Sequestration of the
tumor suppressor
retinoblastoma protein (RB) by the Merkel cell polyomavirus (MCV) is a crucial step in the pathogenesis of
Merkel cell carcinoma (MCC). RB expression is frequently lost, particularly in MCV-negative MCC
tumors, through yet unknown mechanisms. We compared the genomic copy number changes of 13 MCV-positive and 13 -negative MCC
tumors by array comparative genomic hybridization. The analysis revealed increased
genomic instability, amplification of 1p34.3-1p34.2, and losses of 11p in the absence of MCV
infection. Deletions of the RB1 locus were also detected at high rates in MCV-negative
tumors. None of the
tumors with heterozygous RB1 losses expressed RB in immunohistochemistry. RB1 promoter hypermethylation was studied with a methylation-specific multiplex
ligation-dependent probe amplification technique. The RB1 promoter was methylated in all
tumor specimens at CpG islands located close to the ATG
start codon, albeit at low levels. The pattern of hypermethylation was similar in all MCC samples, despite RB expression, survival or MCV status. In conclusion, the frequent heterozygous losses of the RB1 locus could partly explain the decreased RB expression in MCV-negative MCC
tumors, although the effects of RB1 mutations, coinciding promoter hypermethylation and, for example,
miRNA regulation, cannot be excluded.