Genetic and molecular analysis of recessive alleles at the pink-eyed dilution (p) locus of the mouse

Identification of Candidate Genes for Red-Eyed (Albinism) Domestic Guppies Using Genomic and Transcriptomic Analyses

Guppies are small tropical fish with brightly colored bodies and variable tail shapes. There are two phenotypes of domestic guppy eye color: red and black. The wild type is black-eyed. The main object of this study was to identify candidate genes for the red-eyed phenotype in domestic guppies. We hope to provide molecular genetic information for the development of new domestic guppy strains. Additionally, the results also contribute to basic research concerning guppies. In this study, 121 domestic guppies were used for genomic analysis (GWAS), and 44 genes were identified. Furthermore, 21 domestic guppies were used for transcriptomic analysis, and 874 differentially expressed genes (DEGs) were identified, including 357 upregulated and 517 downregulated genes. Through GO and KEGG enrichment, we identified some important terms or pathways mainly related to melanin biosynthesis and ion transport. qRT-PCR was also performed to verify the differential expression levels of four important candidate genes (TYR, OCA2, SLC45A2, and SLC24A5) between red-eyed and black-eyed guppies. Based on the results of genomic and transcriptomic analyses, we propose that OCA2 is the most important candidate gene for the red-eyed phenotype in guppies.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Intragenic recombination within the pun allele of the pink-eyed dilution locus in pre-melanocytes and primordial germ cells of embryonic mice treated with N-ethyl-N-nitrosourea

The forward or reverse processes of intragenic recombination (IGR), which occur through the addition or deletion of duplicated homologous exons of the p^un allele in Pun mice, was observed in vivo, after introducing an homozygous p^un allele in a C57BL/6 background. We assessed the frequency of IGR upon N-ethyl-N-nitrosourea (ENU) treatment of pre-melanocytes (PMCs: somatic cells) and primordial germ cells (PGCs: germ cells) of embryonic mice at 10.5 days of development (E10.5). We simultaneously examined IGR and other mutations at the p locus of PMCs responsible for coat color in the offspring obtained by crossing p^un/p^un with p^un/P mice. The frequencies of both spontaneous and ENU-induced IGR were markedly higher than that of the recessive mutation (RM) in PMCs obtained from crossing C57BL/6 and PW strains (Shibuya et al., 1982). ENU also induces IGR at a higher frequency in PGCs at E10.5, which was observed in the next generation. These results indicate that ENU, which preferentially induces gene mutations through base substitution, also induces IGR at a high frequency in the p^un allele in both somatic and germ cells of embryonic mice at the E10.5 developmental stage.

A custom capture sequence approach for oculocutaneous albinism identifies structural variant alleles at the OCA2 locus

Oculocutaneous albinism (OCA) is a heritable disorder of pigment production that manifests as hypopigmentation and altered eye development. Exon sequencing of known OCA genes is unsuccessful in producing a complete molecular diagnosis for a significant number of affected individuals. We sequenced the DNA of individuals with OCA using short-read custom capture sequencing that targeted coding, intronic, and noncoding regulatory regions of known OCA genes, and genome-wide association study-associated pigmentation loci. We identified an OCA2 complex structural variant (CxSV), defined by a 143 kb inverted segment reintroduced in intron 1, upstream of the native location. The corresponding CxSV junctions were observed in 11/390 probands screened. The 143 kb CxSV presents in one family as a copy number variant duplication for the 143 kb region. In the remaining 10/11 families, the 143 kb CxSV acquired an additional 184 kb deletion across the same region, restoring exons 3-19 of OCA2 to a copy-number neutral state. Allele-associated haplotype analysis found rare SNVs rs374519281 and rs139696407 are linked with the 143 kb CxSV in both OCA2 alleles. For individuals in which customary molecular evaluation does not reveal a biallelic OCA diagnosis, we recommend preliminary screening for these haplotype-associated rare variants, followed by junction-specific validation for the OCA2 143 kb CxSV.

