Samd7, short for Sterile alpha motif domain containing 7, is a component of the Polycomb repressive complex 1 (PRC1). It plays a crucial role in inhibiting the transcription of many genes, especially those activated by the transcription factor Cone-Rod Homeobox (CRX) [1]. This function is vital for the proper regulation of gene expression in the retina and is associated with the epigenetic regulation of photoreceptor cell gene expression [3]. Genetic models, such as gene knockout mouse models, have been instrumental in studying Samd7's function.

In Samd7-null mutant mice, non-rod genes like S-opsin are ectopically expressed in rod photoreceptor cells, leading to rod photoreceptor cell dysfunction. This shows that Samd7 is essential for silencing non-rod gene expression through H3K27me3 regulation in rod photoreceptor cells [3]. In zebrafish, loss of Samd7 leads to the transformation of red cones into hybrid red/ultraviolet (UV) cones, absence of green cones, an approximate doubling of blue cones, and a significant reduction in the number of rods. In mice, Samd7 also represses S-opsin expression in dorsal M-cones, similar to its role in zebrafish [2]. Additionally, Samd7/11 double-knock-out (DKO) mouse retina exhibits shortened photoreceptor outer segments, and more retinal genes are up-regulated compared to Samd7 -/- alone, indicating partial functional redundancy between Samd7 and Samd11 [4].

In conclusion, Samd7 is a key regulator in establishing and maintaining the identity of rod and cone photoreceptor cells. Studies using gene knockout mouse models have revealed its role in preventing the ectopic expression of non-rod genes in rod cells and ensuring the appropriate patterns of gene expression in different photoreceptor subtypes. Mutations in SAMD7 in humans can cause autosomal-recessive macular dystrophy with or without cone dysfunction, highlighting its significance in human retinal function [1].