Membrane-Associated Transporter Protein (MATP) Regulates Melanosomal pH and Influences Tyrosinase Activity

Detection of two non-synonymous SNPs in SLC45A2 on BTA20 as candidate causal mutations for oculocutaneous albinism in Braunvieh cattle

BACKGROUND

Cases of albinism have been reported in several species including cattle. So far, research has identified many genes that are involved in this eye-catching phenotype. Thus, when two paternal Braunvieh half-sibs with oculocutaneous albinism were detected on a private farm, we were interested in knowing whether their phenotype was caused by an already known gene/mutation.

RESULTS

Analysis of genotyping data (50K) of the two albino individuals, their mothers and five other relatives identified a 47.61-Mb candidate haplotype on Bos taurus chromosome BTA20. Subsequent comparisons of the sequence of this haplotype with sequence data from four Braunvieh sires and the Aurochs genome identified two possible candidate causal mutations at positions 39,829,806 bp (G/A; R45Q) and 39,864,148 bp (C/T; T444I) that were absent in 1682 animals from various bovine breeds included in the 1000 bull genomes project. Both polymorphisms represent coding variants in the SLC45A2 gene, for which the human equivalent harbors numerous variants associated with oculocutaneous albinism type 4. We demonstrate an association of R45Q and T444I with the albino phenotype by targeted genotyping.

CONCLUSIONS

Although the candidate gene SLC45A2 is known to be involved in albinism in different species, to date in cattle only mutations in the TYR and MITF genes were reported to be associated with albinism or albinism-like phenotypes. Thus, our study extends the list of genes that are associated with bovine albinism. However, further research and more samples from related animals are needed to elucidate if only one of these two single nucleotide polymorphisms or the combination of both is the actual causal variant.

Delineating the genetic heterogeneity of OCA in Hungarian patients

BACKGROUND

Oculocutaneous albinism (OCA) is a clinically and genetically heterogenic group of pigmentation abnormalities characterized by variable hair, skin, and ocular hypopigmentation. Six known genes and a locus on human chromosome 4q24 have been implicated in the etiology of isolated OCA forms (OCA 1-7).

METHODS

The most frequent OCA types among Caucasians are OCA1, OCA2, and OCA4. We aimed to investigate genes responsible for the development of these OCA forms in Hungarian OCA patients (n = 13). Mutation screening and polymorphism analysis were performed by direct sequencing on TYR, OCA2, SLC45A2 genes.

RESULTS

Although the clinical features of the investigated Hungarian OCA patients were identical, the molecular genetic data suggested OCA1 subtype in eight cases and OCA4 subtype in two cases. The molecular diagnosis was not clearly identifiable in three cases. In four patients, two different heterozygous known pathogenic or predicted to be pathogenic mutations were present. Seven patients had only one pathogenic mutation, which was associated with non-pathogenic variants in six cases. In two patients no pathogenic mutation was identified.

CONCLUSIONS

Our results suggest that the concomitant screening of the non-pathogenic variants-which alone do not cause the development of OCA, but might have clinical significance in association with a pathogenic variant-is important. Our results also show significant variation in the disease spectrum compared to other populations. These data also confirm that the concomitant analysis of OCA genes is critical, providing new insights to the phenotypic diversity of OCA and expanding the mutation spectrum of OCA genes in Hungarian patients.

SLC45A2: A Melanoma Antigen with High Tumor Selectivity and Reduced Potential for Autoimmune Toxicity

Cytotoxic T lymphocyte (CTL)-based immunotherapies have had remarkable success at generating objective clinical responses in patients with advanced metastatic melanoma. Although the melanocyte differentiation antigens (MDA) MART-1, PMEL, and tyrosinase were among the first melanoma tumor-associated antigens identified and targeted with immunotherapy, expression within normal melanocytes of the eye and inner ear can elicit serious autoimmune side effects, thus limiting their clinical potential as CTL targets. Using a tandem mass spectrometry (MS) approach to analyze the immunopeptidomes of 55 melanoma patient-derived cell lines, we identified a number of shared HLA class I-bound peptides derived from the melanocyte-specific transporter protein SLC45A2. Antigen-specific CTLs generated against HLA-A*0201- and HLA-A*2402-restricted SLC45A2 peptides effectively killed a majority of HLA-matched cutaneous, uveal, and mucosal melanoma cell lines tested (18/25). CTLs specific for SLC45A2 showed significantly reduced recognition of HLA-matched primary melanocytes that were, conversely, robustly killed by MART1- and PMEL-specific T cells. Transcriptome analysis revealed that SLC45A2 mRNA expression in normal melanocytes was less than 2% that of other MDAs, therefore providing a more favorable melanoma-to-melanocyte expression ratio. Expression of SLC45A2 and CTL sensitivity could be further upregulated in BRAF(V600E)-mutant melanoma cells upon treatment with BRAF or MEK inhibitors, similarly to other MDAs. Taken together, our study demonstrates the feasibility of using tandem MS as a means of discovering shared immunogenic tumor-associated epitopes and identifies SLC45A2 as a promising immunotherapeutic target for melanoma with high tumor selectivity and reduced potential for autoimmune toxicity. Cancer Immunol Res; 5(8); 618-29. ©2017 AACR.