Feather follicles transcriptome profiles in Bashang long-tailed chickens with different plumage colors

Despite the rich variety in plumage color found in nature, genetic studies on how feather follicles affect pigmentation are often limited to animals that have black and white pigment. To test how gene expression influences plumage color, transcriptomes of chicken feather follicles with white, black, hemp, reed catkins, silvery grey, and landscape plumage colors were generated using Illumina sequencing. We generated six RNA-Seq libraries with over 25 million paired-end clean reads per library with percentage of paired-end clean reads ranging from 96.73 to 96.98%. 78% of the reads mapped to the chicken genome, and approximately 70% of the reads were mapped to exons and 6% mapped to introns. Transcriptomes of feather follicles producing hemp and land plumage were similar, but these two showed moderate differences compared with gray and reed colored plumage. The black and white follicle transcriptomes were most divergent from the other colors. We identified several candidate genes, including GPNMB, PMEL, TYRP1, GPR143, OCA2, SOX10, SLC45A2, KRT75, and TYR. All of these genes are known to induce pigment formation in mice. White feathers result from the lack of pigment formation, and our results suggest that the white chickens due to the recessive insertion mutation of TYR. The formation of black area size and color depth may be due to the expression levels of GPNMB, PMEL, TYRP1, GPR143, OCA2, SOX10, SLC45A2, KRT75, and TYR. The GO analysis of the differentially expressed genes (DEGs) revealed that DEGs in our transcriptome analysis were enriched in cytoskeleton and cell structure related pathways. The black plumage transcriptome showed significant differences in melanogenesis, tyrosine metabolism, and riboflavin metabolism compared with transcriptomes of other plumage colors. The transcriptome profiles of the different chicken plumage colors provide a valuable resource to understand how gene expression influences plumage color, and will be an important resource for identifying candidate genes in breeding programs.

Amelanism in the corn snake is associated with the insertion of an LTR-retrotransposon in the OCA2 gene

The corn snake (Pantherophis guttatus) is a new model species particularly appropriate for investigating the processes generating colours in reptiles because numerous colour and pattern mutants have been isolated in the last five decades. Using our captive-bred colony of corn snakes, transcriptomic and genomic next-generation sequencing, exome assembly, and genotyping of SNPs in multiple families, we delimit the genomic interval bearing the causal mutation of amelanism, the oldest colour variant observed in that species. Proceeding with sequencing the candidate gene OCA2 in the uncovered genomic interval, we identify that the insertion of an LTR-retrotransposon in its 11^th intron results in a considerable truncation of the p protein and likely constitutes the causal mutation of amelanism in corn snakes. As amelanistic snakes exhibit white, instead of black, borders around an otherwise normal pattern of dorsal orange saddles and lateral blotches, our results indicate that melanocytes lacking melanin are able to participate to the normal patterning of other colours in the skin. In combination with research in the zebrafish, this work opens the perspective of using corn snake colour and pattern variants to investigate the generative processes of skin colour patterning shared among major vertebrate lineages.

Genetic variation in regulatory DNA elements: the case of OCA2 transcriptional regulation

Mutations within the OCA2 gene or the complete absence of the OCA2 protein leads to oculocutaneous albinism type 2. The OCA2 protein plays a central role in melanosome biogenesis, and it is a strong determinant of the eumelanin content in melanocytes. Transcript levels of the OCA2 gene are strongly correlated with pigmentation intensities. Recent studies demonstrated that the transcriptional level of OCA2 is to a large extent determined by the noncoding SNP rs12913832 located 21.5 kb upstream of the OCA2 gene promoter. In this review, we discuss current hypotheses and the available data on the mechanism of OCA2 transcriptional regulation and how this is influenced by genetic variation. Finally, we will explore how future epigenetic studies can be used to advance our insight into the functional biology that connects genetic variation to human pigmentation.

