Response to Roberts et al. 2018: is breast cancer truly caused by MSH6 and PMS2 variants or is it simply due to a high prevalence of these variants in the population?

Mismatch repair protein MLH1 suppresses replicative stress in BRCA2-deficient breast tumors

Loss of BRCA2 (breast cancer 2) is lethal for normal cells. Yet it remains poorly understood how, in BRCA2 mutation carriers, cells undergoing loss of heterozygosity overcome the lethality and undergo tissue-specific neoplastic transformation. Here, we identified mismatch repair gene mutL homolog 1 (MLH1) as a genetic interactor of BRCA2 whose overexpression supports the viability of Brca2-null cells. Mechanistically, we showed that MLH1 interacts with Flap endonuclease 1 (FEN1) and competes to process the RNA flaps of Okazaki fragments. Together, they restrained the DNA2 nuclease activity on the reversed forks of lagging strands, leading to replication fork (RF) stability in BRCA2-deficient cells. In these cells, MLH1 also attenuated R-loops, allowing the progression of stable RFs, which suppressed genomic instability and supported cell viability. We demonstrated the significance of their genetic interaction by the lethality of Brca2-mutant mice and inhibition of Brca2-deficient tumor growth in mice by Mlh1 loss. Furthermore, we described estrogen as inducing MLH1 expression through estrogen receptor α (ERα), which might explain why the majority of BRCA2 mutation carriers develop ER-positive breast cancer. Taken together, our findings reveal a role of MLH1 in relieving replicative stress and show how it may contribute to the establishment of BRCA2-deficient breast tumors.

MutL homolog 1 germline mutation c.(453+1_454-1)_(545+1_546-1)del identified in lynch syndrome: A case report and review of literature

BACKGROUND

Lynch syndrome (LS) is an autosomal dominant hereditary disorder because of germline mutations in DNA mismatch repair genes, such as MutL homolog 1 (MLH1), PMS1 homolog 2, MutS homolog 2, and MutS homolog 6. Gene mutations could make individuals and their families more susceptible to experiencing various malignant tumors. In Chinese, MLH1 germline mutation c.(453+1_454-1)_(545+1_546-1)del-related LS has been infrequently reported. Therefore, we report a rare LS patient with colorectal and endometrioid adenocarcinoma and describe her pedigree characteristics.

CASE SUMMARY

A 57-year-old female patient complained of irregular postmenopausal vaginal bleeding for 6 mo. She was diagnosed with LS, colonic malignancy, endometrioid adenocarcinoma, secondary fallopian tube malignancy, and intermyometrial leiomyomas. Then, she was treated by abdominal hysterectomy, bilateral oviduct oophorectomy, and sentinel lymph node resection. Genetic testing was performed using next-generation sequencing technology to detect the causative genetic mutations. Moreover, all her family members were offered a free genetic test, but no one accepted it.

CONCLUSION

No tumor relapse or metastasis was found in the patient during the 30-mo follow-up period. The genetic panel sequencing showed a novel pathogenic germline mutation in MLH1, c.(453+1_454-1)_(545+1_546-1)del, for LS. Moreover, cancer genetic counseling and testing are still in the initial development state in China, and maybe face numerous challenges in the further.

Lynch Syndrome-Associated Variants and Cancer Rates in an Ancestrally Diverse Biobank

PURPOSE

Limited data are available on the prevalence and clinical impact of Lynch syndrome (LS)-associated genomic variants in non-European ancestry populations. We identified and characterized individuals harboring LS-associated variants in the ancestrally diverse BioMe Biobank in New York City.

PATIENTS AND METHODS

Exome sequence data from 30,223 adult BioMe participants were evaluated for pathogenic, likely pathogenic, and predicted loss-of-function variants in MLH1, MSH2, MSH6, and PMS2. Survey and electronic health record data from variant-positive individuals were reviewed for personal and family cancer histories.

RESULTS

We identified 70 individuals (0.2%) harboring LS-associated variants in MLH1 (n = 12; 17%), MSH2 (n = 13; 19%), MSH6 (n = 16; 23%), and PMS2 (n = 29; 41%). The overall prevalence was 1 in 432, with higher prevalence among individuals of self-reported African ancestry (1 in 299) than among Hispanic/Latinx (1 in 654) or European (1 in 518) ancestries. Thirteen variant-positive individuals (19%) had a personal history, and 19 (27%) had a family history of an LS-related cancer. LS-related cancer rates were highest in individuals with MSH6 variants (31%) and lowest in those with PMS2 variants (7%). LS-associated variants were associated with increased risk of colorectal (odds ratio [OR], 5.0; P = .02) and endometrial (OR, 30.1; P = 8.5 × 10-9) cancers in BioMe. Only 2 variant-positive individuals (3%) had a documented diagnosis of LS.