Requirement of Zinc Transporter SLC39A7/ZIP7 for Dermal Development to Fine-Tune Endoplasmic Reticulum Function by Regulating Protein Disulfide Isomerase

Skin is the first area that manifests zinc deficiency. However, the molecular mechanisms by which zinc homeostasis affects skin development remain largely unknown. Here, we show that zinc-regulation transporter-/iron-regulation transporter-like protein 7 (ZIP7) localized to the endoplasmic reticulum plays critical roles in connective tissue development. Mice lacking the Slc39a7/Zip7 gene in collagen 1-expressing tissue exhibited dermal dysplasia. Ablation of ZIP7 in mesenchymal stem cells inhibited cell proliferation thereby preventing proper dermis formation, indicating that ZIP7 is required for dermal development. We also found that mesenchymal stem cells lacking ZIP7 accumulated zinc in the endoplasmic reticulum, which triggered zinc-dependent aggregation and inhibition of protein disulfide isomerase, leading to endoplasmic reticulum dysfunction. These results suggest that ZIP7 is necessary for endoplasmic reticulum function in mesenchymal stem cells and, as such, is essential for dermal development.

Pheomelanogenesis is promoted at a weakly acidic pH

The diversity of pigmentation in the skin, hair, and eyes of humans has been largely attributed to the diversity of pH in melanosomes with an acidic pH being proposed to suppress melanin production, especially eumelanogenesis. We previously showed that an acidic pH greatly suppresses the late stage of eumelanogenesis after the dopachrome stage. The oxidation of tyrosine by tyrosinase in the presence of cysteine forms cysteinyldopa isomers, which are further oxidized to give rise to pheomelanin via benzothiazine intermediates. However, how those steps are controlled by pH has not been characterized. We therefore examined whether pheomelanin synthesis is chemically promoted at an acidic pH. We found that pheomelanin production either from dopa or tyrosine in the presence of cysteine by tyrosinase was greatest at pH values of 5.8-6.3, while eumelanin production was suppressed at pH 5.8. This suggests that mixed melanogenesis is chemically shifted to more pheomelanic states at a weakly acidic pH.

Identification of a novel locus associated with skin colour in African-admixed populations

Skin pigmentation is a complex trait that varies largely among populations. Most genome-wide association studies of this trait have been performed in Europeans and Asians. We aimed to uncover genes influencing skin colour in African-admixed individuals. We performed a genome-wide association study of melanin levels in 285 Hispanic/Latino individuals from Puerto Rico, analyzing 14 million genetic variants. A total of 82 variants with p-value ≤1 × 10⁻⁵ were followed up in 373 African Americans. Fourteen single nucleotide polymorphisms were replicated, of which nine were associated with skin colour at genome-wide significance in a meta-analysis across the two studies. These results validated the association of two previously known skin pigmentation genes, SLC24A5 (minimum p = 2.62 × 10⁻¹⁴, rs1426654) and SLC45A2 (minimum p = 9.71 × 10⁻¹⁰, rs16891982), and revealed the intergenic region of BEND7 and PRPF18 as a novel locus associated with this trait (minimum p = 4.58 × 10⁻⁹, rs6602666). The most significant variant within this region is common among African-descent populations but not among Europeans or Native Americans. Our findings support the advantages of analyzing African-admixed populations to discover new genes influencing skin pigmentation.

Identification of two novel mutations in the SLC45A2 gene in a Hungarian pedigree affected by unusual OCA type 4

Background

Oculocutaneous albinism (OCA) is a clinically and genetically heterogenic group of pigmentation abnormalities. OCA type IV (OCA4, OMIM 606574) develops due to homozygous or compound heterozygous mutations in the solute carrier family 45, member 2 (SLC45A2) gene. This gene encodes a membrane-associated transport protein, which regulates tyrosinase activity and, thus, melanin content by changing melanosomal pH and disrupting the incorporation of copper into tyrosinase.

Methods

Here we report two Hungarian siblings affected by an unusual OCA4 phenotype. After genomic DNA was isolated from peripheral blood of the patients, the coding regions of the SLC45A2 gene were sequenced. In silico tools were applied to identify the functional impact of the newly detected mutations.

