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A frame-shift mutation in COMTD1 is associated with impaired pheomelanin pigmentation in chicken [1]

['Huijuan Bi', 'Science For Life Laboratory', 'Department Of Medical Biochemistry', 'Microbiology', 'Uppsala University', 'Uppsala', 'Jonas Tranell', 'Dawn C. Harper', 'Department Of Pathology', 'Laboratory Medicine']

Date: 2023-06

The biochemical pathway regulating the synthesis of yellow/red pheomelanin is less well characterized than the synthesis of black/brown eumelanin. Inhibitor of gold (IG phenotype) is a plumage colour variant in chicken that provides an opportunity to further explore this pathway since the recessive allele (IG) at this locus is associated with a defect in the production of pheomelanin. IG/IG homozygotes display a marked dilution of red pheomelanin pigmentation, whilst black pigmentation (eumelanin) is only slightly affected. Here we show that a 2-base pair insertion (frame-shift mutation) in the 5 th exon of the Catechol-O-methyltransferase containing domain 1 gene (COMTD1), expected to cause a complete or partial loss-of-function of the COMTD1 enzyme, shows complete concordance with the IG phenotype within and across breeds. We show that the COMTD1 protein is localized to mitochondria in pigment cells. Knockout of Comtd1 in a mouse melanocytic cell line results in a reduction in pheomelanin metabolites and significant alterations in metabolites of glutamate/glutathione, riboflavin, and the tricarboxylic acid cycle. Furthermore, COMTD1 overexpression enhanced cellular proliferation following chemical-induced transfection, a potential inducer of oxidative stress. These observations suggest that COMTD1 plays a protective role for melanocytes against oxidative stress and that this supports their ability to produce pheomelanin.

Vertebrates possess two types of melanin, red/yellow pheomelanin and black/brown eumelanin. In this study, we report that the recessive Inhibitor of gold phenotype in chicken, which causes a severe defect in pheomelanin pigmentation, is associated with a mutation that most likely inactivates the COMTD1 gene. This gene encodes an O-methyltransferase enzyme and is present throughout vertebrate evolution, but is one of the many genes in vertebrate genomes for which the biological function is still poorly understood. This is the first report of a COMTD1 mutation associated with a phenotypic effect. We show that the COMTD1 protein is present in mitochondria in pigment cells. Furthermore, inactivation of the gene in a mouse pigment cell line results in a significant reduction in metabolites that are important for the synthesis of pheomelanin. We hypothesize that COMTD1 activity protects pigment cells from oxidative stress and that inactivation of this function impairs the production of pheomelanin. It is likely that COMTD1 has a similar function in other cell types. This study establishes this chicken mutation as a model for further studies of COMTD1 function.

Funding: The study was funded by the Swedish Research Council (2017-02907), the Knut and Alice Wallenberg Foundation (KAW 2016.0361) (to LA), and the National Institute for Arthritis, Musculoskeletal and Skin Diseases of the United States National Institutes of Health (R01 AR076241) (to MSM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

In the present study we present genetic data demonstrating that the IG phenotype shows complete concordance with a frame-shift mutation in the catechol-O-methyltransferase containing domain 1 gene (COMTD1). COMTD1 is associated with mitochondrial fractions of several cell types [ 24 ]. COMTD1 is ubiquitously expressed in human and mouse [ 25 ] but is expressed most highly in gall bladder, small intestine, parathyroid gland, and renal tubes of the kidney ( http://www.proteinatlas.org/ENSG00000165644 ). The crystal structure of the fragment of human COMTD1 lacking its N-terminal transmembrane domain (Protein Database accession number: 2AVD) suggests that it forms a dimer and binds the co-factor S-adenosyl-methionine (SAM). COMTD1 is predicted to contain an O-methyltransferase domain and shows strong sequence similarity with the well-characterized catechol-O-methyltransferase (COMT). COMT is an O-methyltransferase that catalyzes the transfer of a methyl group from SAM [ 26 ] to a wide number of catechol-substrates including neurotransmitters [ 27 ], catechol estrogens [ 28 ], ascorbic acid [ 29 ], catecholic xenobiotics [ 30 ], and dihydroxyindole intermediates of melanin [ 31 ]. Comt-deficient mice show dramatic shifts in catecholamine metabolites [ 32 – 34 ]. COMT has been reported to play a role in oxidative stress pathways, mainly through the regulation of ROS levels. Knockdown of COMT expression in human endometrial glandular cells increased the propensity of estradiol or catecholestrogens to induce ROS [ 35 ]. Inadequate COMT activity in substantia nigra pars compacta results in increased dopamine and its catechol-containing metabolites, which consequently lead to oxidative damage to neuronal cells [ 36 ]. COMT has been suggested to protect melanocytes from the toxic indole intermediates produced during melanogenesis [ 37 – 43 ], and in vitro COMT catalyzes O-methylation of the melanin substrate L-Dopa to 3-O-methoxytyrosine [ 44 ]. Our data suggest that COMTD1 might also regulate melanogenesis, but that its function is limited to mitochondria and not to melanosomes in which L-Dopa is generated. Moreover, we document a complex interplay between melanogenesis and cellular metabolism, suggesting that both COMT and COMTD1 protect melanocytes from oxidative stress.