Pleiotropic effects of coat colour-associated mutations in humans, mice and other mammals

The characterisation of the pleiotropic effects of coat colour-associated mutations in mammals illustrates that sensory organs and nerves are particularly affected by disorders because of the shared origin of melanocytes and neurocytes in the neural crest; e.g. the eye-colour is a valuable indicator of disorders in pigment production and eye dysfunctions. Disorders related to coat colour-associated alleles also occur in the skin (melanoma), reproductive tract and immune system. Additionally, the coat colour phenotype of an individual influences its general behaviour and fitness. Mutations in the same genes often produce similar coat colours and pleiotropic effects in different species (e.g., KIT [reproductive disorders, lethality], EDNRB [megacolon] and LYST [CHS]). Whereas similar disorders and similar-looking coat colour phenotypes sometimes have a different genetic background (e.g., deafness [EDN3/EDNRB, MITF, PAX and SNAI2] and visual diseases [OCA2, RAB38, SLC24A5, SLC45A2, TRPM1 and TYR]). The human predilection for fancy phenotypes that ignore disorders and genetic defects is a major driving force for the increase of pleiotropic effects in domestic species and laboratory subjects since domestication has commenced approximately 18,000 years ago.

Gabra5-gene haplotype block associated with behavioral properties of the full agonist benzodiazepine chlordiazepoxide

The gabra5 gene is associated with pharmacological properties (myorelaxant, amnesic, anxiolytic) of benzodiazepines. It is tightly located (0.5 cM) close to the pink-eyed dilution (p) locus which encodes for fur color on mouse chromosome 7. We tested the putative role of the gabra5 gene in pharmacological properties of the full non specific agonist chlordiazepoxide (CDP), using behavioral and molecular approaches in mutated p/p mice and wild type F2 from crosses between two multiple markers inbred strain ABP/Le and C57BL/6By strain. From our results, using rotarod, light-dark box, elevated maze and radial arm maze tests, we demonstrate that p/p mice are more sensitive than WT to the sensory motor, anxiolytic and amnesic effect of CDP. This is associated with the presence of a haplotypic block on the murine chromosome 7 and with an up regulation of gabra5 mRNAs in hippocampi of p/p F2 mice.

The effect of dose rate on the frequency of specific-locus mutations induced in mouse spermatogonia is restricted to larger lesions; a retrospective analysis of historical data

A series of 19 large-scale germ-cell mutagenesis experiments conducted several decades ago led to the conclusion that low-LET radiation delivered to mouse spermatogonia at dose rates of 0.8 R/min and below induced only about one-third as many specific-locus mutations as did single, acute exposures at 24 R/min and above. A two-hit origin of the mutations was deemed unlikely in view of the then prevailing evidence for the small size of genetic lesions in spermatogonia. Instead, the dose-rate effect was hypothesized to be the result of a repair system that exists in spermatogonia, but not in more mature male reproductive cells. More recent genetic and molecular studies on the marker genes have identified the phenotypes associated with specific states of the mutant chromosomes, and it is now possible retrospectively to classify individual past mutations as "large lesions" or "other lesions". The mutation-frequency difference between high and low dose rates is restricted to the large lesion mutations, for which the dose-curve slopes differ by a factor exceeding 3.4. For other lesion mutations, there is essentially no difference between the slopes for protracted and acute irradiations; induced other lesions frequencies per unit dose remain similar for dose rates ranging over more than 7 orders of magnitude. For large lesions, these values rise sharply at dose rates >0.8 R/min, though they remain similar within the whole range of protracted doses, failing to provide evidence for a threshold dose rate. The downward bend at high doses that had been noted for X-ray-induced specific-locus mutations as a whole and ascribed to a positive correlation between spermatogonial death and mutation load is now found to be restricted to large lesion mutations. There is a marked difference between the mutation spectra (distributions among the seven loci) for large lesions and other lesions. Within each class, however, the spectra are similar for acute and protracted irradiation.

