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      GnaqM1J: An ENU induced mutant allele affecting pigmentation in the mouse

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          Abstract

          To The Editor Over the last decade, large-scale, primarily ENU based, chemical mutagenesis projects (Clark et al, 2004) inspired the development of high-throughput, broad coverage means to analyze mutant mouse phenotypes. The dark skin mutations, GnaqMhdadsk1 and GnaqMhdadsk10 , arose in just such an ENU mutagenesis screen. These dark skin mutations are due to missense mutations in the Gnaq gene (Van Raamsdonk et al, 2004; Fitch et al, 2003). A third dominant mutation, GnaqM1J , arose an ENU mutagenesis screen using the C57BL/6J mouse strain at The Jackson Laboratory. We report here the phenotype and molecular defect of this new allelic mutation. The GnaqM1J deviant mice presented with abnormally pigmented tails inherited in a dominant manner (Figure 1 A). Further observations indicated pigmentary changes throughout the skin of deviant mice (Figure 1 Bi-iv). GnaqM1J mice exhibited unusually dark pigmented tails (Figure 1 Bi), ears (Figure 1 Bii), lower legs and foot pads, (Figure 1 Biii) and dorsal skin (Figure 1 Biv) as compared to wild-type litter mates. To define histological lesions, a complete set of tissues were harvested from 3 males and 3 females, mutant and control, 12 week old mice and processed routinely. Paraffin sections were stained with hematoxylin and eosin (H&E), and examined by an experienced board certified veterinary anatomic pathologist (JPS) (Silva and Sundberg, 2012). Histological analysis of tail skin taken from wild type C57BL/6J mice demonstrated low levels of interfollicular epidermal pigment and dermal hyper pigmentation (Figure 2 A,B). By contrast, GnaqM1J mutant mice tail sections exhibited high levels of dermal hyper pigmentation and interfollicular epidermal pigmentation in similar regions of the tail (Figure 2 C,D). This was more evident in cross sections of GnaqM1J tail skin (Figure 2E, arrows), especially in oblique sections of the dermis (Figure 2 F). In addition, sections through ventral leg skin (Figure 2G,H) and foot pads with eccrine glands (Figure 2I,J) revealed dermal hyper pigmentation, but no interfollicular pigment in the epidermis. Also, wild-type dorsal skin sections and hair follicles (Figure 2Ki-iii) exhibited no pigment evident in the dermal papilla or residual pigment in the dermis below the hair follicle in the fibrous track of the former catagen follicle, while dorsal skin sections from mutant mouse (Figure 2Li-iii) exhibit abnormal pigmentation in the dermal papilla region extending into the underlying dermis in the fibrous streak remaining at the end of catagen. Immunohistochemistry was also performed whereby paraffin sections from mutant and control animals were stained with antibodies specific to the following antigens: Smooth Muscle Actin, CD31, LYVE1, and VEGFA. The immunohistochemical stains were used to examine vascular abnormalities in these mice, yet no differences were observed between mutant and control mice (data not shown). SNP mapping was performed to isolate the genomic region containing the mutant allele by backcrossing the original C57BL6/J deviant mouse to the inbred mouse strain FVB/NJ. Mapping studies revealed linkage to proximal mouse chromosome 19 near SNP marker 21,436,276 bp (GRCm38) with a logarithm of odds (LOD) score equal to 5.3. Targeted exome sequencing of Gnaq identified a single base pair variant resulting in an A to G missense mutation in exon 2 at 16,219,611 bp (Figure 1 C). This variant is predicted to result in a Threonine to Alanine change at amino acid 54 (T54A). GNAQ consists of an alpha- helical domain, three flexible switch domains and a GTPase domain (Kumar et al, 2009). The T54A resides in the N-terminus of the alpha-helical domain. The “Sorting Tolerant From Intolerant” (SIFT) algorithm identified the T54A variant as a deleterious mutation in the helical domain of GNAQ (SIFT score 0.01), which may affect proper domain functioning. (The Uniprot Consortium, 2014) (Figure 1 C). Previous ENU induced mutations GnaqMhdadsk1 and GnaqMhdadsk10 , affected the C-terminus of the alpha-helical domain and GTPase-domain, respectively (Van Raamsdonk et al 2004, Fitch et al, 2003). All three ENU-induced mutations, affecting different areas of the GNAQ protein, resulted in similar, but not identical, pigmentary changes to the skin of affected mice. Also, the differences in phenotype between the previously described alleles (GnaqMhdadsk1 and GnaqMhdadsk10 ) and GnaqM1J , may be due to genetic background of the mouse strain, whereby the previous alleles were found on the C3HeB/FeJ strain, with a wild-type agouti locus (A/A), and the new allele was found on the C57BL/6J background, with a mutant agouti allele (a/a) producing only black pigment. A multitude of signaling pathways operate in the skin, whose coordination is essential for proper skin development and differentiation (Supplementary Figure 1 and 2). Yet, the molecular mechanisms governing these processes are still largely unknown. We report here a new, publicly available, ENU induced mutant (GnaqM1J ) that results in similar, but more extensive, skin hyperpigmentation defects as the GnaqMhdadsk1 and GnaqMhdadsk10 alleles (Fitch et al, 2003). Early in mouse development migrating melanoblasts localize to the dermis and epidermis (Fitch et al, 2003). Previously described mutations in Gnaq (GnaqMhdadsk1 and GnaqMhdadsk10 ) cause increases in the number of differentiated melanoblasts resulting in increased pigmentation in the dermis of mutant mice (Fitch et al, 2003). Although GnaqM1J is similar to the previous mutations, in that increased dermal pigmentation is observed in adult mice, unlike the previous mutants, it results in both dermal and epidermal hyperpigmentation in tail skin. In light of these mutations, we hypothesize that the genes highlighted in Supplementary Figures 1 and 2 work in a coordinate manner to direct development, localization, and differentiation of skin melanoblasts. Studies using hairless mice with various degrees of cutaneous pigmentation suggested that pigment may play a role in barrier function (Man et al, 2014). As such, mutations in genes regulating skin pigmentation may seriously affect the function of the skin. Gnaq can be tied to barrier formations by way of its interactions with other barrier genes, like Cav1 (Supplementary Figure 2). Mutations affecting skin color are infrequent in the mice, as most affect hair color (Dadras et al, 2014; Sundberg and Silva, 2012). Having single gene mutations on the inbred and very commonly used C57BL/6J background provides a new tool for refined evaluation of pigment in the skin of mice. Supplementary Material 01

