Therapeutic validation of MMR-associated genetic modifiers in a human ex vivo model of Huntington disease.

The pathological huntingtin (HTT) trinucleotide repeat underlying Huntington disease (HD) continues to expand throughout life. Repeat length correlates both with earlier age at onset (AaO) and faster progression, making slowing its expansion an attractive therapeutic approach. Genome-wide association studies have identified candidate variants associated with altered AaO and progression, with many found in DNA mismatch repair (MMR)-associated genes. We examine whether lowering expression of these genes affects the rate of repeat expansion in human ex vivo models using HD iPSCs and HD iPSC-derived striatal medium spiny neuron-enriched cultures. We have generated a stable CRISPR interference HD iPSC line in which we can specifically and efficiently lower gene expression from a donor carrying over 125 CAG repeats. Lowering expression of each member of the MMR complexes MutS (MSH2, MSH3, and MSH6), MutL (MLH1, PMS1, PMS2, and MLH3), and LIG1 resulted in characteristic MMR deficiencies. Reduced MSH2, MSH3, and MLH1 slowed repeat expansion to the largest degree, while lowering either PMS1, PMS2, or MLH3 slowed it to a lesser degree. These effects were recapitulated in iPSC-derived striatal cultures where MutL factor expression was lowered. CRISPRi-mediated lowering of key MMR factor expression to levels feasibly achievable by current therapeutic approaches was able to effectively slow the expansion of the HTT CAG tract. We highlight members of the MutL family as potential targets to slow pathogenic repeat expansion with the aim to delay onset and progression of HD and potentially other repeat expansion disorders exhibiting somatic instability.

Network Rewiring of Homologous Recombination Enzymes during Mitotic Proliferation and Meiosis.

Homologous recombination (HR) is essential for high-fidelity DNA repair during mitotic proliferation and meiosis. Yet, context-specific modifications must tailor the recombination machinery to avoid (mitosis) or enforce (meiosis) the formation of reciprocal exchanges-crossovers-between recombining chromosomes. To obtain molecular insight into how crossover control is achieved, we affinity purified 7 DNA-processing enzymes that channel HR intermediates into crossovers or noncrossovers from vegetative cells or cells undergoing meiosis. Using mass spectrometry, we provide a global characterization of their composition and reveal mitosis- and meiosis-specific modules in the interaction networks. Functional analyses of meiosis-specific interactors of MutLγ-Exo1 identified Rtk1, Caf120, and Chd1 as regulators of crossing-over. Chd1, which transiently associates with Exo1 at the prophase-to-metaphase I transition, enables the formation of MutLγ-dependent crossovers through its conserved ability to bind and displace nucleosomes. Thus, rewiring of the HR network, coupled to chromatin remodeling, promotes context-specific control of the recombination outcome.

Lack of mismatch repair enhances resistance to methylating agents for cells deficient in oxidative demethylation.

The human alkylation B (AlkB) homologs, ALKBH2 and ALKBH3, respond to methylation damage to maintain genomic integrity and cellular viability. Both ALKBH2 and ALKBH3 are direct reversal repair enzymes that remove 1-methyladenine (1meA) and 3-methylcytosine (3meC) lesions commonly generated by alkylating chemotherapeutic agents. Thus, the existence of deficiencies in ALKBH proteins can be exploited in synergy with chemotherapy. In this study, we investigated possible interactions between ALKBH2 and ALKBH3 with other proteins that could alter damage response and discovered an interaction with the mismatch repair (MMR) system. To test whether the lack of active MMR impacts ALKBH2 and/or ALKBH3 response to methylating agents, we generated cells deficient in ALKBH2, ALKBH3, or both in addition to Mlh homolog 1 (MLH1), another MMR protein. We found that MLH1koALKBH3ko cells showed enhanced resistance toward SN1- and SN2-type methylating agents, whereas MLH1koALKBH2ko cells were only resistant to SN1-type methylating agents. Concomitant loss of ALKBH2 and ALKBH3 (ALKBH2ko3ko) rendered cells sensitive to SN1- and SN2-agents, but the additional loss of MLH1 enhanced resistance to both types of damage. We also showed that ALKBH2ko3ko cells have an ATR-dependent arrest at the G2/M checkpoint, increased apoptotic signaling, and replication fork stress in response to methylation. However, these responses were not observed with the loss of functional MLH1 in MLH1koALKBH2ko3ko cells. Finally, in MLH1koALKBH2ko3ko cells, we observed elevated mutant frequency in untreated and temozolomide treated cells. These results suggest that obtaining a more accurate prognosis of chemotherapeutic outcome requires information on the functionality of ALKBH2, ALKBH3, and MLH1.

