Genomic Characterization of Recurrent Chromosomal Aberrations in Retinoblastoma

Abstract

Retinoblastoma is the second most common childhood intraocular malignancy and development of the tumour is initiated by bi-allelic loss of the RB1 gene. Bi-allelic RB1 loss may not always directly lead to malignant retinoblastoma. Subsequent mutational events, specifically gain and loss of chromosomal regions harbouring key oncogenes and tumour suppressor genes, appear to underlie the progression of precursor lesions such as retinoma to retinoblastoma. Research thus far has implicated several cytogenetic aberrations in this sequence of molecular events, the two most recurrent and notable being the augmented copy number status of KIF14 and MDM4 on 1q and of DEK and E2F3 on 6p. A small subset of retinoblastoma exhibits high-level amplification of the MYCN gene on chromosome 2p and is characterized by pRb expression and aggressive histology. 1p36 deletion is also associated with MYCN amplification in neuroblastoma. Our project is testing the hypothesis that 1q and 6p gain and 2p amplification – defined by extra copies of KIF14 and MDM4, DEK and E2F3 and MYCN respectively, and 1p36 deletion – are biomarkers of retinoblastoma progression. This study reports the results of formalin-fixed paraffin-embedded fluorescence in situ hybridization (FFPE-FISH) analysis of the aforementioned genes in two pre-constructed tissue microarrays (TMAs) comprising 270 retinoblastoma patient tumours. Results show 1q gain (3-10 copies) in 136/262 (52%), 6p gain in 127/262 (48.7%), MYCN gain in 18/265 (6.8%) and MYCN amplification (>10 copies) in 20/265 (7.5%) patient tumours. MYCN amplification was also observed in a sample which retained pRb protein expression. Additionally we have demonstrated statistically significant associations between 1q and 6p gain and MYCN amplification and 1p36 deletion. The large cohort and consequent statistical power of our results has enabled a better understanding of the M3 to Mn sequence of genomic aberrations in the progression from retinoma to retinoblastoma. Statistically significant associations between 1q and 6p, indicates synergy within these two regions of gain that would be particularly beneficial to tumours. Owing to the biological interactions among the proteins encoded by DEK, E2F3, KIF14 and MDM4, tumour cells manifesting both 1q and 6p aberrations would have a selective survival and proliferative advantage.