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          Most cited references 10

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          Predicting the effects of coding non-synonymous variants on protein function using the SIFT algorithm.

          The effect of genetic mutation on phenotype is of significant interest in genetics. The type of genetic mutation that causes a single amino acid substitution (AAS) in a protein sequence is called a non-synonymous single nucleotide polymorphism (nsSNP). An nsSNP could potentially affect the function of the protein, subsequently altering the carrier's phenotype. This protocol describes the use of the 'Sorting Tolerant From Intolerant' (SIFT) algorithm in predicting whether an AAS affects protein function. To assess the effect of a substitution, SIFT assumes that important positions in a protein sequence have been conserved throughout evolution and therefore substitutions at these positions may affect protein function. Thus, by using sequence homology, SIFT predicts the effects of all possible substitutions at each position in the protein sequence. The protocol typically takes 5-20 min, depending on the input. SIFT is available as an online tool (http://sift.jcvi.org).
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            Effects of G-protein mutations on skin color.

            A new class of dominant dark skin (Dsk) mutations discovered in a screen of approximately 30,000 mice is caused by increased dermal melanin. We identified three of four such mutations as hypermorphic alleles of Gnaq and Gna11, which encode widely expressed Galphaq subunits, act in an additive and quantitative manner, and require Ednrb. Interactions between Gq and Kit receptor tyrosine kinase signaling can mediate coordinate or independent control of skin and hair color. Our results provide a mechanism that can explain several aspects of human pigmentary variation and show how polymorphism of essential proteins and signaling pathways can affect a single physiologic system.
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              Genetics of dark skin in mice.

              Chemical mutagenesis in the mouse is a powerful approach for phenotype-driven genetics, but questions remain about the efficiency with which new mutations ascertained by their phenotype can be localized and identified, and that knowledge applied to a specific biological problem. During a global screen for dominant phenotypes in about 30,000 animals, a novel class of pigmentation mutants were identified by dark skin (Dsk). We determined the genetic map location, homozygous phenotype, and histology of 10 new Dsk and 2 new dark coat (Dcc) mutations, and identified mutations in Agouti (Met1Leu, Dcc4), Sox18 (Leu220ter, Dcc1), Keratin 2e (Thr500Pro, Dsk2), and Egfr (Leu863Gln, Dsk5). Cutaneous effects of most Dsk mutations are limited to melanocytes, except for the Keratin 2e and Egfr mutations, in which hyperkeratosis and epidermal thickening precede epidermal melanocytosis by 3-6 wk. The Dsk2 mutation is likely to impair intermediate filament assembly, leading to cytolysis of suprabasal keratinocytes and secondary hyperkeratosis and melanocytosis. The Dsk5 mutation causes increased tyrosine kinase activity and a decrease in steady-state receptor levels in vivo. The Dsk mutations represent genes or map locations not implicated previously in pigmentation, and delineate a developmental pathway in which mutations can be classified on the basis of body region, microscopic site, and timing of pigment accumulation.
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                Author and article information

                Journal
                0426720
                4839
                J Invest Dermatol
                J. Invest. Dermatol.
                The Journal of investigative dermatology
                0022-202X
                1523-1747
                1 October 2015
                January 2016
                01 July 2016
                : 136
                : 1
                : 334-336
                NIHMS726476
                10.1038/JID.2015.382
                4731045
                26763459

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                Dermatology

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