The birds in (A) and (B) carry the bottom recessive wheaten allele (Y) at the MC1R locus and shows red pheomelanin-based pigmentation. The birds in (C) and (D) carry the brown allele (B) at the same locus that allows expression of both eumelanin and pheomelanin and IG dilution is apparent as regards pheomelanin pigmentation. (A) and (B) depict F 2 birds from the mapping pedigree with the wild-type phenotype or the recessive IG phenotype (IG/IG), respectively. (C) and (D) depict two IG/IG birds from the Lemon Millefleur Sabelpoot ( Fig 1C ) and Sebright-Lemon ( Fig 1D ) breeds, respectively. Photo by Michèle Tixier-Boichard (A and B) and C and D were taken by Nicolas Bruneau, INRAE.

While the mouse is the most widely-used mammalian model organism for understanding the genetic basis of pigment variation ( http://www.espcr.org/micemut/ ), the chicken is the prime avian model organism, and the immense phenotypic variation in plumage and skin colour within and between chicken breeds has yielded numerous genetic insights into the regulation of pigmentation [ 20 ]. The Inhibitor of gold (IG) or Cream phenotype was first described by Taylor [ 21 ] and later analysed by Punnett [ 22 ]. The autosomal recessive IG allele mainly dilutes the red/pheomelanic components of the plumage with minor effects on black/eumelanic components ( Fig 1 ). A general dilution of pigmentation is present in homozygous IG/IG birds that are also homozygous for the recessive wheaten allele (Y) at the MC1R locus and therefore exhibit only red pheomelanin pigmentation ( Fig 1B ). The phenotype is apparent already at hatch. In contrast, IG birds carrying other MC1R alleles that allow expression of both eumelanin and pheomelanin show a dilution of red pheomelanic pigmentation but no visible dilution of black eumelanic pigmentation ( Figs 1C and 1D ). Male IG chickens display a greater dilution of pheomelanin pigmentation than IG females, suggesting the involvement of sex-specific factors. The IG allele is fairly widespread among chicken breeds around the world including some White Leghorn lines [ 23 ], some broiler lines, Lemon Millefleur Sabelpoot ( Fig 1C ), Sebright-Lemon ( Fig 1D ), and Lemon Spangled Hamburg. However, the mutated gene responsible for the IG phenotype has not yet been identified.

Melanins are produced within epidermal melanocytes within membrane-bound compartments called melanosomes [ 6 , 7 ]. The rate-limiting reaction in melanogenesis, the oxidation of L-tyrosine to dopaquinone (DQ), is catalysed by tyrosinase. A non-functional tyrosinase leads to disrupted synthesis of melanin and is associated with oculocutaneous albinism type 1 [ 8 ]. Two tyrosinase-related proteins, TYRP1 and TYRP2/dopachrome tautomerase (DCT), also contribute to the synthesis of eumelanins. By contrast, pheomelanins are thought to be spontaneously produced from L-tyrosine in the presence of the amino acid cysteine, low levels of tyrosinase, and a lower pH [ 9 – 11 ]. During melanin synthesis, superoxide anion (O 2 - ) and hydrogen peroxide (H 2 O 2 ) are generated at various steps, potentially exposing melanocytes to oxidative stress. Indeed, excessive ROS production from either endogenous or exogenous sources can disrupt melanocyte homeostasis, compromising their survival or leading to their malignant transformation [ 12 , 13 ]. In most cell types, ROS are mainly generated in the oxidative reaction process of mitochondrial respiratory chain as by-products of normal cellular metabolism [ 14 ]. Modification of the redox state and increased ROS production within mitochondria have major consequences for both mitochondrial and extramitochondrial processes. Disrupted cell proliferation is one of the consequences of mitochondrial stress, as mitochondria are the primary source of energy production for fundamental cellular reactions [ 15 , 16 ]. In addition, accumulated ROS resulting from mitochondrial stress can cause DNA damage and disrupt signalling pathways that regulate cell proliferation [ 17 , 18 ]. Additionally, mitochondrial fission activity is associated with cell proliferation [ 19 ].

Deposition of melanins in skin, hair, or feathers is essential for pigmentation in birds and mammals [ 1 ]. Melanins come in two types: black/brown eumelanins and yellow/red pheomelanins. Skin, hair, and feather colour is determined by the amount and ratio of the type of melanin that is deposited [ 2 ]. Pigmentation serves many different functions in animals, such as camouflage, mate attraction, and protection against ultraviolet radiation (UVR). The latter is mainly achieved by eumelanins, whereas pheomelanins have phototoxic properties because they generate reactive oxygen species (ROS) upon UVR exposure [ 3 – 5 ].