MutLα suppresses error-prone DNA mismatch repair and preferentially protects noncoding DNA from mutations.

The DNA mismatch repair (MMR) system promotes genome stability and protects humans from certain types of cancer. Its primary function is the correction of DNA polymerase errors. MutLα is an important eukaryotic MMR factor. We have examined the contributions of MutLα to maintaining genome stability. We show here that loss of MutLα in yeast increases the genome-wide mutation rate by ∼130-fold and generates a genome-wide mutation spectrum that consists of small indels and base substitutions. We also show that loss of yeast MutLα leads to error-prone MMR that produces T > C base substitutions in 5'-ATA-3' sequences. In agreement with this finding, our examination of human whole-genome DNA sequencing data has revealed that loss of MutLα in induced pluripotent stem cells triggers error-prone MMR that leads to the formation of T > C mutations in 5'-NTN-3' sequences. Our further analysis has shown that MutLα-independent MMR plays a role in suppressing base substitutions in N3 homopolymeric runs. In addition, we describe that MutLα preferentially protects noncoding DNA from mutations. Our study defines the contributions of MutLα-dependent and independent mechanisms to genome-wide MMR.

Clonal origin and genomic diversity in Lynch syndrome-associated endometrial cancer with multiple synchronous tumors: Identification of the pathogenicity of MLH1 p.L582H.

Lynch syndrome-associated endometrial cancer patients often present multiple synchronous tumors and this assessment can affect treatment strategies. We present a case of a 27-year-old woman with tumors in the uterine corpus, cervix, and ovaries who was diagnosed with endometrial cancer and exhibited cervical invasion and ovarian metastasis. Her family history suggested Lynch syndrome, and genetic testing identified a variant of uncertain significance, MLH1 p.L582H. We conducted immunohistochemical staining, microsatellite instability analysis, and Sanger sequencing for Lynch syndrome-associated cancers in three generations of the family and identified consistent MLH1 loss. Whole-exome sequencing for the corpus, cervical, and ovarian tumors of the proband identified a copy-neutral loss of heterozygosity (LOH) occurring at the MLH1 position in all tumors. This indicated that the germline variant and the copy-neutral LOH led to biallelic loss of MLH1 and was the cause of cancer initiation. All tumors shared a portion of somatic mutations with high mutant allele frequencies, suggesting a common clonal origin. There were no mutations shared only between the cervix and ovary samples. The profiles of mutant allele frequencies shared between the corpus and cervix or ovary indicated that two different subclones originating from the corpus independently metastasized to the cervix or ovary. Additionally, all tumors presented unique mutations in endometrial cancer-associated genes such as ARID1A and PIK3CA. In conclusion, we demonstrated clonal origin and genomic diversity in a Lynch syndrome-associated endometrial cancer, suggesting the importance of evaluating multiple sites in Lynch syndrome patients with synchronous tumors.

Clinical features and impact of p53 status on sporadic mismatch repair deficiency and Lynch syndrome in uterine cancer.

The clinical features of sporadic mismatch repair deficiency (MMRd) and Lynch syndrome (LS) in Japanese patients with endometrial cancer (EC) were examined by evaluating the prevalence and prognostic factors of LS and sporadic MMRd in patients with EC. Targeted sequencing of five LS susceptibility genes (MLH1, MSH2, MSH6, PMS2, and EPCAM) was carried out in 443 patients with EC who were pathologically diagnosed with EC at the National Cancer Center Hospital between 2011 and 2018. Pathogenic variants in these genes were detected in 16 patients (3.7%). Immunohistochemistry for MMR proteins was undertaken in 337 of the 433 (77.9%) EC patients, and 91 patients (27.0%) showed absent expression of at least one MMR protein. The 13 cases of LS with MMR protein loss (93.8%) showed a favorable prognosis with a 5-year overall survival (OS) rate of 100%, although there was no statistically significant difference between this group and the sporadic MMRd group (p = 0.27). In the MMRd without LS group, the 5-year OS rate was significantly worse in seven patients with an aberrant p53 expression pattern than in those with p53 WT (53.6% vs. 93.9%, log-rank test; p = 0.0016). These results suggest that p53 abnormalities and pathogenic germline variants in MMR genes could be potential biomarkers for the molecular classification of EC with MMRd.

