Mismatch repair gene deficiency and genetic anticipation in Lynch syndrome: myth or reality?

Anticipation in families with MLH1-associated Lynch syndrome

BACKGROUND

Lynch syndrome (LS) is an autosomal-dominant, hereditary cancer predisposition syndrome caused by pathogenic variants (PVs) in one of the mismatch-repair genes MLH1, MSH2/EPCAM, MSH6, or PMS2. Individuals who have MLH1 PVs have high lifetime risks of colorectal cancer (CRC) and endometrial cancer (EC). There is controversy regarding whether a younger age at diagnosis (or anticipation) occurs in MLH1-associated LS. The objective of this study was to assess anticipation in families with MLH1-associated LS by using statistical models while controlling for potential confounders.

METHODS

Data from 31 families with MLH1 PVs were obtained from an academic registry. Wilcoxon signed-rank tests on parent-child-pairs as well as parametric Weibull and semiparametric Cox proportional hazards and Cox mixed-effects models were used to calculate hazard ratios or to compare mean ages at CRC/EC diagnosis by generation. Models were also corrected for ascertainment bias and birth-cohort effects.

RESULTS

A trend toward younger ages at diagnosis of CRC/EC in successive generations, ranging from 3.2 to 15.7 years, was observed in MLH1 PV carrier families. A greater hazard for cancer in younger generations was not precluded by the inclusion of birth cohorts in the model. Individuals who had MLH1 variants with no Mlh1 activity were at a 78% greater hazard for CRC/EC than those who retained Mlh1 activity.

CONCLUSIONS

The current results demonstrated evidence in support of anticipation in families with MLH1-associated LS across all statistical models. Mutational effects on Mlh1 activity influenced the hazard for CRC/EC. Screening based on the youngest age of cancer diagnosis in MLH1-LS families is recommended.

Telomere Instability in Lynch Syndrome Families Leads to Some Shorter Telomeres in MSH2+/- Carriers

Lynch syndrome (LS) is an inherited predisposition to early onset of various cancers, caused by mutation in a DNA mismatch repair (MMR) gene. In heterozygous MMR^+/− carriers, somatic mutation, loss or silencing of the wild type allele increases the mutation rate, facilitating the initiation of MMR-defective cancers. These cancers are characterized by instability at short tandem repeats (STRs) and in telomeric DNA. We have investigated telomere length in saliva DNA from LS and control families, using single telomere analysis at XpYp and 12q and by qPCR to measure total telomeric DNA. Single telomere analysis showed a trend for shorter XpYp telomeres in MSH2^+/− carriers compared to MLH1^+/− carriers or controls, but this was masked in the comparative analysis of total telomeric DNA. Comparison of age-adjusted telomere length within families showed that neither MSH2^+/− or MLH1^+/− children had consistently shorter or longer telomeres than their MMR^+/− parent, indicating the absence of an inter-generational effect on telomere length. Unexpectedly however, wildtype children in families with MSH2 mutations, had significantly longer XpYp telomeres than their MMR^+/− parent. Altogether our data suggest that MMR insufficiency, particularly in MSH2^+/− carriers, increases telomere instability and somatic cell turnover during the lifetime of LS mutation carriers but has minimal consequences for telomere length in the germline.

The Apparent Genetic Anticipation in PMS2-Associated Lynch Syndrome Families Is Explained by Birth-cohort Effect

BACKGROUND

PMS2-associated Lynch syndrome is characterized by a relatively low colorectal cancer penetrance compared with other Lynch syndromes. However, age at colorectal cancer diagnosis varies widely, and a strong genetic anticipation effect has been suggested for PMS2 families. In this study, we examined proposed genetic anticipation in a sample of 152 European PMS2 families.

METHODS

The 152 families (637 family members) that were eligible for analysis were mainly clinically ascertained via clinical genetics centers. We used weighted Cox-type random effects model, adjusted by birth cohort and sex, to estimate the generational effect on the age of onset of colorectal cancer. Probands and young birth cohorts were excluded from the analyses. Weights represented mutation probabilities based on kinship coefficients, thus avoiding testing bias.