Results

Direct sequencing of the SLC45A2 gene revealed two novel, heterozygous mutations, one missense (c.1226G > A, p.Gly409Asp) and one nonsense (c.1459C > T, p.Gln437*), which were present in both patients, suggesting the mutations were compound heterozygous. In silico tools suggest that these variations are disease causing mutations.

Conclusions

The newly identified mutations may affect the transmembrane domains of the protein, and could impair transport function, resulting in decreases in both melanosomal pH and tyrosinase activity. Our study provides expands on the mutation spectrum of the SLC45A2 gene and the genetic background of OCA4.

Retinal axon guidance at the midline: Chiasmatic misrouting and consequences

The visual representation of the outside world relies on the appropriate connectivity between the eyes and the brain. Retinal ganglion cells are the sole neurons that send an axon from the retina to the brain, and thus the guidance decisions of retinal axons en route to their targets in the brain shape the neural circuitry that forms the basis of vision. Here, we focus on the choice made by retinal axons to cross or avoid the midline at the optic chiasm. This decision allows each brain hemisphere to receive inputs from both eyes corresponding to the same visual hemifield, and is thus crucial for binocular vision. In achiasmatic conditions, all retinal axons from one eye project to the ipsilateral brain hemisphere. In albinism, abnormal guidance of retinal axons at the optic chiasm leads to a change in the ratio of contralateral and ipsilateral projections with the consequence that each brain hemisphere receives inputs primarily from the contralateral eye instead of an almost equal distribution from both eyes in humans. In both cases, this misrouting of retinal axons leads to reduced visual acuity and poor depth perception. While this defect has been known for decades, mouse genetics have led to a better understanding of the molecular mechanisms at play in retinal axon guidance and at the origin of the guidance defect in albinism. In addition, fMRI studies on humans have now confirmed the anatomical and functional consequences of axonal misrouting at the chiasm that were previously only assumed from animal models. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 844-860, 2017.

An Acrodermatitis Enteropathica-Associated Zn Transporter, ZIP4, Regulates Human Epidermal Homeostasis

Acrodermatitis enteropathica is an autosomal recessive disorder characterized by scaly eczematous dermatosis accompanied by alopecia and diarrhea. Various mutations in the SLC39A4 gene (ZIP4), which encodes a zinc transporter, are responsible for this disorder. However, the molecular mechanism underlying the involvement of ZIP4 in the pathogenesis of this condition has yet to be established. In this study, we report the role of ZIP4 in human epidermis. ZIP4 is predominantly expressed in human keratinocytes, and its expression is dramatically reduced on epidermal differentiation. ZIP4 knockdown in human keratinocytes down-regulates zinc (Zn) levels and the transcriptional activity of a key epidermal Zn-binding protein, ΔNp63, and dysregulates epidermal differentiation in a reconstituted human skin model, resulting in the appearance of proliferating keratinocytes even in the uppermost layers of the skin. We verified that, among the amino acid residues in its Zn-binding motif, Cys205 is critical for the processing and nuclear distribution of ΔNp63 and, therefore, Zn-dependent transcriptional activity. Our results suggest that ZIP4 is essential for maintaining human epidermal homeostasis through the regulation of Zn-dependent ΔNp63 activity and can provide insight into the molecular mechanisms responsible for the cutaneous symptoms observed in Acrodermatitis enteropathica patients.

Local Adaptation of Sun-Exposure-Dependent Gene Expression Regulation in Human Skin

Sun-exposure is a key environmental variable in the study of human evolution. Several skin-pigmentation genes serve as classical examples of positive selection, suggesting that sun-exposure has significantly shaped worldwide genomic variation. Here we investigate the interaction between genetic variation and sun-exposure, and how this impacts gene expression regulation. Using RNA-Seq data from 607 human skin samples, we identified thousands of transcripts that are differentially expressed between sun-exposed skin and non-sun-exposed skin. We then tested whether genetic variants may influence each individual’s gene expression response to sun-exposure. Our analysis revealed 10 sun-exposure-dependent gene expression quantitative trait loci (se-eQTLs), including genes involved in skin pigmentation (SLC45A2) and epidermal differentiation (RASSF9). The allele frequencies of the RASSF9 se-eQTL across diverse populations correlate with the magnitude of solar radiation experienced by these populations, suggesting local adaptation to varying levels of sunlight. These results provide the first examples of sun-exposure-dependent regulatory variation and suggest that this variation has contributed to recent human adaptation.