Given the alterations in glutathione metabolites observed in Comtd1 KO B16F10 cells and the critical role of glutathione in resistance to oxidative stress, we tested whether COMTD1 might impact cellular responses to stress. Chemical-mediated transfection methods have been reported to induce acute inadvertent toxic effects including oxidative stress [ 52 – 55 ]. We therefore treated wild-type and Comtd1 KO melanocytes with jetPRIME transfection reagent and either empty pcDNA3.1 vector or pcDNA3.1 driving COMTD1 expression, and assessed growth over six days. Transfection of either WT or Comtd1 KO cells with empty vector resulted in a drastically reduced growth rate compared with untreated cells (compare Fig 6E with Fig 8 , filled triangles). By contrast, transfection with the COMTD1 expression vector strikingly restored wild-type growth rates to both WT and KO cells ( Fig 8A and 8B , respectively, empty circles). This indicates that overexpression of COMTD1 mitigates cellular stress caused by chemical transfection.

(A) Schematic representation of possible impact on biosynthesis of pheomelanin by COMTD1. Upregulated metabolites are highlighted in red and downregulated in blue. The solid and dashed lines denote significant and nonsignificant difference, respectively. ( B-D) Mass spectrometric peak intensity of the corresponding metabolites in wild-type and knockout cell lines. Data are presented as mean ± SD from experimental replicates (N = 6 in KO; N = 12 in WT). ns: not significant, P > 0.05; *: P < 0.05; **: P <0.01; ***: P <0.001. Peak intensity of metabolites was acquired by UPLC-MS analysis.

To determine if COMTD1 impacts mitochondrial metabolism, we used ultra-high pressure liquid chromatography fractionation and mass spectrometry (UPLC-MS) to compare the metabolic signatures of WT and Comtd1 KO B16F10 cells. Unsupervised multivariate analysis (PCA) was used to explore potential metabolic differences associated with Comtd1 inactivation. Additionally, supervised multivariate analysis (PLS-DA) revealed a clear distinction in the metabolome of WT and Comtd1 KO cells after cross validation (CV) of the model ( S3A–S3D Fig ). Metabolic pathway analysis documented significant differences in multiple metabolic pathways ( S3E Fig ), particularly those that impact responses to oxidative stress including glutamate/glutathione metabolism, riboflavin metabolism, and the tricarboxylic acid (TCA) cycle ( Fig 7 ). Furthermore, we noted up-regulated levels of biochemical indicators of cell redox balance, glutathione (GSH) and glutathione disulfide (GSSG) ( Fig 7A ). The two upstream metabolites in glutathione biosynthesis, glutamine and glutamate, are also significantly up-regulated in the Comtd1-deleted cell lines ( Fig 7B and S5 Table ). The significant down-regulation of cysteinyldopa ( Fig 7B and S5 Table ), an intermediate in pheomelanin synthesis, is of particular interest in relation to the IG phenotype. The majority of metabolites involved in the Krebs cycle were significantly reduced in the Comtd1 KO cells except succinate, which showed the opposite trend ( Fig 7C and S5 Table ), indicating impaired entry into the Krebs cycle and a block in succinate dehydrogenase—the only enzyme in the pathway that requires reduction of oxidized FAD to FADH2. This is consistent with the large reduction in riboflavin and FMN ( Fig 7D ), both precursors of FAD, and suggest a defect in mitochondrial function. In addition, the content of riboflavin, a key component of the mitochondrial respiratory chain (RC), was 3.4-fold reduced in Comtd1 KO cells relative to wild-type cells ( Fig 7D ).

(A) Schematic description of CRISPR/Cas9 mediated inactivation of murine Comtd1. Black bars indicate the target sites of two gRNAs in the exons of Comtd1. Yellow arrow indicates a 236bp deletion introduced by paired sgRNAs. The primer pair indicated by red arrows is used for amplifying genomic DNA. (B) Six colonies retrieved from Comtd1 knockout in B16F10 cells. The PAM sequence is in red. Dash (-) indicates deleted nucleotides. The top one is wild type and the others are KO clones, of which, three lines (KO1, KO2, KO3) were generated using sgRNA1, one line was generated using sgRNA2, and the last two lines carrying 236bp deletion were generated by the sgRNA pair. (C) Western blot analysis of whole-cell lysates prepared from six COMTD1 knockout clones and one WT cell line using antibodies against COMTD1 and the control β-actin. (D) Quantitative RT-PCR analysis of Comtd1 expression in KO and WT cell lines. Date are presented as mean ± SD (n = 3 biological replicates). (E) Cell growth curves of KO (grey) and WT (black) were recorded by the Incucyte Zoom live-cell imaging system and data are expressed as cell confluence (%; mean ± SEM, n = 6 in KO, n = 3 in WT).

To assess the impact of COMTD1 on cellular metabolism and explore links between metabolism and melanogenesis, we used CRISPR/Cas9-mediated gene targeting to generate a Comtd1 knockout (KO) in the pigmented mouse B16F10 melanoma cell line ( Fig 6A ). We targeted exons 3 and 4 that are present in both chicken COMTD1 transcripts and that are expected to encode part of the O-methyltransferase domain. Cells were transfected with Cas9 and either a single sgRNA or paired-gRNAs to generate different knockout clones. PCR screening of B16F10 clones revealed four clones carrying deletions around either the sgRNA1 or sgRNA2 targeting site from cells transfected with single sgRNAs and two clones carrying a 236 bp deletion between sgRNA1 and sgRNA2 from cells transfected with both sgRNAs. Comtd1 inactivation in these clones was validated by Sanger sequencing, RT-qPCR analysis, and immunoblotting ( Fig 6B–6D ). Growth curves of WT and KO cell lines in standard culture medium over six days did not differ significantly ( Fig 6E ), suggesting that loss of Comtd1 does not substantially impact cell viability, in accordance with the phenotype of Comt-deficient mice [ 51 ].

