Lynch Syndrome Screening
Cascade testing for Lynch Syndrome: Analysis of MLH1 promoter hypermethylation by pyrosequencing
Lynch syndrome (LS) is an inherited disorder involving germ-line mutation of DNA mismatch repair genes. The mismatch repair system involves a series of proteins including MSH2, MLH1, PMS2 and MSH6, which maintain DNA integrity. A germ-line mutation in any of these genes therefore increases the risk of a number of cancers, primarily colorectal cancers (50-70% lifetime risk), and endometrial cancers (40-60% lifetime risk) (Sehgal et al, 2014). As the age of cancer onset is younger than expected for sporadic cancers, diagnosis of LS families has relied on age and family history in the past. However, lack of reliability using this approach has led to an improvement in NICE guide-lines to detect LS families, and the current diagnostic strategy in Leeds is out-lined below (Table i).
Table i. NICE guidelines for Lynch syndrome diagnostic screening when immunohistochemistry 1-panel testing is available. From NICE guidelines, 2017 (www.nice.org.uk).
MLH1 loss (visualised by immunohistochemistry performed in Histopathology), is observed in mismatch repair deficient colorectal tumours from LS patients. However, it is also observed in a significant minority of sporadic colorectal tumours resulting from somatic hypermethylation of the MLH1 promoter. Given that MLH1 promoter hypermethylation is rarely seen in LS cases harbouring MLH1 deficient tumours, this test can be used to discriminate these two possibilities. In cases where MLH1 loss is likely to have arisen from a germ-line mutation, (i.e. MLH1 protein loss shown not to result from MLH1 promoter hypermethylation), families can then undergo further constitutional genetic screening of mismatch repair genes.
Our laboratory now offer MLH1 promoter hypermethylation testing by pyrosequencing to MLH1 deficient, BRAF wild-type, colorectal cancer referrals as requested.
Please note, this is a routine test, currently not accredited by UKAS, with variable turn-around times as samples are batched, typically taking between 2 to 4 weeks.