RESULTS

Family data across three generations, including 123 colorectal cancers, were analyzed. When compared with the first generation, the crude HR for anticipation was 2.242 [95% confidence interval (CI), 1.162-4.328] for the second generation and 2.644 (95% CI, 1.082-6.464) for the third generation. However, after correction for birth cohort and sex, the effect vanished [HR = 1.302 (95% CI, 0.648-2.619) and HR = 1.074 (95% CI, 0.406-2.842) for second and third generations, respectively].

CONCLUSIONS

Our study did not confirm previous reports of genetic anticipation in PMS2-associated Lynch syndrome. Birth-cohort effect seems the most likely explanation for observed younger colorectal cancer diagnosis in subsequent generations, particularly because there is currently no commonly accepted biological mechanism that could explain genetic anticipation in Lynch syndrome.

IMPACT

This new model for studying genetic anticipation provides a standard for rigorous analysis of families with dominantly inherited cancer predisposition.

Genetic anticipation in Swedish Lynch syndrome families

Among hereditary colorectal cancer predisposing syndromes, Lynch syndrome (LS) caused by mutations in DNA mismatch repair genes MLH1, MSH2, MSH6 or PMS2 is the most common. Patients with LS have an increased risk of early onset colon and endometrial cancer, but also other tumors that generally have an earlier onset compared to the general population. However, age at first primary cancer varies within families and genetic anticipation, i.e. decreasing age at onset in successive generations, has been suggested in LS. Anticipation is a well-known phenomenon in e.g neurodegenerative diseases and several reports have studied anticipation in heritable cancer. The purpose of this study is to determine whether anticipation can be shown in a nationwide cohort of Swedish LS families referred to the regional departments of clinical genetics in Lund, Stockholm, Linköping, Uppsala and Umeå between the years 1990–2013. We analyzed a homogenous group of mutation carriers, utilizing information from both affected and non-affected family members. In total, 239 families with a mismatch repair gene mutation (96 MLH1 families, 90 MSH2 families including one family with an EPCAM–MSH2 deletion, 39 MSH6 families, 12 PMS2 families, and 2 MLH1+PMS2 families) comprising 1028 at-risk carriers were identified among the Swedish LS families, of which 1003 mutation carriers had available follow-up information and could be included in the study. Using a normal random effects model (NREM) we estimate a 2.1 year decrease in age of diagnosis per generation. An alternative analysis using a mixed-effects Cox proportional hazards model (COX-R) estimates a hazard ratio of exp(0.171), or about 1.19, for age of diagnosis between consecutive generations. LS-associated gene-specific anticipation effects are evident for MSH2 (2.6 years/generation for NREM and hazard ratio of 1.33 for COX-R) and PMS2 (7.3 years/generation and hazard ratio of 1.86). The estimated anticipation effects for MLH1 and MSH6 are smaller.

Genetic anticipation is a phenomenon where symptoms of a hereditary disease appear at an earlier age and/or are more severe in successive generations. In genetic disorders such as Fragile X syndrome, Myotonic dystrophy type 1 and Huntington disease, anticipation is caused by the expansion of unstable trinucleotide repeats during meiosis. Anticipation is also reported to occur in some hereditary cancers though the underlying mechanism behind this observation is unknown. Several studies have investigated anticipation in Lynch syndrome, the most common hereditary colorectal cancer syndrome, yet there is a debate concerning whether anticipation occurs and what underlying mechanism there is. The objective of this project is to study if anticipation is part of the clinical picture in Swedish families with LS, with the long term goal to enable better prediction of age at onset in family members. Our results suggest that anticipation occurs in families with mutation in MSH2 and PMS2, while the evidence is equivocal for MLH1 and MSH6.

Segregation of a novel MLH1 mutation in an Iranian Lynch syndrome family

Lynch syndrome is an autosomal dominant disorder caused by germline mutations in mismatch repair (MMR) genes. The main feature of this disorder is an early onset of hereditary colorectal cancer in addition to other cancers arising from different tissues. Here, we report an Iranian family with several members affected with Lynch syndrome related cancers. Exome sequencing with focus on 14 genes related with hereditary colorectal cancer has shown a novel mutation in exon 19 of MLH1 gene (c.2133delC, p.Trp712Gly fs*71). This mutation is located in a region coding for the functional domain for the interaction with MLH3/PMS1/PMS2. As some clinical aspects of the disorder have been shown to be associated with certain mutations, identification of causative mutation in each family has implications for surveillance protocols.