in the proto-oncogene Braf that conforms to the mutation signature of urethane, suggesting that the mutational bias of the carcinogen may account for this non-canonical mutation, similar to that for canonical Q61L/R mutations in Kras. Second, G12D and Q61H mutations in Kras that did not fit this mutation signature, and instead shared similarity with Single Nucleotide Variations detected by Maximum-Depth Sequencing from normal cells, suggesting that perhaps these mutations were pre-existing. We thus posit that when canonical Kras mutations are selected against that the carcinogen may instead promote the expansion of pre-existing genomic driver mutations, although admittedly we cannot rule out other mechanisms. Interrogating the mutation signatures of human lung cancers similarly identified KRAS genomic driver mutations that failed to match the mutation signature of the tumor. Thus, we also speculate that the selection for non-canonical genomic driver mutations during urethane carcinogenesis may reflect the process by which discordance between genomic driver mutations and mutational signatures arises in human cancers.

Abstract P229: Pre-clinical evaluation of next-generation inhibitor targeting a wide spectrum of oncogenic BRAF dimers

Abstract The canonical BRAF V600E (Class I) mutation is a potent oncogene uniquely active as a RAS-independent monomer, successfully targeted by several FDA-approved inhibitors. However, these first-generation BRAF inhibitors are not active against non-canonical BRAF oncogenic mutations, including BRAF-fusions, that drive RAS-independent (Class II) or RAS-dependent (Class III) dimers. As such, developing inhibitors directed against dimeric BRAF oncogenic mutations that avoid paradoxical activation is a major unmet clinical need. We applied proprietary Mutation-Allostery-Pharmacology (MAP) platform technology developed by Black Diamond Therapeutics to identify and validate a group of previously uncharacterized non-canonical oncogenic Class II and Class III BRAF mutation clusters. We further demonstrate that this ensemble of novel and previously validated non-canonical oncogenic BRAF mutants can form the basis of a differentiated drug discovery program aimed at identifying small molecules that potently and selectively target this family of dimeric BRAF mutations. Herein, we describe a small molecule inhibitor, BDTX BRAF-A, with potent anti-proliferative activity directed against tumor cells expressing a wide spectrum of non-canonical Class II/III mutations. This broad activity (“MasterKey” profile) of BDTX BRAF-A is further demonstrated in cell lines that harbor endogenous oncogenic dimer-inducing BRAF mutations and in various solid tumor patient-derived xenograft (PDX) models ex vivo. Importantly, BDTX BRAF-A did not induce paradoxical RAF activation characteristic of Class I BRAF inhibitors. Finally, BDTX BRAF-A achieves robust anti-tumor efficacy and target engagement of dimeric BRAF oncogenes in mouse models. These data support the continued development of rationally designed molecules targeting a broad range of non-canonical BRAF dimer-promoting mutations to extend the prospect of precision medicine in patients. Citation Format: Yoon-Chi Han, Pui Yee Ng, Ryan Schulz, Shao Ning Yang, Alana Lelo, Luisa Shin, Matthew O'Connor, Ivan Jewett, Noboru Ishiyama, Darlene Romashko, Shalabh Thakur, Andrei Salomatov, Sherri Smith, Elizabeth Buck, Christopher Roberts, Matthew Lucas, Tai-An Lin. Pre-clinical evaluation of next-generation inhibitor targeting a wide spectrum of oncogenic BRAF dimers [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P229.

Clinical and Molecular Features of Patients with Gliomas Harboring IDH1 Non-canonical Mutations: A Systematic Review and Meta-Analysis.

The canonical isocitrate dehydrogenase1 R132 mutation (IDH1 R132) is the most frequent mutation among IDH-mutated gliomas. Non-canonical IDH1 mutations or IDH2 mutations are unusual and their clinical and biological role is still unclear. We performed a systematic review and meta-analysis to assess the clinical role of IDH non-canonical mutations. Overall, we selected 13 of 3513 studies reporting data of 4007 patients with a diagnosis of grade2 and grade3 glioma including 3091 patients with a molecularly proven IDH1 or IDH2 mutation. Patients with non-canonical IDH1 mutations were younger and presented a higher DNA methylation level as compared to those with canonical IDH1 R132H alteration. The overall incidence of non-canonical IDH1 mutations was 7.9% (95%CI 5.4-10.7%) in patients with IDH-mutated gliomas. There was no statistical difference in terms of incidence between patients with grade2 or grade3 glioma. Patients with non-canonical IDH mutations had a lower rate of 1p19q codeletion (risk difference 31%, 95%CI 23-38%) and presented a significantly prolonged survival (pooled HR 0.47, 95%CI 0.28-0.81) as compared to those with IDH1 R132H mutation. Non-canonical IDH1 mutations occur in 7.9% of IDH-mutated gliomas and identify a specific subgroup of patients with an improved survival despite a lower rate of 1p19q codeletion. Data about the type of IDH mutation should be collected in clinical practice and within interventional trials as this could be a critical variable for improved stratification and selection of patients.

Therapeutic implications of activating noncanonical PIK3CA mutations in head and neck squamous cell carcinoma.