(A-D) Immortalized melan-Ink4a cells from Ink4a-deficient C57BL/6J mice were transiently transfected to express COMTD1 fused with the HA11 epitope at either the N-terminus (HA-COMTD1; A, C) or C-terminus (COMTD1-HA; B, D). Two days later, cells were fixed and analyzed by bright field (BF) and immunofluorescence microscopy for HA and either the mitochondrial resident protein MAVS (A, B) or the ER resident protein calnexin (CNX; C, D). Individual images of labelled cells or the bright field image are shown in addition to an overlay of HA (green) with MAVS (red; HA/ MAVS), CNX (red; HA/ CNX), or the pseudocolored bright field image (magenta; HA/BF). Insets show a 5-fold magnified image of the boxed region to emphasize overlap or lack thereof. Main scale bar, 10 μm; inset scale bar, 2 μm. (E) Quantification of the degree of overlap of COMTD1-HA or HA-COMTD1, as indicated, with markers of the ER (CNX; N = 29 for COMTD1-HA, N = 17 for HA-COMTD1), mitochondria (MAVS; N = 25 for COMTD1-HA, N = 16 for HA-COMTD1), mature melanosomes (TYRP1; N = 16), immature melanosomes (PMEL; N = 17), late endosomes/ lysosomes (LAMP2; N = 15), or early endosomes (STX13; N = 21). Data from 4–5 individual experiments are presented as a box and whiskers plot in which the area of overlap is shown relative to the total area occupied by HA (e.g., CNX vs. HA) or by the indicated marker (e.g., HA vs. CNX). See S2 Fig for examples of the data for TYRP1, PMEL, LAMP2 and STX13. Statistical significance was determined by ordinary one-way ANOVA with Tukey’s tests for multiple comparisons; ****, P < 0.0001.

To define the intracellular localization of COMTD1 in melanocytes, human COMTD1 was fused to the HA11 epitope at either the N-terminus (to generate HA-COMTD1) or the C-terminus (to generate COMTD1-HA), and the tagged proteins were expressed by transient transfection in melan-Ink4a melanocytes, a highly pigmented immortalized melanocyte cell line derived from C57BL/6J-Arf-Ink4a -/- mice. Cells were then analyzed by bright field microscopy (to detect pigment granules) and immunofluorescence microscopy for the HA-tagged proteins and markers of multiple compartments ( Figs 5 and S2 ) . Labelling for the HA epitope fused to COMTD1 at either the N-terminus or C-terminus failed to overlap significantly with pigment granules (1.5 ± 1.3% and 2.2 ± 1.8%, respectively; Fig 5 ) or with markers of mature melanosomes (TYRP1; 8.8 ± 6.3% for COMTD1-HA) or immature melanosomes (PMEL; 6.6 ± 3.8% for COMTD1-HA) ( Figs 5E , S2A , S2B , S2E and S2F ), indicating that COMTD1 is unlikely to localize to melanosomes. Moreover, COMTD1 labelling failed to overlap substantially with labelling for markers of the endoplasmic reticulum (calnexin; 29.3 ± 7.5% for HA-COMTD1, 28.5 ± 8.8% for COMTD1-HA)( Fig 5C – 5E ) late endosomes/ lysosomes (LAMP2; 12.1 ± 5.4% for COMTD1-HA; Figs 5E , S2C , and S2G ), or early endosomes (syntaxin 13; 15.0 ± 6.3% for COMTD1-HA; Figs 5E and S2D , S2H )–all compartments that contribute to assembly, processing, and trafficking of melanosomal proteins [ 6 ]. However, COMTD1, fused to HA at either the N- or C-terminus, overlapped extensively with labelling for the mitochondrial outer membrane protein, MAVS (76.8 ± 5.0% for HA-COMTD1, 83.4 ± 8.7% for COMTD1-HA; Fig 5A , 5 B and 5 E ). These data strongly suggest that COMTD1 localizes predominantly, if not exclusively, to mitochondria, consistent with a tissue-wide mouse mitochondrial proteomics study [ 24 ].

Fig 4C displays the alignment of the COMTD1 protein sequences from eight distantly related organisms including Gallus gallus wild-type COMTD1 and predicted COMTD1 IG1 and COMTD1 IG2 (denoted as Chicken_IG1 and Chicken_IG2, respectively). The Gallus gallus wild-type COMTD1 N protein consists of 260 amino acids with a C-terminal Methyltransf_3 O-methyltransferase domain (based on the NCBI Conserved Domains Database; E-value: 1.7x10 -79 ; pfam:01596). The protein encoded by the COMTD1 IG1 transcript deviates from COMTD1 N at codon 163 and contains a premature stop at codon 201. Thus, the protein encoded by COMTD1 IG1 lacks the C-terminal 98 amino acids present in the predicted wild-type protein, and these are replaced by 38 amino acids due the frameshift. The protein sequence derived from the COMTD1 IG2 transcript aligned well to wild-type protein sequence except for the 4 out-of-frame amino acids derived from exon 5 and the 44 missing amino acids encoded by exon 6. As indicated in Fig 4C , the mutant isoforms lack considerable portions of the O-methyltransferase domain and are therefore predicted to be non-functional.

