Repetitive DNA Hypomethylation in Multiple Myeloma

Multiple myeloma (MM) is a malignant proliferation of bone marrow plasma cells characterized by a wide spectrum of genetic and epigenetic changes. Global hypomethylation of repetitive genomic sequences such as long interspersed nuclear elements-1 (LINE-1) and Alu repetitive elements (approximately 500.000 and 1.4 million in the human genome) has been associated with chromosomal instability. Additionally, satellite alfa DNA (SAT-alpha DNA) hypomethylation has been reported to be associated to karyotypic instability in human cancer, possibly playing a role in centromere function. So far, the LINE-1/Alu and centromeric SAT-alpha DNA methylation patterns have not been investigated in the context of the different clinical and molecular MM subtypes. Global DNA methylation changes were investigated in a panel of 53 newly diagnosed, untreated MMs, 7 plasma cell leukemias (PCL) and 11 healthy subjects as controls. DNA was extracted from purified plasma cells, treated with bisulfite and analyzed by bisulfite-PCR and Pyrosequencing. Methylation of LINE-1 and Alu elements was shown to correlate with total 5mC content and thus used to estimate global DNA methylation. MMs showed a decrease of Alu (21.1%) and LINE-1 (70.0%) methylation average levels compared with controls (25.2% and 79.5% respectively). Lower median methylation levels were also found in centromeric SAT-alpha DNA of MMs (77.95%) compared to controls (89.5%). The median methylation level of PCLs was lower than MMs (16.7% versus 21.1% for Alu; 45.5% versus 70.0% for LINE-1; and 33.3% versus 77.9% for SATalpha DNA). Notably, a statistically significant association between SAT-alpha DNA and LINE-1 methylation (Spearman’s rank correlation, ρ = 0.94; P < 0.001) was found in MM. The comparison between methylation pattern and different molecular MM subgroups by means of non parametric tests, revealed that LINE-1 and SAT-alpha DNA methylation was significantly lower in the nonhyperdiploid versus hyperdiploid (HD) tumors (P = 0.01 and 0.02 respectively). Alu and SAT-alpha were significantly lower in the MMs with t(4;14) (P = 0.02 and 0.004 respectively). Finally, in the context of translocation/cyclin D (TC) classification, a statistically significant differences inside the five different groups were found for SAT-alpha DNA methylation (P = 0.008, Kruskal-Wallis test). These findings may provide insights into the molecular mechanisms of MM pathogenesis and suggest that our approach may contribute toward a more exhaustive stratification of the disease.