Altered expression of the iron transporter Nramp1 (Slc11a1) during fetal development of the retinal pigment epithelium in microphthalmia-associated transcription factor Mitf^mi and Mitf^vitiligo mouse mutants

Microphthalmia-associated transcription factor (Mitf) is expressed in neural crest cell-derived melanocytes, and in the retinal pigment epithelium (RPE) during ocular development. Mutations in Mitf are associated with auditory/visual/pigmentary syndromes in humans. Mitf^mi/mi mouse mutants lack pigmentation, and are microphthalmic, while Mitf^vit/vit mouse mutants display abnormal RPE pigmentation, and progressive retinal degeneration. Microarray analysis was used to identify novel downstream gene targets/pathways in the RPE that are altered by mutations in the transcription factor Mitf. Using the Affymetrix platform, gene expression profiles were generated using the eyes of E13.5 mouse fetuses that were wildtype, heterozygous, or homozygous for the Mitf^mi mutation. In a separate experiment, eyes from E13.5 mouse fetuses homozygous for the Mitf^vit mutation were compared to eyes from the C57BL/6 control background strain. Statistical analyses were performed using robust multiarray average, mixed-effects ANOVA and random-variance t-tests. Altered expression of genes involved in pigment formation, melanosome biogenesis/transport, and redox homeostasis were observed. Twelve genes were commonly mis-regulated in the eyes of both Mitf mutants: 10 of these genes were downregulated in both mutants relative to controls, while 2 of the genes (Nramp1 (Slc11a1) and epoxide hydrolase) were downregulated in Mitf^mi/mi mutants, and conversely, upregulated in Mitf^vit/vit mutants. Quantitative RT-PCR and immunohistochemistry were used to confirm altered gene/protein expression. RPE expression of the Fe⁺² iron transporter Nramp1 (Slc11a1) has not previously been reported. Fe⁺² is an important co-factor utilized by the iron-dependent isomerohydrolase RPE65 in the retinoid visual cycle. However, excess accumulation of Fe⁺² in the RPE has recently been associated with oxidative damage and age-related macular degeneration. Abnormal pigmentation and increased activity of Slc11a1 in the RPE of Mitf^vit mice may contribute to the pathology and progressive retinal degeneration observed in these mutants.