Oculocutaneous albinism (OCA) is a heterogeneous and autosomal recessive disorder with hypopigmentation in the eye, hair, and skin color. Four genes, TYR, OCA2, TYRP1, and SLC45A2, have been identified as causative genes for nonsyndromic OCA1–4, respectively. The genetic identity of OCA5 locus on 4q24 is unknown. Additional unknown OCA genes may exist as at least 5% of OCA patients have not been characterized during mutational screening in several populations. We used exome sequencing with a family-based recessive mutation model to determine that SLC24A5 is a previously unreported candidate gene for nonsyndromic OCA, which we designate as OCA6. Two deleterious mutations in this patient, c.591G>A and c.1361insT, were identified. We found apparent increase of immature melanosomes and less mature melanosomes in the patient’s skin melanocytes. However, no defects in the platelet dense granules were observed, excluding typical Hermansky–Pudlak syndrome (HPS), a well-known syndromic OCA. Moreover, the SLC24A5 protein was reduced in steady-state levels in mouse HPS mutants with deficiencies in BLOC-1 and BLOC-2. Our results suggest that SLC24A5 is a previously unreported nonsyndromic OCA candidate gene and that the SLC24A5 transporter is transported into mature melanosomes by HPS protein complexes.

To characterise the genotypic and phenotypic spectrum of foveal hypoplasia (FH) Multi-centre, observational study 907 patients with a confirmed molecular diagnosis of albinism, PAX6, SLC38A8, FRMD7, AHR or achromatopsia (ACHM) from twelve centres in nine countries (n=523), or, extracted from publicly available datasets from previously reported literature (n=384). Individuals with a confirmed molecular diagnosis and availability of foveal optical coherence tomography (OCT) scans were identified from twelve centres or from the literature, between January 2011 and March 2021. A genetic diagnosis was confirmed by sequence analysis. Grading of FH was derived from OCTs. Grade of FH, presence or absence of photoreceptor specialisation (PRS+ vs PRS−), molecular diagnosis and visual acuity (VA). The most common genetic etiology for typical FH in our cohort was albinism (67.5%), followed by PAX6 (21.8%), SLC38A8 (6.8%) and FRMD7 (3.5%) variants. AHR variants were rare (0.4%). Atypical FH was seen in 67.4% of ACHM cases. Atypical FH in ACHM had significantly worse VA compared to typical FH (p<0.0001). There was a significant difference in the spectrum of FH grades based on the molecular diagnosis (X²=60.4, p<0.0001). All SLC38A8 cases were PRS− (p=0.003), while all FRMD7 cases were PRS+ (p<0.0001). Analysis of albinism sub-types revealed a significant difference in the grade of FH (X²=31.4, p<0.0001) and VA (p=0.0003) between oculocutaneous albinism (OCA) compared to ocular albinism (OA) and Hermansky-Pudlak syndrome (HPS). OA and HPS demonstrated higher grades of FH and worse VA than OCA. There was a significant difference (p<0.0001) in VA between FRMD7 variants compared to other diagnoses associated with FH. We characterised the phenotypic and genotypic spectrum of FH. Atypical FH is associated with much worse prognosis compared to all other forms of FH. In typical FH, our data suggests that arrested retinal development occurs earlier in SLC38A8, OA, HPS and AHR variants and much later in FRMD7 variants. The defined time-period of foveal developmental arrest for OCA and PAX6 variants appears to demonstrate more variability. Our findings provide mechanistic insight into disorders associated with FH and also have significant prognostic and diagnostic value.

Hermansky–Pudlak syndrome (HPS) is characterized by defects of multiple tissue‐specific lysosome‐related organelles (LROs), typically manifesting with oculocutaneous albinism or ocular albinism, bleeding tendency, and in some cases with pulmonary fibrosis, inflammatory bowel disease or immunodeficiency, neuropsychological disorders. Eleven HPS subtypes in humans and at least 15 subtypes in mice have been molecularly identified. Current understanding of the underlying mechanisms of HPS is focusing on the defective biogenesis of LROs. Compelling evidences have shown that HPS protein‐associated complexes (HPACs) function in cargo transport, cargo recycling, and cargo removal to maintain LRO homeostasis. Further investigation on the molecular and cellular mechanism of LRO biogenesis and secretion will be helpful for better understanding of its pathogenesis and for the precise intervention of HPS.

Oculocutaneous albinism (OCA) is a heterogeneous and autosomal recessive hypopigmentation disorder, which is caused by mutations of genes involved in pigment biosynthesis or melanosome biogenesis. We have previously identified NCKX5 (also known as SLC24A5) as a causative gene for OCA type 6 (OCA6). However, the pathogenesis of OCA6 is unknown. We found that NCKX5 is localized to mitochondria, not to melanosomes. Pharmacological inhibition of mitochondrial function or NCKX exchanger activity reduced pigment production. Loss of NCKX5 attenuated Ca²⁺ enrichment in melanosomes, which compromised PMEL fibril formation, melanosome maturation and pigment production. Thus, we have defined a new class of hypopigmentation attributable to dysfunctional mitochondria and an impairment of mitochondrial Ca²⁺ transfer into melanosomes. Thus, it is possible that mitochondrial function could have a role in the graying of hair in older people and formation of hypopigmented lesions in vitiligo patients.