Background: Retinoblastoma (Rb) is the commonest eye cancer of childhood. Although considered to be developmental, the cell-of-origin is not fully established. The seven retinal cell types all derive from a pool of retinal progenitor cells. The distalless (DLX) family of evolutionarily-conserved homeobox genes encode transcription factors (TF) expressed in the developing and mature retina. The DLX TF are necessary for retinal ganglion cells (RGC) differentiation, in part due to direct regulation of other TF that are either activated or repressed during eye development.
Results: We have established that DLX2 directly binds to the promoters and activate the expression of Brn3a and Brn3b required for retinal ganglion cell (RGC) differentiation. In utero electroporation of DLX2 in the embryonic retina results in ectopic expression of BRN3A and BRN3B in vivo. Furthermore, DLX2 directly binds to the promoters and represses the expression of the cone rod homeobox gene, Crx. Crossing a Crx-lacZ reporter mouse with the Dlx1/Dlx2 double knockout (DKO) mouse results in increased lacZ expression, consistent with de-repression. Coupling embryonic mouse retina ChIP to a CpG island microarray, we have identified the Rb-like family member p107 as a direct DLX2 target in vivo. DLX2 is expressed in all three layers of the human retina and is expressed almost uniformly in the Rb1;p107 DKO mouse model of retinoblastoma. Furthermore, DLX2 is co-expressed with PAX6, PROX1 and syntaxin, markers of horizontal and amacrine progenitor cells, respectively. Over 80% of human Rb tumours examined to date express DLX2.
Conclusions: Our work supports the developmental origins of human retinoblastoma and the re-assessment of differentiation based therapies as an adjunct to current treatment options.