Coexistence of Constitutional Mismatch Repair Deficiency syndrome and Lynch syndrome in a family of seven : MSH6 mutation and childhood colorectal cancer - a case series.

Purpose: To present a case series of two fraternal twin girls who passed away from brain and colorectal cancers attributed to Constitutional Mismatch Repair Deficiency syndrome (CMMRD). A review of literature for CMMRD-related pediatric malignancies is also presented.

Methods: The two girls were diagnosed with cancer at the age of 11 and 13 respectively. The early onset of multiple malignancies in the family raised clinical suspicion for a potential genetic mutation. The presence of café-au-lait spots at clinical examination led to further investigations for neurofibromatosis.

Results: Neurofibromatosis type 1 testing was negative in both children. Genetic analysis turned out positive for biallelic MSH6 mutations in the two girls, leading to CMMRD syndrome diagnosis. Both parents and two out of three alive siblings were diagnosed with Lynch syndrome.

Conclusions: Colorectal cancer is a very rare finding in childhood and should raise suspicion for CMMRD syndrome and should be followed by regular screening.