Colours of domestication

During the last decade, coat colouration in mammals has been investigated in numerous studies. Most of these studies addressing the genetics of coat colouration were on domesticated animals. In contrast to their wild ancestors, domesticated species are often characterized by a huge allelic variability of coat-colour-associated genes. This variability results from artificial selection accepting negative pleiotropic effects linked with certain coat-colour variants. Recent studies demonstrate that this selection for coat-colour phenotypes started at the beginning of domestication. Although to date more than 300 genetic loci and more than 150 identified coat-colour-associated genes have been discovered, which influence pigmentation in various ways, the genetic pathways influencing coat colouration are still only poorly described. On the one hand, similar coat colourations observed in different species can be the product of a few conserved genes. On the other hand, different genes can be responsible for highly similar coat colourations in different individuals of a species or in different species. Therefore, any phenotypic classification of coat colouration blurs underlying differences in the genetic basis of colour variants. In this review we focus on (i) the underlying causes that have resulted in the observed increase of colour variation in domesticated animals compared to their wild ancestors, and (ii) the current state of knowledge with regard to the molecular mechanisms of colouration, with a special emphasis on when and where the different coat-colour-associated genes act.

Mouse mutagenesis and disease models for neuropsychiatric disorders

In this chapter, mutant mouse resources which have been developed by classical genetics as well as by modern large-scale mutagenesis projects are summarized. Various spontaneous and induced mouse mutations have been archived since the rediscovery of Mendel's genetics in 1900. Moreover, genome-wide, large-scale mutagenesis efforts have recently been expanding the available mutant mouse resources. Forward genetics projects using ENU mutagenesis in the mouse were started in the mid-1990s. The widespread adoption of reverse genetics, using knockouts and conditional mutagenesis based on gene-targeting technology, followed. ENU mutagenesis has now evolved to provide a further resource for reverse genetics, with multiple point mutations in a single gene and this new approach is described. Researchers now have various options to obtain mutant mice: point mutations, transgenic mouse strains, and constitutional or conditional knockout mice. The established mutant strains have already contributed to modeling human diseases by elucidating the underlying molecular mechanisms as well as by providing preclinical applications. Examples of mutant mice, focusing on neurological and behavioral models for human diseases, are reviewed. Human diseases caused by a single gene or a small number of major genes have been well modeled by corresponding mutant mice. Current evidence suggests that quantitative traits based on polygenes are likely to be associated with a range of psychiatric diseases, and these are now coming within the range of modeling by mouse mutagenesis.

Functional interactions between OCA2 and the protein complexes BLOC-1, BLOC-2, and AP-3 inferred from epistatic analyses of mouse coat pigmentation

The biogenesis of melanosomes is a multistage process that requires the function of cell-type-specific and ubiquitously expressed proteins. OCA2, the product of the gene defective in oculocutaneous albinism type 2, is a melanosomal membrane protein with restricted expression pattern and a potential role in the trafficking of other proteins to melanosomes. The ubiquitous protein complexes AP-3, BLOC-1, and BLOC-2, which contain as subunits the products of genes defective in various types of Hermansky-Pudlak syndrome, have been likewise implicated in trafficking to melanosomes. We have tested for genetic interactions between mutant alleles causing deficiency in OCA2 (pink-eyed dilution unstable), AP-3 (pearl), BLOC-1 (pallid), and BLOC-2 (cocoa) in C57BL/6J mice. The pallid allele was epistatic to pink-eyed dilution, and the latter behaved as a semi-dominant phenotypic enhancer of cocoa and, to a lesser extent, of pearl. These observations suggest functional links between OCA2 and these three protein complexes involved in melanosome biogenesis.