Alpelisib selectively inhibits the p110α catalytic subunit of PI3Kα and is approved for treatment of breast cancers harboring canonical PIK3CA mutations. In head and neck squamous cell carcinoma (HNSCC), 63% of PIK3CA mutations occur at canonical hotspots. The oncogenic role of the remaining 37% of PIK3CA noncanonical mutations is incompletely understood. We report a patient with HNSCC with a noncanonical PIK3CA mutation (Q75E) who exhibited a durable (12 months) response to alpelisib in a phase II clinical trial. Characterization of all 32 noncanonical PIK3CA mutations found in HNSCC using several functional and phenotypic assays revealed that the majority (69%) were activating, including Q75E. The oncogenic impact of these mutations was validated in 4 cellular models, demonstrating that their activity was lineage independent. Further, alpelisib exhibited antitumor effects in a xenograft derived from a patient with HNSCC containing an activating noncanonical PIK3CA mutation. Structural analyses revealed plausible mechanisms for the functional phenotypes of the majority of the noncanonical PIK3CA mutations. Collectively, these findings highlight the importance of characterizing the function of noncanonical PIK3CA mutations and suggest that patients with HNSCC whose tumors harbor activating noncanonical PIK3CA mutations may benefit from treatment with PI3Kα inhibitors.

PATH-15. NON-CANONICAL IDH 1 AND IDH 2 MUTATIONS ARE ASSOCIATED WITH IMPROVED SURVIVAL IN PATIENTS WITH GLIOMAS: RESULTS OF A META-ANALYSIS

Abstract BACKGROUND Non-canonical isocitrate dehydrogenase (IDH) mutations are uncommon among patients with grade 2 and 3 gliomas and their clinical significance is still unclear. METHOD We carried out a systematic review and meta-analysis to evaluate the prognostic impact of IDH1 and IDH2 non-canonical mutations. We searched English-written articles published on PubMed/Medline, Cochrane library, and Scopus until the 1st May 2021. Keywords adopted for the research were: ‘’IDH’’ OR ‘’IDH1’’ OR ‘’IDH2’’ OR ‘’Isocitrate dehydrogenase’’ AND ‘’glioma’’. RESULTS We selected 13 of 3513 studies reporting data of 4007 patients with a diagnosis of grade 2 and grade 3 including 3091 patients with a molecular proven IDH 1 or IDH 2 mutations. Between patients with IDH mutated gliomas, 479 (15.5%) patients showed non-canonical IDH 1 or IDH 2 mutations. The presence of IDH 1 non-canonical mutation occurred in younger age and in non codeleted 1p19q as compared to canonical IDH 1 mutation. However, patients with IDH 2 mutations showed more frequently 1p19q codeletion as compared to those with canonical IDH 1 mutated glioma. Four studies reported complete data survival for patients with non-canonical (n= 160) and canonical mutations (n= 1019). The pooled HR of these studies was 0.47 (95% CI, 0.28-0.81) confirming a positive prognostic role for non-canonical IDH 1 and IDH 2 mutations. CONCLUSION The presence of non-canonical IDH 1 and IDH 2 mutations defines a specific subgroup of gliomas occurring at younger age with improved survival. Patients with a non-canonical IDH 1 mutation showed less frequently 1p19q codeletion as compared to those patients harboring a canonical or IDH 2 mutation.

Functional Consequences of Mutations in Myeloproliferative Neoplasms.

Driver mutations occur in Janus kinase 2 (JAK2), thrombopoietin receptor (MPL), and calreticulin (CALR) in BCR-ABL1 negative myeloproliferative neoplasms (MPNs). From mutations leading to one amino acid substitution in JAK2 or MPL, to frameshift mutations in CALR resulting in a protein with a different C-terminus, all the mutated proteins lead to pathologic and persistent JAK2-STAT5 activation. The most prevalent mutation, JAK2 V617F, is associated with the 3 entities polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF), while CALR and MPL mutations are associated only with ET and MF. Triple negative ET and MF patients may harbor noncanonical mutations in JAK2 or MPL. One major fundamental question is whether the conformations of JAK2 V617F, MPL W515K/L/A, or CALR mutants differ from those of their wild type counterparts so that a specific treatment could target the clone carrying the mutated driver and spare physiological hematopoiesis. Of great interest, a set of epigenetic mutations can co-exist with the phenotypic driver mutations in 35%–40% of MPNs. These epigenetic mutations, such as TET2, EZH2, ASXL1, or DNMT3A mutations, promote clonal hematopoiesis and increased fitness of aged hematopoietic stem cells in both clonal hematopoiesis of indeterminate potential (CHIP) and MPNs. Importantly, the main MPN driver mutation JAK2 V617F is also associated with CHIP. Accumulation of several epigenetic and splicing mutations favors progression of MPNs to secondary acute myeloid leukemia. Another major fundamental question is how epigenetic rewiring due to these mutations interacts with persistent JAK2-STAT5 signaling. Answers to these questions are required for better therapeutic interventions aimed at preventing progression of ET and PV to MF, and transformation of these MPNs in secondary acute myeloid leukemia.