(A) Agarose gel electrophoresis of COMTD1 RT-PCR products from feather follicles representing all three possible Ig genotypes. The wild-type (N) allele expresses only the full-length transcript while two transcripts is expressed from the IG allele. (B) The annotated COMTD1 gene with the wild-type transcript (COMTD1 N ) together with the two COMTD1 transcripts (COMTD1 IG1 and COMTD1 IG2 ) transcribed from the IG-allele. The bottom track shows vertebrate sequence conservation scores from the USCS browser. (C) Clustal X alignment of predicted COMTD1 sequence in chicken (Gallus gallus)–including the putative protein products of COMTD1 IG1 and COMTD1 IG2 and homologs in zebra finch (Taeniopygia guttata), anolis lizard (Anolis carolinensis), green pufferfish (Tetraodon nigroviridis), dog (Canis familiaris), mouse (Mus musculus), human (Homo sapiens) and salmon (Salmo salar). Dots (.) indicate identity to the full-length chicken master sequence. The arrows indicate amino acids that are in direct contact with the cofactor, SAM, based on the crystal structure model of COMTD1 (Protein Database accession number: 2AVD). The black square indicates the putative O-methyltransferase domain.

RT-PCR analysis of RNA derived from feather follicles representing the three possible genotypes at the IG locus revealed that there are two COMTD1 transcripts associated with the IG allele. A larger transcript of 3,042 bp, referred to as COMTD1 IG1 , is similar in size to the wild-type COMTD1 N transcript (3,040 bp) that encodes the full-length protein. A second transcript of 2,908 bp (COMTD1 IG2 ) is ~100 bp smaller than COMTD1 IG1 and COMTD1 N ( Fig 4A ). Sequencing showed that both COMTD1 IG1 and COMTD1 IG2 contained the 2-bp insertion but that COMTD1 IG2 lacks exon 6 and is thus 134 bp smaller than COMTD1 IG1 ( Fig 4B ).

We also searched for the presence of the 2-bp insertion in COMTD1 using publicly available chicken whole genome sequencing (WGS) data, representing 20 different populations of non-IG birds (displaying red plumage phenotypes), and 28 populations of other plumage colours ( S3 and S4 Tables ). Among the 20 non-IG populations, all individuals were homozygous for the wild-type allele except one red junglefowl and one Red Porcelain Booted Bantam which were both heterozygous for the 2-bp insertion. This observation does not rule out this mutation as being causative since the phenotype is a recessive trait. In addition, the COMTD1 2-bp insertion mutation was present at a high frequency in White Leghorns ( S4 Table ), and may contribute to the pure white phenotype in this breed. This is possible because it is well known that the PMEL mutation underlying dominant white colour in this breed effectively inhibits expression of black eumelanin but is less effective in inhibiting the production of red pheomelanin [ 50 ]. One black Java was homozygous for the 2-bp insertion ( S4 Table ), but since black Javas have an entirely eumelanic plumage we do not expect the effect of the IG mutation to be noted without measuring melanin content.

The 2-bp insertion in COMTD1 was also genotyped in (i) IG birds from five different populations and (ii) wild-type birds representing 7 domestic breeds and three junglefowl species all displaying red plumage phenotypes. This screen revealed a complete concordance between homozygosity for the 2-bp insertion (CT/CT) and the IG phenotype ( Table 3 ). This highly significant association across breeds and complete fixation within breeds for the IG phenotype confirmed that the allele present among this sample of IG birds must have been inherited from a common ancestor. The presence of the CT-allele at a low frequency in the Marans population, showing red plumage, does not rule out this mutation as causative since we cannot exclude the possibility that the recessive IG allele occurs at a low frequency in this breed. One of the SASSO birds expected to show the IG phenotype was heterozygous for the wild-type allele. This could be explained by a rare phenotype misclassification since alleles at other loci may also dilute pigmentation, or to the presence of genetic heterogeneity at this locus such that other COMTD1 mutations with a very similar phenotypic effect may occur in this breed.

The 2-bp insertion (CT allele) was genotyped in a large pedigree of an intercross between a White Leghorn line (Obese Line) and red junglefowl, which segregated at the IG and sex-linked Silver loci. Sex-linked Silver birds were excluded from the analysis through genotyping of the putative Silver mutation in White Leghorns (S*S_WL) [ 49 ]. There was a highly significant correlation between the CT/CT genotype and dilution of pheomelanin (χ 2 = 38.6, df = 1, P = 5.2x 10 −10 ; S2 Table ). None of the 25 CT/CT homozygotes displayed a red plumage while 58 (70.7%) of the 82 birds in the other two genotype classes did. The lack of red plumage in the 29.3% of birds that were not CT/CT likely reflects allelic variation at other loci, such as recessive white, champagne, lavender, and/or Dominant white, which all dilute pheomelanin pigmentation. Thus, these data support the contention that the 2-bp insertion in COMTD1 is causative for the IG phenotype.

