Loss of photoreceptorness and gain of genomic alterations in retinoblastoma reveal tumor progression

BACKGROUND Retinoblastoma is a pediatric eye cancer associated with RB1 loss or MYCN amplification (RB1 (+/+) MYCN(A) ). There are controversies concerning the existence of molecular subtypes within RB1(-/-) retinoblastoma. To test whether these molecular subtypes exist, we performed molecular profiling. METHODS Genome-wide mRNA expression profiling was performed on 76 primary human retinoblastomas. Expression profiling was complemented by genome-wide DNA profiling and clinical, histopathological, and ex vivo drug sensitivity data. FINDINGS RNA and DNA profiling identified major variability between retinoblastomas. While gene expression differences between RB1 (+/+) MYCN(A) and RB1(-/-) tumors seemed more dichotomous, differences within the RB1(-/-) tumors were gradual. Tumors with high expression of a photoreceptor gene signature were highly differentiated, smaller in volume and diagnosed at younger age compared with tumors with low photoreceptor signature expression. Tumors with lower photoreceptor expression showed increased expression of genes involved in M-phase and mRNA and ribosome synthesis and increased frequencies of somatic copy number alterations. INTERPRETATION Molecular, clinical and histopathological differences between RB1(-/-) tumors are best explained by tumor progression, reflected by a gradual loss of differentiation and photoreceptor expression signature. Since copy number alterations were more frequent in tumors with less photoreceptorness, genomic alterations might be drivers of tumor progression. RESEARCH IN CONTEXT Retinoblastoma is an ocular childhood cancer commonly caused by mutations in the RB1 gene. In order to determine optimal treatment, tumor subtyping is considered critically important. However, except for very rare retinoblastomas without an RB1 mutation, there are controversies as to whether subtypes of retinoblastoma do exist. Our study shows that retinoblastomas are highly diverse but rather than reflecting distinct tumor types with a different etiology, our data suggests that this diversity is a result of tumor progression driven by cumulative genetic alterations. Therefore, retinoblastomas should not be categorized in distinct subtypes, but be described according to their stage of progression.