Retained PAX2 expression associated with DNA mismatch repair deficiency in endometrial endometrioid adenocarcinoma.

AIMS: Loss of expression of tumour suppressor PAX2 and MMR deficiency (dMMR) has been frequently seen in endometrial endometrioid adenocarcinoma (EEC). However, the relationship between PAX2 expression and MMR status is unknown. METHODS AND RESULTS: We studied the PAX2 expression and examined its association with MMR status at the protein and genetic levels in 180 cases of EEC. Overall, total loss of PAX2 expression was found in about 70%, while retained PAX2 expression was seen in 30% of EEC. Among 125 cases with loss of PAX2, 68.8% were found in EECs with pMMR, while 31.2% were seen in those with dMMR. Among 55 cases of EECs with retained PAX2 expression, 92.7% were EECs with dMMR and 7.3% were those with pMMR (P < 0.001). While dMMR cases with MLH1 hypermethylation show almost equal retained or loss of PAX2 expression (52% versus 48%), dMMR with genetic alterations had significantly more retained PAX2 expression than loss of PAX2 (92.3% versus 7.7%), regardless of somatic or germline mutations. Loss of PAX2 was observed in 97.3% of dMMR with MLH1 hypermethylation compared to 2.7% of dMMR with genetic alterations (P < 0.001). Aggressive features such as higher tumour grades (FIGO 2-3) and advanced clinical stage (T2-T4) were significantly more frequently seen in dMMR with retained PAX2 expression, compared those to pMMR with loss of PAX2 expression. CONCLUSION: Our study demonstrates a close correlation between retained PAX2 expression and dMMR in EEC. The molecular mechanism and clinical significance linking these two pathways in EEC remains to be unravelled.

Association between colorectal cancer, the frequency of Bacteroides fragilis, and the level of mismatch repair genes expression in the biopsy samples of Iranian patients.

BACKGROUND: Deficient DNA mismatch repair (MMR) can cause microsatellite instability (MSI) and is more common in colorectal cancer (CRC) patients. Understanding the carcinogenic mechanism of bacteria and their impact on cancer cells is crucial. Bacteroides fragilis (B. fragilis) has been identified as a potential promoter of tumorigenesis through the alteration of signaling pathways. This study aims to assess the expression levels of msh2, msh6, mlh1, and the relative frequency of B. fragilis in biopsy samples from CRC patients. MATERIALS AND METHODS: Based on the sequence of mlh1, msh2, and msh6 genes, B. fragilis specific 16srRNA and bacterial universal 16srRNA specific primers were selected, and the expression levels of the target genes were analyzed using the Real-Time PCR method. RESULTS: Significant increases in the expression levels of mlh1, msh2, and msh6 genes were observed in the cancer group. Additionally, the expression of these MMR genes showed a significant elevation in samples positive for B. fragilis presence. The relative frequency of B. fragilis in the cancer group demonstrated a significant rise compared to the control group. CONCLUSION: The findings suggest a potential correlation between the abundance of B. fragilis and alterations in the expression of MMR genes. Since these genes can play a role in modifying colon cancer, investigating microbial characteristics and gene expression changes in CRC could offer a viable solution for CRC diagnosis.

Investigation of sperm hsa-mir-145-5p and MLH1 expressions, seminal oxidative stress and sperm DNA fragmentation in varicocele.