Varying levels of sun-exposure across the world have significantly shaped human evolution. Previous analyses have found several skin pigmentation genes with evidence of strong evolutionary pressures throughout human history, manifesting as large differences in the frequency of genomic variants across populations. But even within populations, individuals respond differently to sun-exposure, suggesting variation in addition to the major differences in skin pigmentation across populations. Here we investigated whether genetic variants associate with response to sun-exposure within Europeans. To measure the response we analyzed gene expression in sun-exposed and non-sun-exposed skin, and identified ten genetic variants that associated with the sun-exposure response of nearby genes. One of these genetic variants, which associated with the sun-exposure response of the gene RASSF9, showed evidence of adaptation in humans in response to solar radiation. Together this evidence suggests that the regulation of gene expression is influenced by sun-exposure and that the sun-exposure dependent effect on RASSF9 expression may have had an effect on human fitness. To our knowledge, this is the first example of an environment-dependent regulatory variant with evidence of adaptation in humans.

TPC2 controls pigmentation by regulating melanosome pH and size

Melanin is responsible for pigmentation of skin and hair and is synthesized in a specialized organelle, the melanosome, in melanocytes. A genome-wide association study revealed that the two pore segment channel 2 (TPCN2) gene is strongly linked to pigmentation variations. TPCN2 encodes the two-pore channel 2 (TPC2) protein, a cation channel. Nevertheless, how TPC2 regulates pigmentation remains unknown. Here, we show that TPC2 is expressed in melanocytes and localizes to the melanosome-limiting membrane and, to a lesser extent, to endolysosomal compartments by confocal fluorescence and immunogold electron microscopy. Immunomagnetic isolation of TPC2-containing organelles confirmed its coresidence with melanosomal markers. TPCN2 knockout by means of clustered regularly interspaced short palindromic repeat/CRISPR-associated 9 gene editing elicited a dramatic increase in pigment content in MNT-1 melanocytic cells. This effect was rescued by transient expression of TPC2-GFP. Consistently, siRNA-mediated knockdown of TPC2 also caused a substantial increase in melanin content in both MNT-1 cells and primary human melanocytes. Using a newly developed genetically encoded pH sensor targeted to melanosomes, we determined that the melanosome lumen in TPC2-KO MNT-1 cells and primary melanocytes subjected to TPC2 knockdown is less acidic than in control cells. Fluorescence and electron microscopy analysis revealed that TPC2-KO MNT-1 cells have significantly larger melanosomes than control cells, but the number of organelles is unchanged. TPC2 likely regulates melanosomes pH and size by mediating Ca(2+) release from the organelle, which is decreased in TPC2-KO MNT-1 cells, as determined with the Ca(2+) sensor tyrosinase-GCaMP6. Thus, our data show that TPC2 regulates pigmentation through two fundamental determinants of melanosome function: pH and size.

The Development of Sugar-Based Anti-Melanogenic Agents

The regulation of melanin production is important for managing skin darkness and hyperpigmentary disorders. Numerous anti-melanogenic agents that target tyrosinase activity/stability, melanosome maturation/transfer, or melanogenesis-related signaling pathways have been developed. As a rate-limiting enzyme in melanogenesis, tyrosinase has been the most attractive target, but tyrosinase-targeted treatments still pose serious potential risks, indicating the necessity of developing lower-risk anti-melanogenic agents. Sugars are ubiquitous natural compounds found in humans and other organisms. Here, we review the recent advances in research on the roles of sugars and sugar-related agents in melanogenesis and in the development of sugar-based anti-melanogenic agents. The proposed mechanisms of action of these agents include: (a) (natural sugars) disturbing proper melanosome maturation by inducing osmotic stress and inhibiting the PI3 kinase pathway and (b) (sugar derivatives) inhibiting tyrosinase maturation by blocking N-glycosylation. Finally, we propose an alternative strategy for developing anti-melanogenic sugars that theoretically reduce melanosomal pH by inhibiting a sucrose transporter and reduce tyrosinase activity by inhibiting copper incorporation into an active site. These studies provide evidence of the utility of sugar-based anti-melanogenic agents in managing skin darkness and curing pigmentary disorders and suggest a future direction for the development of physiologically favorable anti-melanogenic agents.

Making the invisible visible

In this review, I will discuss how careful scrutiny of genetic skin disorders could help us to understand human biology. Like other organs, the skin and its appendages, such as hairs and teeth, experience fundamental biological processes ranging from lipid metabolism to vesicular transport and cellular migration. However, in contrast to other organ systems, they are accessible and can be studied with relative ease. By visually revealing the functional consequences of single gene defects, genetic skin diseases offer a unique opportunity to study human biology. Here, I will illustrate this concept by discussing how human genetic disorders of skin pigmentation reflect the mechanisms underlying this complex and vital process.