Long anticipation complicates identifying Lynch syndrome in monozygotic twins

Anticipation is a term used to express an earlier age of disease onset in successive generations. Patients with Lynch syndrome (LS) harbor mutations in MMR genes that appear to be associated with the phenomenon where disease is diagnosed 5–12 years earlier in mutation carrying children compared with their affected parent. This has the potential to complicate the recognition of LS as defined by the Amsterdam criteria (or iterations of it) in young probands affected by colorectal cancer. In this report, we describe a case of an LS family with an MLH1 mutation (c.1748 del T). The phenomenon of anticipation was observed in the son of a monozygotic twin who developed two colon cancers that were diagnosed at an age 20 years earlier than the first diagnosis of cancer in his mother. Both the mother and her twin went on to simultaneously develop a rectal carcinoma.