All detected polymorphisms were intersected with the three ENSEMBL gene predictions in the IG minimum shared haplotype ( S1 Table ). We also determined the allele frequency of all detected sequence variants among 46 previously sequenced birds all assumed to be wild-type (N/-) at the IG locus. There were only two variants that were not found in the homozygous state in non-IG birds: a synonymous substitution in LRMDA and a 2-bp insertion of CT at nucleotide position 15,675,521 bp in exon 5 of COMTD1. The COMTD1 insertion is predicted to cause a frame shift, and was thus considered to be the strongest candidate mutation to explain the IG allele due to its predicted severity. The synonymous substitution in LRMDA (chr6: 14,861,168 bp) has a negative conservation score (-4.2) based on the 77 vertebrate PhastCons Conserved Elements Sequences, suggesting that it is not in a conserved site and thus may not be functionally important.

To find candidate coding sequence mutations that might result in a loss-of-function responsible for the IG phenotype within the relatively large IBD region associated with IG, we performed whole genome sequencing of an IG/IG homozygote. A 300 bp fragment library was generated and short paired-end reads (100 bp each) were mapped to the chicken galGal6 genome assembly, giving on average 30X coverage. Large structural variants and polymorphic sites, including single nucleotide polymorphisms (SNPs), insertions, and deletions (InDels), were identified in the high confidence minimum shared IG region in comparison to the red junglefowl reference sequence. The causative mutation was assumed to be in the homozygous state in these data due to the recessive mode of inheritance of the IG phenotype. Polymorphisms present in the heterozygous state were excluded from the analyses.

We used an Identical-by-Descent (IBD) mapping approach to refine the localization of the IG locus, under the assumption that most, if not all, IG chromosomes trace back to one common ancestor. We partially re-sequenced the associated 3.71 Mb region in six birds, representing five different IG populations, one of which was an F 2 IG/IG homozygote from the mapping-population. Combining these data with previously published sequence data from birds not carrying the IG allele (red junglefowl and Rhode Island Red) revealed a 432,231 bp minimum shared haplotype (between nucleotide position 15,257,589 and 15,689,820 ) among IG chromosomes ( Table 2 ). An even smaller IBD-region (261,682 bp, between 15,428,138 and 15, 689,820 bp) was supported by one recombinant chromosome found in one Lemon Spangled Hamburg bird. The larger, high confidence, minimum shared region harboured three ENSEMBL protein-coding gene annotations and two lncRNAs ( Fig 3 ). The three protein-coding genes displayed a high sequence similarity to genes in other vertebrates: LRMDA, ZNF503, and COMTD1 (Refseq gene predictions) and were thus considered to be true protein-coding genes. The smaller 261,682 bp IBD region contained only one of these protein-coding genes, COMTD1, and the two lncRNAs.

To identify the genetic region responsible for the IG phenotype, linkage analysis was performed using 159 F 2 individuals from a three-generation intercross segregating at the IG-locus. Close linkage between IG and the genetic marker MCW250 on chicken chromosome 6 at nucleotide position 16,581,142 bp was identified (LOD score = 39.3; θ = 0.1%). The marker AP3UP3_5Mb, located approximately 3.7 Mb away from MCW250, showed complete linkage with IG (LOD score = 49.1; θ = 0.0%; Table 1 ). Addition of more markers in the proximity of AP3UP3_5Mb allowed us to assign the IG locus to a 3.71 Mb region on chicken chromosome 6 defined by the closest flanking markers IG1250Kb and AP3UP500Kb, which showed recombination with the IG locus ( Table 1 ). Further addition of genetic markers within the 3.71 Mb region revealed complete linkage between these markers and IG. This was a much larger region lacking recombination events than expected given the number of informative meioses in this study and the average recombination rate on chicken chromosome 6, which has been estimated to be around 3.0 cM/Mb [ 48 ]. However, a low rate of recombination in this chromosomal region is also evident from the consensus map for chicken [ 48 ]. Thus, the low rate of recombination in the present study does not imply that the IG allele may be associated with a chromosomal rearrangement, e.g., an inversion, which suppresses recombination.

We also compared pigmentation between R+ and R- birds from the Rhode Island Red breed. R+ birds produced nearly two-fold more pheomelanin, both BT-type and BZ-type, as compared to R- birds (P<0.001; Fig 2C ). Variations in melanin content were noted between males and females in the three subgroups of birds (R+, R-, and IG), with females showing higher values ( S1 Fig ).

