A gain-of-function variant in SREBF1 causes generalized skin hyperpigmentation with congenital cataracts.

ABSTRACT

BACKGROUND: Lipid metabolism has essential roles in skin barrier formation and the regulation of skin inflammation. Lipid homeostasis regulates skin melanogenesis, although the underlying mechanism remains largely unknown. Sterol regulatory element binding protein 1 (SREBP-1) is a key transcription factor essential for cellular lipid metabolism. Loss-of-function variants in SREBF1 are responsible for autosomal-dominant ichthyosis follicularis, alopecia and photophobia syndrome, emphasizing the significance of lipid homeostasis in skin keratinization. OBJECTIVES: To identify the genetic basis of a new entity featuring diffuse skin hyperpigmentation with congenital cataracts, and to unravel the underlying mechanism for the pathogenesis of the SREBF1 variant. METHODS: Whole-exome sequencing was performed to identify underlying genetic variants. Quantitative polymerase chain reaction, Western blot and immunofluorescence staining were used to assess the expression and the subcellular localization of the SREBF1 variant. The transcriptional activity of mutant SREBP-1 was determined by a luciferase reporter assay. A transgenic zebrafish model was constructed. RESULTS: Two unrelated patients presented with generalized skin hyperpigmentation with skin xerosis, congenital cataracts and extracutaneous symptoms. We identified a de novo nonsense variant c.1289C>A (p.Ser430*) in SREBF1 in both patients. The variant encoded a truncated protein that showed preferential nucleus localization, in contrast to wild-type SREBP-1 which - in sterol-sufficient conditions - is mainly localized in the cytoplasm. The luciferase reporter assay revealed that the p.Ser430* mutant exhibited enhanced transcriptional activity. Cultured patient primary melanocytes showed increased melanin synthesis vs. those from healthy controls. At 35 days postfertilization, the p.Ser430* transgenic zebrafish model exhibited more black spots, along with upregulated expression of melanogenic genes. CONCLUSIONS: We demonstrated that a gain-of-function variant of SREBF1 causes a previously undescribed disorder characterized by generalized skin hyperpigmentation and congenital cataracts. Our study reveals the involvement of SREBP-1 in melanogenesis and lens development, and paves the way for the development of novel therapeutic targets for skin dyspigmentation or cataracts.

The genetic basis of many diseases that cause skin hyperpigmentation are not fully understood. Hyperpigmentation means that some patches of skin are darker than others. This is caused by the overproduction of a pigment called melanin. We report on two patients who were born with skin hyperpigmentation and cataracts. The cause of the patients' disease was unknown, so we carried out genetic testing in the patients. The tests showed that both patients had a change ('mutation') in a gene called 'SREBF1'. This gene encodes for a protein called SREBF-1. Other mutations in this protein are involved in other skin diseases. A different test showed that the mutated SREBF1 gene was activated more often than normal. Skin cells taken from both patients also produced more pigment than cells taken from people without hyperpigmentation. To confirm this gene mutation causes more skin pigmentation, we did an experiment with zebrafish with the same mutation. At 35 days after fertilization, the zebrafish showed more black spots on their skin. Our study reveals the involvement of SREBP-1 in the production of melanin and lens development in the eye. The findings may offer a new approach to treating hyperpigmentation in skin diseases.