High level of microsatellite instability but not hypermethylation of mismatch repair genes in therapy-related and secondary acute myeloid leukaemia and myelodysplastic syndrome Mohammad H. Sheikhha, Khalid Tobal and John A. Liu Yin 1 Molecular Oncology Group, University Department of Haematology, Manchester Royal infirmary, Manchester, UK Received 20 August 2001; accepted for publication 4 December 2001 Summary. Microsatellite instability (MSI) is associated with defects in the DNA mismatch repair (MMR) system, such as mutation or epigenetic silencing of the genes by promoter hypermethylation. We investigated the presence of MSI and promoter hypermethylation of hMLH1 and hMSH2 genes in 82 patients (68 acute myeloid leukaemia, AML; 14 myelo- dysplastic syndromes, MDS). Twelve separate microsatellite loci, including three mononucleotide repeat markers, were used. Mutator phenotype (RER+) was detected in 20 AML (29Æ4%) and 3 MDS (21Æ4%) patients. RER+ rate was much higher in the therapy-related and secondary cases compared with the de novo cases. Three out of 7 (42Æ9%) secondary (s-AML) and 8 out of 17 (47Æ1%) therapy-related (t-AML) showed RER+ in comparison with 9 out of 44 (20Æ5%) de novo cases. Similar rates were detected in MDS patients (2/2 therapy-related and 1/12 de novo). The promoter hypermethylation was found in three hMLH1 (3Æ7%) and two hMSH2 (2Æ4%) genes. All these five patients had AML and were older than 60 years of age. Two of them had s-AML and one had t-AML. RER+ was detected in three of these five patients. Our data suggest that genetic instability is associated with AML and MDS, especially t-AML and s-AML. In addition, our results indicate that the hMSH2 and hMLH1 promoter hypermethylation is not a common event in these malignancies, but may play a role in the develop- ment of AML in elderly patients. Keywords: microsatellite instability, mismatch repair genes, hypermethylation, AML, MDS. DNA mismatch repair genes (MMR) play important roles in genomic stability, including correction of chromosomal errors associated with DNA replication and recombination. Malfunction of the MMR system results in a mutator phenotype, which is characterized by microsatellite insta- bility (MSI). Microsatellites are highly polymorphic repeat- ing units of 1–6 bp in eukaryotic genomes (Weber & May, 1989). MSI occurs as a result of insertion/deletion in these simple repeated sequences and has been associated with genetic abnormalities of the MMR genes (Zhu et al, 1999; Shin & Park, 2000). MSI in various human tumours has been shown to be caused by abnormalities such as germline mutations or hypermethylation of the promoter region in MMR genes, principally hMSH2 and hMLH1 (Kane et al, 1997; Cun- ningham et al, 1998; Deng et al, 1999; Bevilacqua & Simpson, 2000). Recently, hypermethylation of CpG-rich areas located in the promoter of genes (CpG islands) has been shown to be involved in silencing tumour suppressor genes in human malignancies such as acute myeloid leukaemia (AML) (Wong et al, 2000). In addition, abnormal expression and mutation of hMSH2 was shown in AML patients (Horiike et al, 1999; Zhu et al, 1999). In haema- tological malignancies, MSI have been observed in a subset of myeloid and lymphoid malignancies (Wada et al, 1994; Kaneko et al, 1995; Roblrdo et al, 1995; Takeuchi et al, 1997; Inoue et al, 2000). The rates of MSI reported in different studies have been highly variable. Ben-Yehuda et al (1996) have reported a very high rate in therapy- related (t-) leukaemia and t-MDS (94%), while Das-Gupta et al (2001) reported MSI in 44% of t-AML and, in other studies, a low rate or no MSI was found in de novo AML (0%) and secondary AML (s-AML) (1Æ6%) (Tasaka et al, 1996; Rimsza et al, 2000). These results indicate the need for further investigation of MSI in AML. The overall objectives of our study were: (1) to determine the frequency of mutator phenotype (RER+) in different types of AML and MDS Correspondence: K. Tobal, Molecular Oncology Group, University Department of Haematology, Manchester Royal infirmary, Cobbett House, Oxford Road, Manchester, M13 9WL, UK. E-mail: ktobal@labmed.cmht.nwest.nhs.uk British Journal of Haematology, 2002, 117, 359–365 Ó 2002 Blackwell Science Ltd 359