The melanin content in feathers was analyzed in RIR (homozygous wild-type, N/N) and IG (IG/IG) birds. All birds show the recessive wheaten phenotype (MC1R*Y/Y) and produce predominantly red pheomelanin ( Fig 1A and 1B ). The RIR birds came from two different lines (R+ and R-) that were divergently selected for feed efficiency and that exhibit more and less intensive pheomelanin pigmentation, respectively. Solubilized feathers from the neck of both sexes were analyzed for melanin content by spectrophotometry for absorbance at 500 nm (A500) and 650 nm (A650). The A500 value reflects the total amount of eumelanin and pheomelanin combined (total melanin) while the A650/A500 ratio provides a rough estimate of the ratio of eumelanin to pheomelanin–the lower the ratio, the higher the pheomelanin content [ 45 ]. In IG birds, the level of total melanin was reduced significantly to about 20% that of R- birds and 10% that of R+ birds ( Fig 2A ). The A650/A500 ratio indicates that IG birds show a higher proportion of eumelanin pigmentation compared to the high pheomelanin content of R- and R+ birds ( Fig 2B ), consistent with the severely reduced pheomelanin pigmentation in the IG birds. To better quantify pheomelanin and eumelanin content, products of chemical degradations were analyzed by high-performance liquid chromatography (HPLC). Homogenized feathers in water were subjected to alkaline hydrogen peroxide oxidation (AHPO) [ 46 ] and hydroiodic acid (HI) hydrolysis [ 47 ]; AHPO generates pyrrole-2,3,5-tricarboxylic acid (PTCA) from eumelanin and thiazole-2,4,5-tricarboxylic acid (TTCA) from benzothiazole (BZ)-type pheomelanin [ 46 ], and HI hydrolysis generates 4-amino-3-hydroxyphenylalanine (4-AHP) from benzothiazine (BT)-type pheomelanin. AHPO and HI analysis showed that in IG birds, the levels of eumelanin, BT pheomelanin, and BZ pheomelanin were all reduced significantly to 15%, 20%, and 20% compared to R- birds, respectively ( Fig 2C ). Total melanin values (eumelanin and pheomelanin combined) by HPLC and by spectrophotometry corresponded well. Taken together, these results show that feather content of total melanin and particularly pheomelanin is strongly diminished in IG birds as compared to R- birds and even more strongly as compared to R+ birds.

A three-generation intercross was set up between three Rhode Island Red (RIR) birds carrying the wild-type allele at the IG locus (N) and four individuals from an experimental population fixed for the Inhibitor of Gold allele to produce a pedigree for mapping the IG locus. A total of 413 F 2 chickens were born, of which 83 (20.1%) displayed the dilution of pheomelanin pigmentation caused by IG homozygosity. This represents a minor but significant deviation from the 25% expected based on Mendelian segregation (χ 2 = 5.30, df = 1, P = 0.02).

Discussion

The dilution of red pigmentation in chickens homozygous for the IG mutation has been documented for over 90 years, but the responsible mutation had not been identified. This study provides evidence that the causal mutation for the IG phenotype is a 2-bp insertion in exon 5 of COMTD1, encoding a mitochondrial transmembrane protein with O-methyltransferase activity. The mutation results in a frame shift in the middle of the enzymatic domain, likely inactivating it. As a consequence, mitochondrial metabolism is altered, with a consequent increase in glutathione synthesis and potentially increased susceptibility to oxidative stress. The changes in mitochondrial metabolism affect pheomelanin production in melanosomes, likely indirectly. Together, the data suggest an important role for COMTD1 in controlling the production of oxidative intermediates in melanocytes and thus for controlling melanin production.

Our genetic data showed that the IG locus mapped to a 433 kb interval on chicken chromosome 6 containing three protein-coding genes (LRMDA, ZNF503, and COMTD1) and two lncRNA genes (Fig 3); a single recombinant chromosome further reduced the IBD interval to 262 kb containing only COMTD1. The genetic dissection of the IG locus was hampered by an unusual low recombination rate in this chromosomal region resulting in poor resolution in linkage mapping and a relatively large IBD region. However, both LMRDA and COMTD1 were candidates for a pigmentation phenotype. A nonsense mutation in LMRDA causes oculocutaneous albinism type 7 in humans [56], and knockout of LRMDA in a human melanoma line causes alterations in PMEL processing and in the expression of melanogenic genes [57], suggesting that LRMDA plays an important role in eumelanin synthesis. However, disruption of this gene was not expected to preferentially impair pheomelanin production. COMTD1 was considered an IG candidate because the protein encoded by its paralog, COMT, has been shown in vitro to catalyze O-methylation of the melanin substrate L-Dopa to 3-O-methoxytyrosine [58]. Due to the large IBD interval we restricted our search for a candidate causal mutation to the three protein-coding genes, and identified a 2-bp insertion in exon 5 of COMTD1. While we cannot formally exclude that non-coding changes in the interval cause or contribute to the phenotypic effect, causality of the 2-bp insertion is supported by (i) a complete genotype-phenotype correlation across chicken breeds in which no chicken with a clear non-IG phenotype was homozygous for the 2-bp insertion, and (ii) the dramatic reduction in pheomelanin metabolites detected in mouse B16F10 cells with a targeted inactivation of Comtd1 (Figs 7 and S3). The nature of this mutation and the functional data presented in this study make the inactivation of COMTD1 a plausible causal mutation for the IG phenotype in the chicken. This is the first COMTD1 mutation associated with a phenotypic effect in any organism.

Two different COMTD1 transcripts were detected in IG birds: a full-length transcript (IG1), which is out of frame after the 2 bp insertion in exon 5 leading to a premature stop codon, and an alternative splice form (IG2), which carries four out-of-frame codons from exon 5, lacks exon 6, but contains exon 7 in frame. It is likely that this splice variant is only noted in IG birds because the full-length transcript is degraded by nonsense-mediated RNA decay [59] allowing a rare splice variant to be visible after standard PCR.

