Defects of the mismatch repair gene MSH2 are implicated in the development of murine and human lymphoblastic lymphomas and are associated with the aberrant expression of rhombotin-2 (Lmo-2) and Tal-1 (SCL).

Mutations in the DNA mismatch repair (MMR) gene hMSH2 underlie a novel pathway of tumorigenesis for some cancers of epithelial origin. Mice deficient in MSH2 are susceptible to lymphomas but defects in this gene have not been identified in human lymphoid tumors. To determine if the lymphomas these mice develop are related to a particular subtype of human lymphoma we evaluated 20 clinically ill homozygous MSH2-/- mice ranging in age from 2 to 13 months. The murine tumors comprised a single histopathologic entity representing the malignant counterpart of precursor thymic T cells and closely resembled human precursor T-cell lymphoblastic lymphoma (LBL). Evaluation of the expression of three T-cell malignancy associated genes showed that Rhombotin-2 (RBTN-2 also known as Lmo-2), TAL-1 (also known as SCL), and HOX-11 were expressed in 100%, 40%, and 0% of the murine tumors, respectively. The MSH2-/- murine model of precursor T-cell LBL was substantiated by the finding of a nearly identical expression profile of RBTN-2, TAL-1, and HOX-11 in 10 well-characterized cases of human LBL. Direct evidence for MSH2 abnormalities in human LBL was established by sequence analysis of exon 13 of hMSH2, which revealed coding region mutations in 2 of 10 cases. Our findings implicate defects in the MMR system with the aberrant expression of T-cell specific proto-oncogenes and define a new pathway of human lymphomagenesis.