BACKGROUND: Mechanisms by which varicocele causes infertility are not clear and few studies have reported that some miRNAs show expression alterations in men with varicocele. Recently, sperm promoter methylation of MLH1 has been shown to be higher in men diagnosed with varicocele. This study aimed to assess the potential effects of miR-145, which was determined to target MLH1 mRNA in silico on sperm quality and function in varicocele. METHODS: Sperm miR-145 and MLH1 expressions of six infertile men with varicocele (Group 1), nine idiopathic infertile men (Group 2), and nine fertile men (control group) were analyzed by quantitative PCR. Sperm DNA fragmentation was evaluated by TUNEL and the levels of seminal oxidative damage and total antioxidant capacity were analyzed by ELISA. RESULTS: Our results have shown that sperm expression of miR-145 was decreased in Group 1 compared to Group 2 (P = 0.029). MLH1 expression was significantly higher in Group 2 than the controls (P = 0.048). Total antioxidant level and sperm DNA fragmentations of Group 1 and Group 2 were decreased (P = 0.001 and P = 0.011, respectively). Total antioxidant capacity was positively correlated with sperm concentration (ρ = 0.475, P = 0.019), total sperm count (ρ = 0.427, P = 0.037), motility (ρ = 0.716, P < 0.0001) and normal morphological forms (ρ = 0.613, P = 0.001) and negatively correlated with the seminal oxidative damage (ρ=-0.829, P = 0.042) in varicocele patients. CONCLUSION: This is the first study investigating the expressions of sperm miR-145 and MLH1 in varicocele patients. Further studies are needed to clarify the potential effect of miR-145 on male fertility.

Incidences of colorectal adenomas and cancers under colonoscopy surveillance suggest an accelerated "Big Bang" pathway to CRC in three of the four Lynch syndromes.

BACKGROUND: Colorectal cancers (CRCs) in the Lynch syndromes have been assumed to emerge through an accelerated adenoma-carcinoma pathway. In this model adenomas with deficient mismatch repair have an increased probability of acquiring additional cancer driver mutation(s) resulting in more rapid progression to malignancy. If this model was accurate, the success of colonoscopy in preventing CRC would be a function of the intervals between colonoscopies and mean sojourn time of detectable adenomas. Contrary to expectations, colonoscopy did not decrease incidence of CRC in the Lynch syndromes and shorter colonoscopy intervals have not been effective in reducing CRC incidence. The prospective Lynch Syndrome Database (PLSD) was designed to examine these issues in carriers of pathogenic variants of the mis-match repair (path_MMR) genes. MATERIALS AND METHODS: We examined the CRC and colorectal adenoma incidences in 3,574 path_MLH1, path_MSH2, path_MSH6 and path_PMS2 carriers subjected to regular colonoscopy with polypectomy, and considered the results based on sojourn times and stochastic probability paradigms. RESULTS: Most of the path_MMR carriers in each genetic group had no adenomas. There was no association between incidences of CRC and the presence of adenomas. There was no CRC observed in path_PMS2 carriers. CONCLUSIONS: Colonoscopy prevented CRC in path_PMS2 carriers but not in the others. Our findings are consistent with colonoscopy surveillance blocking the adenoma-carcinoma pathway by removing identified adenomas which might otherwise become CRCs. However, in the other carriers most CRCs likely arised from dMMR cells in the crypts that have an increased mutation rate with increased stochastic chaotic probabilities for mutations. Therefore, this mechanism, that may be associated with no or only a short sojourn time of MSI tumours as adenomas, could explain the findings in our previous and current reports.

Classification of MLH1 Missense VUS Using Protein Structure-Based Deep Learning-Ramachandran Plot-Molecular Dynamics Simulations Method.

Pathogenic variation in DNA mismatch repair (MMR) gene MLH1 is associated with Lynch syndrome (LS), an autosomal dominant hereditary cancer. Of the 3798 MLH1 germline variants collected in the ClinVar database, 38.7% (1469) were missense variants, of which 81.6% (1199) were classified as Variants of Uncertain Significance (VUS) due to the lack of functional evidence. Further determination of the impact of VUS on MLH1 function is important for the VUS carriers to take preventive action. We recently developed a protein structure-based method named "Deep Learning-Ramachandran Plot-Molecular Dynamics Simulation (DL-RP-MDS)" to evaluate the deleteriousness of MLH1 missense VUS. The method extracts protein structural information by using the Ramachandran plot-molecular dynamics simulation (RP-MDS) method, then combines the variation data with an unsupervised learning model composed of auto-encoder and neural network classifier to identify the variants causing significant change in protein structure. In this report, we applied the method to classify 447 MLH1 missense VUS. We predicted 126/447 (28.2%) MLH1 missense VUS were deleterious. Our study demonstrates that DL-RP-MDS is able to classify the missense VUS based solely on their impact on protein structure.

