Loss of 5hmC identifies a new type of aberrant DNA hypermethylation in glioma

Aberrant DNA hypermethylation is a hallmark of cancer although the underlying molecular mechanisms are still poorly understood. To study the possible role of 5-hydroxymethylcytosine (5hmC) in this process we analyzed the global and locus-specific genome-wide levels of 5hmC and 5-methylcytosine (5mC) in human primary samples from 12 non-tumoral brains and 53 gliomas. We found that the levels of 5hmC identified in non-tumoral samples were significantly reduced in gliomas. Strikingly, hypo-hydroxymethylation at 4627 (9.3%) CpG sites was associated with aberrant DNA hypermethylation and was strongly enriched in CpG island shores. The DNA regions containing these CpG sites were enriched in H3K4me2 and presented a different genuine chromatin signature to that characteristic of the genes classically aberrantly hypermethylated in cancer. As this 5mC gain is inversely correlated with loss of 5hmC and has not been identified with classical sodium bisulfite-based technologies, we conclude that our data identifies a novel 5hmC-dependent type of aberrant DNA hypermethylation in glioma.This work has been financially supported by: the Plan Nacional de I+D+I 2013–2016/FEDER (PI15/00892 to M.F.F. and A.F.F.; RTC-2015-3393-1 to A.F.F.); the ISCIII-Subdirección General de Evaluación y Fomento de la Investigación, and the Plan Nacional de I+D+I 2008–2011/FEDER (CP11/00131 to A.F.F.); IUOPA (to G.F.B. and M.S); the Fundación Científica de la AECC (to R.G.U.); the Fundación Ramón Areces (to M.F.F); FICYT (to E.G.T., M.G.G. and A.C.); and the Asturias Regional Government (GRUPIN14-052 to M.F.F.). Work in P.M. laboratory is supported by the European Research Council (CoG-2014-646903), the Spanish Ministry of Economy-Competitiveness (SAF-SAF2013-43065), the Obra Social La Caixa-Fundaciò Josep Carreras, and the Generalitat de Catalunya. P.M. is an investigator in the Spanish Cell Therapy cooperative network (TERCEL). The IUOPA is supported by the Obra Social Cajastur-Liberbank, Spain.Peer reviewe