Constitutional mismatch repair-deficiency syndrome.

The mismatch repair (MMR) machinery contributes to genome integrity and the MLH1, MSH2, MSH6 and PMS2 genes play a crucial role in this process. MMR corrects single base-pair mismatches and small insertion-deletion loops that arise during replication. Moreover, the MMR system is involved in the cellular response to a variety of agents that damage DNA and in immunoglobulin class switch recombination. Hetero zygous germline mutations in MLH1, MSH2, MSH6 and PMS2 cause Lynch syndrome (LS), an autosomal dominant cancer syndrome associated with hereditary nonpolyposis colorectal cancer (HNPCC), endometrium carcinoma and other malignancies, occurring on average in the fourth and fifth decade of life. Notably, LS associated tumors display somatic loss of the remaining wild type MLH1, MSH2, MSH6 or PMS2 allele and evidence of microsatellite instability (for review see ). In contrast to individuals with LS who harbor a heterozygous mutant MMR gene allele, rare cases with biallelic deleterious germline mutations in MMR genes leading to constitutional mismatch repair-deficiency (CMMRD) have been recognized since 1999. This cancer syndrome is characterized by a broad spectrum of early-onset malignancies and a phenotype that resembles neurofibromatosis type 1. In this issue of Haematologica, Ripperger and colleagues report on a patient with CMMR-D caused by a novel MSH6 mutation leading to a T-cell lymphoma and colonic adenocarcinoma at six and 13 years of age, respectively. In addition, they review 26 leukemia and/or lymphoma cases reported previously.