A tumor focused approach to resolving the etiology of DNA mismatch repair deficient tumors classified as suspected Lynch syndrome.

Routine screening of tumors for DNA mismatch repair (MMR) deficiency (dMMR) in colorectal (CRC), endometrial (EC) and sebaceous skin (SST) tumors leads to a significant proportion of unresolved cases classified as suspected Lynch syndrome (SLS). SLS cases (n = 135) were recruited from Family Cancer Clinics across Australia and New Zealand. Targeted panel sequencing was performed on tumor (n = 137; 80×CRCs, 33×ECs and 24xSSTs) and matched blood-derived DNA to assess for microsatellite instability status, tumor mutation burden, COSMIC tumor mutational signatures and to identify germline and somatic MMR gene variants. MMR immunohistochemistry (IHC) and MLH1 promoter methylation were repeated. In total, 86.9% of the 137 SLS tumors could be resolved into established subtypes. For 22.6% of these resolved SLS cases, primary MLH1 epimutations (2.2%) as well as previously undetected germline MMR pathogenic variants (1.5%), tumor MLH1 methylation (13.1%) or false positive dMMR IHC (5.8%) results were identified. Double somatic MMR gene mutations were the major cause of dMMR identified across each tumor type (73.9% of resolved cases, 64.2% overall, 70% of CRC, 45.5% of ECs and 70.8% of SSTs). The unresolved SLS tumors (13.1%) comprised tumors with only a single somatic (7.3%) or no somatic (5.8%) MMR gene mutations. A tumor-focused testing approach reclassified 86.9% of SLS into Lynch syndrome, sporadic dMMR or MMR-proficient cases. These findings support the incorporation of tumor sequencing and alternate MLH1 methylation assays into clinical diagnostics to reduce the number of SLS patients and provide more appropriate surveillance and screening recommendations.

MLH1-methylated endometrial cancer under 60 years of age as the "sentinel" cancer in female carriers of high-risk constitutional MLH1 epimutation.

OBJECTIVE: Universal screening of endometrial carcinoma (EC) for mismatch repair deficiency (MMRd) and Lynch syndrome uses presence of MLH1 methylation to omit common sporadic cases from follow-up germline testing. However, this overlooks rare cases with high-risk constitutional MLH1 methylation (epimutation), a poorly-recognized mechanism that predisposes to Lynch-type cancers with MLH1 methylation. We aimed to determine the role and frequency of constitutional MLH1 methylation among EC cases with MMRd, MLH1-methylated tumors. METHODS: We screened blood for constitutional MLH1 methylation using pyrosequencing and real-time methylation-specific PCR in patients with MMRd, MLH1-methylated EC ascertained from (i) cancer clinics (n = 4, <60 years), and (ii) two population-based cohorts; "Columbus-area" (n = 68, all ages) and "Ohio Colorectal Cancer Prevention Initiative (OCCPI)" (n = 24, <60 years). RESULTS: Constitutional MLH1 methylation was identified in three out of four patients diagnosed between 36 and 59 years from cancer clinics. Two had mono-/hemiallelic epimutation (∼50% alleles methylated). One with multiple primaries had low-level mosaicism in normal tissues and somatic "second-hits" affecting the unmethylated allele in all tumors, demonstrating causation. In the population-based cohorts, all 68 cases from the Columbus-area cohort were negative and low-level mosaic constitutional MLH1 methylation was identified in one patient aged 36 years out of 24 from the OCCPI cohort, representing one of six (∼17%) patients <50 years and one of 45 patients (∼2%) <60 years in the combined cohorts. EC was the first/dual-first cancer in three patients with underlying constitutional MLH1 methylation. CONCLUSIONS: A correct diagnosis at first presentation of cancer is important as it will significantly alter clinical management. Screening for constitutional MLH1 methylation is warranted in patients with early-onset EC or synchronous/metachronous tumors (any age) displaying MLH1 methylation.

Differential outcomes and immune checkpoint inhibitor response among endometrial cancer patients with MLH1 hypermethylation versus MLH1 "Lynch-like" mismatch repair gene mutation.

