MIT/TFE family transcription factors, Mitf and Tfec, redundantly promote pigment cell differentiation and retinal epithelium pigmentation in teleosts

Background: Transcription factors of the MIT/TFE family—mitfa, mitfb, and tfec—play essential roles in specifying pigment cells derived from neural crest cells in vertebrates. In teleosts, which possess multiple pigment cell types, these factors exhibit partially redundant but largely distinct functions across different pigment lineages. We previously reported that the combined loss of mitfa and mitfb leads to a complete absence of melanophores, xanthophores, and leucophores in medaka.
Results: Here, we describe newly generated hypomorphic alleles of mitfa and mitfb, which result in delayed melanophore development. Using these hypomorphic mutants in combination with a newly generated null allele of tfec, we show that Tfec can partially compensate for the loss of Mitfa and Mitfb in melanophore, xanthophore, and leucophore differentiation. Consistent with previous findings in zebrafish, loss of tfec alone results in the complete absence of iridophores. Importantly, simultaneous null mutations in all three genes cause a complete loss of all four chromatophore types and lead to defective pigmentation of the retinal pigmented epithelium (RPE), which is not derived from the neural crest in both Oryzias species and in zebrafish. Using an RPE-specific marker, we further show that the RPE is specified and structurally intact but fails to melanize.
Conclusions: These findings indicate that the redundant functions of Mitfa, Mitfb, and Tfec are not only essential for the specification of diverse pigment cell types but also critical for RPE pigmentation.