DNA hypermethylation/boundary control loss identified in retinoblastomas associated with genetic and epigenetic inactivation of the gene promoter.

DNA hypermethylation events occur frequently in human cancers, but less is known of the mechanisms leading to their initiation. Retinoblastoma, an intraocular cancer affecting young children, involves bi-allelic inactivation of the gene (). encodes a tumour suppressing, cell cycle regulating transcription factor ( that binds and regulates the core and other responsive promoters with epigenetic functions that include recruitment of histone deacetylases (). Evidence suggests that bi-allelic epigenetic inactivation/hypermethylation of the core promoter (), is specific to sporadic retinoblastomas (frequency10%), whereas heritable promoter variants (, frequency1-2%) are not associated with known epigenetic phenomena. We report heritable retinoblastomas with the expected loss of expression, in which hypermethylation consistent with distal boundary displacement (BD) relative to normal peripheral blood DNAs was detected in 4/4 cases. In contrast, proximal was identified in retinoblastomas while multiple boundaries distal of the core promoter was further identified in and retinoblastomas. However, weak or no DNA hypermethylation/ in peripheral blood DNA was detected in 8/9 patients, with the exception, a carrier of a microdeletion encompassing several promoter elements. These findings suggest that loss of boundary control may be a critical step leading to epigenetic inactivation of the gene and that novel DNA methylation boundaries/profiles identified in the promoter of retinoblastomas, may be the result of epigenetic phenomena associated with epimutation in conjunction with loss of expression/binding and/or promoter interactions with boundary control elements.