OBJECTIVES: To identify differential survival outcomes and immune checkpoint inhibitor (ICI) response in MLH1 hypermethylated versus MLH1 mutated ("Lynch-like") endometrial tumors and determine whether their molecular profiles can elucidate the differential outcomes. METHODS: 1673 mismatch repair deficient endometrial tumors were analyzed by next-generation sequencing and whole transcriptome sequencing (Caris Life Sciences, Phoenix, AZ). PD-L1, ER, and PR were tested by immunohistochemistry and immune cell infiltrates were calculated using MCP-counter. Significance was determined using Chi-square and Mann-Whitney U tests and adjusted for multiple comparisons. Overall survival (OS) was depicted using Kaplan-Meier survival curves. RESULTS: The endometrial cancer cohort comprised 89.2% patients with MLH1 hypermethylated tumors and 10.8% with MLH1 mutated tumors, with median ages of 67 and 60 years, respectively (p < 0.01). Patients with MLH1 hypermethylated tumors had significantly worse OS and trended toward worse OS following ICI treatment than patients with MLH1 mutated tumors. The immune microenvironment of MLH1 hypermethylated relative to MLH1 mutated was characterized by decreased PD-L1 positivity, immune checkpoint gene expression, immune cell infiltration, T cell inflamed scores, and interferon gamma (IFNγ) scores. MLH1 hypermethylation was also associated with decreased mutation rates in TP53 and DNA damage repair genes, but increased rates of JAK1, FGFR2, CCND1, and PTEN mutations, as well as increased ER and PR positivity. CONCLUSIONS: Endometrial cancer patients with MLH1 hypermethylation display significantly decreased survival and discrepant immunotherapy responses compared to patients with MLH1 mutated tumors, which was associated with differential mutational profiles, a more immune cold phenotype, and increased ER/PR expression in MLH1 hypermethylated tumors. Providers may consider early transition from single agent ICI to a multi-agent regimen or hormonal therapy for patients with MLH1 hypermethylated tumors.

Differential operation of MLH1/MSH2 and FANCD2 crosstalk in chemotolerant bladder carcinoma: a clinical and therapeutic intervening study.

We aimed to understand the crosstalk between mismatch repair (MMR) and FA-BRCA pathway in primary bladder carcinoma (BlCa) samples as well as in chemotolerant cell line. We analysed the genetic alterations of MLH1 and MSH2 (MMR-related genes) and after that we correlated it with the nuclear translocation of FANCD2 protein. Next, we evaluated this crosstalk in T24 BlCa cell line in response to doxorubicin treatment. In primary BlCa tumors, infrequent genetic deletion (17-20%) but frequent promoter methylation (28-55%) of MLH1 and MSH2 was observed, where MLH1 was significantly (p < 0.05) more methylated among the early staged samples (NMIBC). However, MSH2 was significantly more altered among the NMIBC samples, signifying the importance of MMR pathway during the early pathogenesis of the disease. Furthermore, BlCa samples with underexpressed MLH1/MSH2 protein possessed cytoplasmic FANCD2 protein; encouraging that inefficiency of MMR proteins might restrict FANCD2 nuclear translocation. Next, we analysed publicly available data in GEO2R tool where we observed that in response to chemotherapeutic drugs, expression of MLH1, MSH2 and FANCD2 were diminishing. Validating this result in doxorubicin tolerant T24 cells, we found that expression of MLH1 and MSH2 was gradually decreased with increasing dose of doxorubicin. Interestingly, FANCD2 mono-ubiquitination (L-form) was also reduced in chemotolerant T24 cells. The crosstalk between MMR and FA-BRCA pathway was substantiated in the primary BlCa tumors. Further, in response to doxorubicin, this crosstalk was found to be hampered due to under-expression of MLH1 and MSH2 gene, thereby rendering chemotolerance.

Patterns of care and outcomes of risk reducing surgery in women with pathogenic variants in non-BRCA and Lynch syndrome ovarian cancer susceptibility genes.

