Mismatch repair and the downstream target genes, PAX5 and Ikaros, in childhood acute lymphoblastic leukemia

The mismatch repair (MMR) pathway is a post-replicative DNA repair process and MMR deficiency is a common feature of ALL cell lines. In this study we have investigated MMR deficiency in a large cohort of primary relapsed ALL (n = 40) and investigated coding microsatellites (MS) of the lymphoid transcription factors, PAX5 and IKZF1 as downstream target genes. Only one patient showed MMR deficiency, as evidenced by microsatellite instability, which was acquired at relapse and was associated with reduced expression of both MLH1 and MSH2. Coding MS in candidate target genes including PAX5, IKZF1, BAX and TGFBRII were all wild type in this patient but the MMR-deficient cell line REH, was confirmed to have a coding MS in both PAX5 and TGFBRII. Whilst MMR deficiency is not highly prevalent in primary ALL, optimisation of the drug regimen to omit/replace thioguanines should be considered for children with MMR deficiency and/or reduced expression of key pathway components.