Here, we provide gain-of-function, loss-of function, and molecular evidence supporting genetic interactions between
metastasis associated
protein 1 (MTA1) and Six3 and between Six3 and
rhodopsin. We discovered that MTA1 physically interacts with the Six3
chromatin in a
histone deacetylase-dependent manner, leading to transcriptional suppression of the Six3 gene. MTA1 is also a Six3-interacting
corepressor that contributes to a self-negative regulation of Six3 transcription by Six3. In contrast, deletion of the MTA1 alleles in murine embryonic fibroblasts or its knockdown in rat retinal ganglion cells stimulates Six3 expression. MTA1 inactivation in the MTA1-null mice results in an elevated Six3 level and proliferation of the retina cells with no obvious abnormities in eye formation. However, unexpectedly, we discovered an enhanced recruitment of Six3 to the
rhodopsin chromatin in retina from the MTA1-null mice; Six3's homeodomain interacts with specific
DNA elements in the
rhodopsin promoter to stimulate its transcription, resulting in increased
rhodopsin expression. Further, in
holoprosencephaly patients,
Six3 protein with a naturally occurring deletion mutation in the helix 3 of the homeodomain does not bind to
rhodopsin DNA or stimulate
rhodopsin transcription, implying a potential defective
rhodopsin pathway in the affected
holoprosencephaly patients. Further Six3 cooperates with Crx or NRL in stimulating transcription from the
rhodopsin-luc. These findings reveal a previously unrecognized role for the MTA1 as an upstream modifier of Six3 and indicate that Six3 is a direct stimulator of
rhodopsin expression, thus revealing a putative role for the MTA1/Six3/
rhodopsin pathway in vertebrate eye.