OBJECTIVES: Guidelines recommend risk-reducing bilateral salpingo-oophorectomy (RRSO) for women with pathogenic variants of non-BRCA and Lynch syndrome-associated ovarian cancer susceptibility genes. Optimal timing and findings at the time of RRSO for these women remains unclear. We sought to characterize practice patterns and frequency of occult gynecologic cancers for these women at our two institutions. METHODS: Women with germline ovarian cancer susceptibility gene pathogenic variants who underwent RRSO between 1/2000-9/2019 were reviewed in an IRB-approved study. All patients were asymptomatic with no suspicion for malignancy at time of RRSO. Clinico-pathologic characteristics were extracted from the medical records. RESULTS: 26 Non-BRCA (9 BRIP1, 9 RAD51C, and 8 RAD51D) and 75 Lynch (36 MLH1, 18 MSH2, 21 MSH6) pathogenic variants carriers were identified. Median age at time of RRSO was 47. There were no occurrences of occult ovarian or fallopian tube cancer in either group. Two patients (3%) in the Lynch group had occult endometrial cancer. Median follow up was 18 and 35 months for non-BRCA and Lynch patients, respectively. No patient developed primary peritoneal cancer upon follow up. Post-surgical complications occurred in 9/101 (9%) of patients. Hormone replacement therapy (HRT) was rarely used despite reported post-menopausal symptoms in 6/25 (23%) and 7/75 (37%) patients, respectively. CONCLUSIONS: No occult ovarian or tubal cancers were observed in either group. No recurrent or primary gynecologic-related cancers occurred upon follow-up. Despite frequent menopausal symptoms, HRT use was rare. Both groups experienced surgical complications when hysterectomy and/or concurrent colon surgery was performed suggesting concurrent surgeries should only be performed when indicated.

Hormone Receptor-Positive Breast Cancer Sensitive to Pembrolizumab: Evidence of the Pathogenicity of the MLH1 Variant 1835del3.

A woman with estrogen/progesterone receptor-positive, ERBB2-negative metastatic breast cancer developed progressive disease despite treatment with multiple hormonal and chemotherapeutic modalities. She carried a germline variant of MLH1 (1835del3), also known as c.1835_1837del and v612del, the pathogenicity of which has not been conclusively determined. MLH1 staining was not seen on immunohistochemical staining of her tumor tissue. The patient experienced a >5-year dramatic response to 4 doses of pembrolizumab. Family studies revealed multiple other relatives with the MLH1 1835del3 variant, as well as multiple relatives with colon cancer. The one relative with colon cancer who underwent genetic testing demonstrated the same variant. Laboratory studies revealed that the patient's tumor showed loss of heterozygosity (LOH) in the MLH1 region, high levels of microsatellite instability, and a high tumor mutational burden. LOH in the MLH1 region, along with the remarkable clinical response to pembrolizumab treatment and the presence of the same MLH1 variant in affected relatives, supports the hypothesis that the MLH1 1835del3 variant is pathogenic. Given the patient's family history, this likely represents an uncommon presentation of Lynch syndrome. Physicians should be alert to evaluate patients for targetable genetic variants even in unlikely clinical situations such as the one described here.

Constitutional MLH1 Methylation Is a Major Contributor to Mismatch Repair-Deficient, MLH1-Methylated Colorectal Cancer in Patients Aged 55 Years and Younger.

BACKGROUND: Most mismatch repair-deficient (MMRd) colorectal cancer (CRC) cases arise sporadically, associated with somatic MLH1 methylation, whereas approximately 20% have germline mismatch repair pathogenic variants causing Lynch syndrome (LS). Universal screening of incident CRC uses presence of MLH1 methylation in MMRd tumors to exclude sporadic cases from germline testing for LS. However, this overlooks rare cases with constitutional MLH1 methylation (epimutation), a poorly recognized mechanism for LS. We aimed to assess the frequency and age distribution of constitutional MLH1 methylation among incident CRC cases with MMRd, MLH1-methylated tumors. METHODS: In retrospective population-based studies, we selected all CRC cases with MMRd, MLH1-methylated tumors, regardless of age, prior cancer, family history, or BRAF V600E status, from the Columbus-area HNPCC study (Columbus) and Ohio Colorectal Cancer Prevention Initiative (OCCPI) cohorts. Blood DNA was tested for constitutional MLH1 methylation by pyrosequencing and real-time methylation-specific PCR, then confirmed with bisulfite-sequencing. RESULTS: Results were achieved for 95 of 98 Columbus cases and all 281 OCCPI cases. Constitutional MLH1 methylation was identified in 4 of 95 (4%) Columbus cases, ages 34, 38, 52, and 74 years, and 4 of 281 (1.4%) OCCPI cases, ages 20, 34, 50, and 55 years, with 3 showing low-level mosaic methylation. Mosaicism in blood and normal colon, plus tumor loss of heterozygosity of the unmethylated allele, demonstrated causality in 1 case with sample availability. Age stratification showed high rates of constitutional MLH1 methylation among younger patients. In the Columbus and OCCPI cohorts, respectively, these rates were 67% (2 of 3) and 25% (2 of 8) of patients aged <50 years but with half of the cases missed, and 75% (3 of 4) and 23.5% (4 of 17) of patients aged ≤55 years with most cases detected. CONCLUSIONS: Although rare overall, a significant proportion of younger patients with MLH1-methylated CRC had underlying constitutional MLH1 methylation. Routine testing for this high-risk mechanism is warranted in patients aged ≤55 years for a timely and accurate molecular diagnosis that will significantly alter their clinical management while minimizing additional testing.