COMTD1 orthologs are present in all vertebrates. However, while the O-methyltransferase domain in COMTD1 shows high sequence identity to the related COMT, a molecular function for the COMTD1 protein has not yet been established. Our findings that COMTD1 localizes to mitochondria in immortalized melanocytes (Fig 5) and that inactivation of Comtd1 in a mouse melanoma cell line alters the pattern of mitochondrial metabolites (Fig 7) provides the first evidence that COMTD1 regulates mitochondrial metabolism. Thus, our work adds to the growing list of links between mitochondrial function and melanin production in melanocytes [60–64].

Many genes are known to regulate eumelanin synthesis or the switch between eumelanin and pheomelanin [65], but only a few have been identified that specifically affect pheomelanin synthesis. Mutations in Slc7a11, Ggt1, Mfsd12, Clcn7, Ostm1, Sox10, and several as yet undefined genes have all been shown to reduce pheomelanin pigmentation in mouse with no or modest impact on eumelanin production [10,66–72]. Several of these genes (Slc7a11, Mfsd12, and Ggt1) impact cysteine uptake, intracellular transport, or incorporation into glutathione [10,73,74], and thus might share a mechanistic basis with Comtd1 for their impact on pheomelanin synthesis. Clcn7 and Ostm1 encode the subunits of the CLC7 chloride transporter that impacts lysosomal pH [75], and thus might alter the pH of melanosomes promoting specifically pheomelanin synthesis. In chicken, two different missense mutations in the SLC45A2 gene are associated with a specific inhibition of pheomelanin pigmentation characteristic for the sex-linked Silver phenotype [49], while SLC45A2 mutations in other organisms inhibit both eumelanin and pheomelanin pigmentation [76–78]. Like CLC7, SLC45A2 regulates melanosome pH [79], but how these SLC45A2 mutations in chicken cause this specific effect on pheomelanin production is unclear.

COMTD1 is an O-methyltransferase based on the presence of the conserved O-methyltransferase domain, the binding of S-adenosylmethionine in the crystal-structure (pdb: 2AVD, https://www.rcsb.org/structure/2AVD), and its structural and sequence similarity to COMT. COMT has been implicated the protection of melanocytes from the cytotoxic dihydroxyindole derivatives that are formed during eumelanogenesis by O-methylation [31]; indeed, O-methylated forms of the eumelanin intermediates 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid have been detected in the urine and serum from human melanoma patients and melanoma-bearing mice [44,80–83]. The pheomelanin precursor 5-S-cysteinyldopa is a known target of O-methylation [84] and its presence in urine is a diagnostic marker for human melanoma [85]. COMTD1 may have higher specificity to toxic compounds that are formed during pheomelanin synthesis, and thus may protect pheomelanogenic melanocytes from damage or death; indeed, such an effect might have been missed in our experiments using B16F10 cells, which predominantly generate eumelanin [86]. If this is the case, then one might predict that pheomelanogenic IG birds will have fewer functional melanocytes than wild-type birds.

Inactivation of Comtd1 in mouse melanocytes had a significant effect on cysteine and methionine metabolism (S3 Fig). Thus, mutations in COMTD1 may affect the synthesis of pheomelanin indirectly by regulating the levels of cysteine through the conversion of its cofactor S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH). SAH can in turn either be metabolized to homocysteine and subsequently to cysteine or be converted to methionine. Polymorphisms in human COMT are associated with decreased levels of homocysteine in blood plasma, and extensive usage of COMT inhibitors is associated with low plasma homocysteine levels [87,88]–which positively correlate with cysteine levels in human [89]. It is possible that a non-functional COMTD1 might result in decreased levels of homocysteine and cysteine in the plasma and thereby inhibit pheomelanogenesis due to limited cysteine accessibility in melanocytes. Such a mechanism would be expected to have a more broad impact on cellular physiology, which is not observed in IG birds [90], but may be compensated by other metabolic alterations in specific cell types. By contrast, melanocytes are highly vulnerable to oxidative stress, and thus might be more sensitive to decreased cysteine concentration than other tissues. During periods of oxidative stress, the limited supply of cysteine in melanocytes may be preferably used to generate glutathione and maintain a healthy redox balance at the cost of reduced pheomelanin synthesis; this hypothesis is supported by the elevated GSH level and decreased cysteinyldopa level observed after Comtd1 inactivation in B16F10 cells (Fig 7B and S5 Table).

Further functional studies of the IG mutation are needed to fully understand how the mutation affects the synthesis of pheomelanin. The IG bird may also be used as a model to investigate the importance of COMTD1 in other physiological processes. We noted a 20% reduction of the number of IG/IG homozygotes at birth in the F2 generation used for linkage mapping, 20% compared with the expected 25%. Given that we have documented that the COMTD1 CT insertion shows complete concordance with the IG phenotype and serves as a diagnostic marker for it, we predict that the reduction in live IG/IG homozygotes is not due to incomplete penetrance but rather to mild sub-lethality due to an important role for COMTD1 outside the pigment system.

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