Genetic and epigenetic alterations of RB2/p130 tumor suppressor gene in human sporadic retinoblastoma: implications for pathogenesis and therapeutic approach

Abstract

Human retinoblastoma occurs in two forms (familial and sporadic) both due to biallelic mutation of the RB1/p105 gene even if its loss is insufficient for malignancy. We have recently reported that loss of expression of the retinoblastoma-related protein pRb2/p130 correlates with low apoptotic index, suggesting that RB2/p130 gene could be involved in retinoblastoma. Mutational analysis of RB2/ p130 in primary tumors showed a tight correlation between Exon 1 mutations and pRb2/p130 expression level in sporadic retinoblastoma. These mutations are located within a CpG-enriched region prone to de novo methylation. Analysis of RB2/p130 methylation status revealed that epigenetic events, most probably consequent to the Exon 1 mutations, determined the observed phenotype. Treatment of Weri-Rb1 cell line by 5-AzadC induced an increase in expression level of pRb2/p130, E2F1, p73 and p53. Overall, our results highlight a crucial role of epigenetic events in sporadic retinoblastoma, which opens a perspective for new therapeutic approaches.

Rb2/p130 is the dominating pocket protein in the p53–p21 DNA damage response pathway leading to senescence

“…However, until now, little is known about the correlation between inactivation of Rb2/p130 and tumor development. It has been observed that low levels of Rb2/p130 expression correlate with high malignancy in primary tumors (Susini et al, 2001;Li et al, 2004), but only some cases of mutations or promoter methylations of Rb2/p130 have been published (Cinti et al, 2005;Tosi et al, 2005). Unlike direct genetic or epigenetic inactivation, functional inactivation of signaling pathways involving Rb2/p130 occurs at high frequency in tumor development.…”

One hit, two hits, three hits, more? Genomic changes in the development of retinoblastoma

“…This locus is outside either MRL discussed earlier, but RBL2 has been suspected of playing a role in retinoblastoma. Its expression is commonly lost in retinoblastoma (Bellan et al, 2002;Tosi et al, 2005), including in the progression from retinoma to retinoblastoma (Dimaras et al, submitted), and it is one of the genes that can be ablated along with Rb1 to cause retinal tumor formation in mice (MacPherson et al, 2004). It is commonly hypermethylated in tumors with reduced expression, and when WERI-Rb1 cells are treated with a demethylating agent, RBL2 expression is restored, along with expression of p53, p73, and consequent apoptosis (Tosi et al, 2005).…”

“…Its expression is commonly lost in retinoblastoma (Bellan et al, 2002;Tosi et al, 2005), including in the progression from retinoma to retinoblastoma (Dimaras et al, submitted), and it is one of the genes that can be ablated along with Rb1 to cause retinal tumor formation in mice (MacPherson et al, 2004). It is commonly hypermethylated in tumors with reduced expression, and when WERI-Rb1 cells are treated with a demethylating agent, RBL2 expression is restored, along with expression of p53, p73, and consequent apoptosis (Tosi et al, 2005). However, the ''mutations'' in the RBL2 gene identified by these authors are polymorphisms that are not unique to tumors (they are present in the Celera normal reference human genome assembly, NW_926462.1), and so are unlikely to have an etiological role in retinoblastoma, although they could serve to modify methylation patterns.…”

A small molecule based on the pRb2/p130 spacer domain leads to inhibition of cdk2 activity, cell cycle arrest and tumor growth reduction in vivo

“…This family includes the product of the Rb susceptibility gene, the pRb/p105 protein and the related p107 and pRb2/130 proteins (Mayol et al, 1993;Du and Pogoriler, 2006;Merola et al, 2006). They share the ability to recruit chromatin-remodeling enzymes and their best characterized targets are the members of the E2F/DP family of transcription factors, generally referred to as E2F (Cinti et al, 2005;Tosi et al, 2005;Genovese et al, 2006;Macaluso et al, 2006;Purev et al, 2006). Both pRb2/p130 and p107 are able to bind cdk2/ cyclins A and E (Claudio et al, 1996).…”