Amplicon-based NGS test for assessing MLH1 promoter methylation and its correlation with BRAF mutation in colorectal cancer patients.

Detecting MLH1 promoter methylation is highly relevant to differentiate between possible Lynch syndrome patients or patients with sporadic causes of MLH1/PMS2 deficiency in colorectal (CRC) and endometrial cancers. Here, we aimed to develop a test for assessing MLH1 promoter methylation based in next generation sequencing (NGS), and to evaluate the concordance of MLH1 methylation and BRAF-V600 mutation status in CRC. For that, we performed a series of experiments with DNA from tumor, saliva and commercial control samples and our in house developed amplicon-based NGS test. In patients' samples, MLH1 methylation above 10% was only observed in tumors with MLH1/PMS2 loss. We confirmed the reproducibility and accuracy of MLH1 promoter analysis performing a serial dilution experiment with completely methylated and unmethylated control DNAs and a comparison between two NGS platforms (Ion Proton and Illumina). In MLH1/PMS2 deficient tumors, the MLH1 methylation status was concordant with the BRAF mutation status in 90% (18/20) of the cases. Our amplicon-based NGS test showed a great sensitivity and specificity for detecting MLH1 methylation in CRC samples, with a high agreement with the evaluation of BRAF mutation. This simple and affordable test could be used as a reflex test to identify patients with sporadic causes of MLH1/PMS2 deficiency in CRC, aiding to genetic test referral and identification of Lynch syndrome patients.

The Control of the Crossover Localization in Allium.

Meiotic crossovers/chiasmata are not randomly distributed and strictly controlled. The mechanisms behind crossover (CO) patterning remain largely unknown. In Allium cepa, as in the vast majority of plants and animals, COs predominantly occur in the distal 2/3 of the chromosome arm, while in Allium fistulosum they are strictly localized in the proximal region. We investigated the factors that may contribute to the pattern of COs in A. cepa, A. fistulosum and their F1 diploid (2n = 2x = 8C + 8F) and F1 triploid (2n = 3x = 16F + 8C) hybrids. The genome structure of F1 hybrids was confirmed using genomic in situ hybridization (GISH). The analysis of bivalents in the pollen mother cells (PMCs) of the F1 triploid hybrid showed a significant shift in the localization of COs to the distal and interstitial regions. In F1 diploid hybrid, the COs localization was predominantly the same as that of the A. cepa parent. We found no differences in the assembly and disassembly of ASY1 and ZYP1 in PMCs between A. cepa and A. fistulosum, while F1 diploid hybrid showed a delay in chromosome pairing and a partial absence of synapsis in paired chromosomes. Immunolabeling of MLH1 (class I COs) and MUS81 (class II COs) proteins showed a significant difference in the class I/II CO ratio between A. fistulosum (50%:50%) and A. cepa (73%:27%). The MLH1:MUS81 ratio at the homeologous synapsis of F1 diploid hybrid (70%:30%) was the most similar to that of the A. cepa parent. F1 triploid hybrid at the A. fistulosum homologous synapsis showed a significant increase in MLH1:MUS81 ratio (60%:40%) compared to the A. fistulosum parent. The results suggest possible genetic control of CO localization. Other factors affecting the distribution of COs are discussed.