CONCLUSION

We found a higher prevalence of LS-associated variants among individuals of African ancestry in New York City. Although cancer risk is significantly increased among variant-positive individuals, the majority do not harbor a clinical diagnosis of LS, suggesting underrecognition of this disease.

No Evidence of Increased Risk of Breast Cancer in Women With Lynch Syndrome Identified by Multigene Panel Testing

PURPOSE

Prior estimates of breast cancer risk in women with Lynch syndrome (LS) range from population risk to 18-fold increased risk with reported differences by gene. Here, breast cancer rates were determined in a large cohort of women with pathogenic variants (PVs) in a mismatch repair (MMR) gene detected through multigene panel testing and compared with rates in the US population and women undergoing panel testing.

METHODS

MMR gene PV carriers were identified among women tested for suspicion of LS or hereditary breast and ovarian cancer (HBOC) who met inclusion criteria. Standardized incidence ratios (SIRs) and 95% CIs of breast cancer were calculated compared with age-matched incidence in the general US female population and with women negative for PVs stratified by the test indication.

RESULTS

In total, 0.8% of women (30,362 of 441,966 women) carried MMR gene PVs. PVs in PMS2 (37.5%) and MSH6 (29.3%) were more common than in MLH1 (13.7%) and MSH2/EPCAM (19.4%). Women with PVs in PMS2 and MSH6 were tested more frequently for HBOC, whereas those with PVs in MLH1 and MSH2/EPCAM were tested more frequently for LS. Breast cancer rates in women with LS were lower than those in the general female population (SIR, 0.88; 95% CI, 0.81 to 0.96) and did not differ compared with women with negative panel testing for HBOC (SIR, 0.90; 95% CI, 0.82 to 0.99) or LS (SIR, 1.02; 95% CI, 0.78 to 1.30).

CONCLUSION

In this large cohort of women with LS identified through panel testing, there was no evidence for increased risk of breast cancer compared with the general US population or women undergoing panel testing. These findings support average-risk breast cancer screening in women with LS.

PMS2 germline mutation c.1577delA (p.Asp526Alafs*69)-induced Lynch syndrome-associated endometrial cancer: A case report

RATIONALE

Lynch syndrome (LS) is an autosomal dominant cancer predisposition condition caused by germline heterozygous mutations in mismatch repair (MMR) genes. However, as one of the MMR genes, PMS2 mutation-induced LS-associated endometrial cancer (LSAEC) was rarely reported.

PATIENT CONCERNS

A 26-year-old female patient suffered from prolonged menstrual period and increased menstrual flow for 2 months.

DIAGNOSES

The patient was diagnosed with cervix CIN III, endometrial cancer (EC), anemia, and LS.

INTERVENTIONS

Total hysterectomy, bilateral salpingectomy, pelvic lymphadenectomy were performed for treating EC, while ovariectomy was refused by the patient. The patient underwent postoperative chemotherapy with paclitaxel combined with carboplatin for 6 courses of treatment. Laparoscopic partial enterectomy was applied for treating colon cancer 5 years later after the surgery treatment for EC. Besides, Sanger sequencing and high-throughput genome sequencing were employed to detect the genetic status of the family that included two generations with four members. Immunohistochemistry (IHC) staining was used to identify the function of PMS2 mutation.

OUTCOMES

The 26-year-old Chinese patient suffered from LSAEC and recovered well after surgery. A PMS2 germline heterozygous mutation (c.1577delA) was confirmed by gene sequencing 5 years later. In addition, PMS2 mutation was verified by IHC. The patient was followed up for 7 years.

LESSONS

Carrying PMS2 germline mutation (c.1577delA) confers an extremely high susceptibility of suffering from LS-associated cancers. Thus, close clinical monitoring and prophylactic surgery are highly recommended to reduce the morbidity and mortality of LS-associated cancers.

PMS2 expression decrease causes severe problems in mismatch repair

PMS2 is one of the four susceptibility genes in Lynch syndrome (LS), the most common cancer syndrome in the world. Inherited mutations in DNA mismatch repair (MMR) genes, MLH1, MSH2, and MSH6, account for approximately 90% of LS, while a relatively small number of LS families segregate a PMS2 mutation. This and the low cancer penetrance in PMS2 families suggest that PMS2 is only a moderate or low‐risk susceptibility gene. We have previously shown that even a partial expression decrease in MLH1, MSH2, or MSH6 suggests that heterozygous LS mutation carriers have MMR malfunction in constitutive tissues. Whether and how PMS2 expression decrease affects the repair capability is not known. Here, we show that PMS2 knockdown cells retaining 19%, 33%, or 53% of PMS2 expression all have significantly reduced MMR efficiency. Surprisingly, the cells retaining expression levels comparable to PMS2 mutation carriers indicate the lowest repair efficiency.