HERC2 is an E3 ligase that targets BRCA1 for degradation

The breast cancer suppressor BRCA1 forms a stable heterodimeric E3 ubiquitin ligase with BARD1. Each protein controls the abundance and stability of the other, and loss of the interaction leads to BRCA1 degradation. Here, we show that HERC2, a protein recently implicated in DNA damage repair, targets BARD1-uncoupled BRCA1 for degradation. HERC2 shuttles between the nucleus and the cytoplasm. Its COOH-terminal HECT-containing domain interacts with an NH(2)-terminal degron domain in BRCA1. HERC2 ubiquitinates BRCA1; this reaction depends on Cys(4762) of HERC2, the catalytic ubiquitin binding site, and the degron of BRCA1. The HERC2-BRCA1 interaction is maximal during the S phase of the cell cycle and rapidly diminishes as cells enter G(2)-M, inversely correlated with the steady-state level of BRCA1. Significantly, HERC2 depletion antagonizes the effects of BARD1 depletion by restoring BRCA1 expression and G(2)-M checkpoint activity. Conversely, BARD1 protects BRCA1 from HERC2-mediated ubiquitination. Collectively, our findings identify a function for HERC2 in regulating BRCA1 stability in opposition to BARD1. The HERC2 expression in breast epithelial cells and breast carcinomas suggests that this mechanism may play a role in breast carcinogenesis.

Predicting phenotype from genotype: normal pigmentation

Genetic information in forensic studies is largely limited to CODIS data and the ability to match samples and assign them to an individual. However, there are circumstances, in which a given DNA sample does not match anyone in the CODIS database, and no other information about the donor is available. In this study, we determined 75 SNPs in 24 genes (previously implicated in human or animal pigmentation studies) for the analysis of single- and multi-locus associations with hair, skin, and eye color in 789 individuals of various ethnic backgrounds. Using multiple linear regression modeling, five SNPs in five genes were found to account for large proportions of pigmentation variation in hair, skin, and eyes in our across-population analyses. Thus, these models may be of predictive value to determine an individual's pigmentation type from a forensic sample, independent of ethnic origin.

Oculocutaneous albinism in Suncus murinus: establishment of a strain and identification of its responsible gene

The house musk shrew Suncus murinus (Insectivora, Soricidae) is referred to as suncus in a laboratory context. Although the capture of albino-like shrews (wild suncus) has been reported previously, albino-like strains have never been established, and the molecular basis of the character has remained elusive. We have established an OCAO mutant strain (oculocutaneous albinism Okinawa), from a wild suncus with a white coat and red eyes, which was captured in 2002. During the course of establishing the strain, it was revealed that the albino-like phenotype was inherited in an autosomal recessive manner. To elucidate the molecular basis of this phenotype, we cloned the suncus cDNAs for tyrosinase (Tyr), pink-eyed dilution (p), and solute carrier family 45, member 2 (Slc45a2), since these genes are involved in oculocutaneous albinism in various species, including humans. Several polymorphisms were identified in these genes; however, linkage analysis excluded the involvement of Tyr and p. On the other hand, two amino acid substitutions (V240A and G366E) were identified in Slc45a2 that cosegregated with the phenotype in the OCAO mutant strain. While V240A was also present in colored suncus collected from Okinawa, G366E was unique to the albino-like suncus and heterozygous carriers. Thus, we conclude that a mutation in Slc45a2 (G366E) is responsible for an albino-like phenotype in Suncus murinus.

A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color

We have previously demonstrated that haplotypes of three single nucleotide polymorphisms (SNPs) within the first intron of the OCA2 gene are extremely strongly associated with variation in human eye color. In the present work, we describe additional fine association mapping of eye color SNPs in the intergenic region upstream of OCA2 and within the neighboring HERC2 (hect domain and RLD2) gene. We screened an additional 92 SNPs in 300-3000 European individuals and found that a single SNP in intron 86 of HERC2, rs12913832, predicted eye color significantly better (ordinal logistic regression R(2) = 0.68, association LOD = 444) than our previous best OCA2 haplotype. Comparison of sequence alignments of multiple species showed that this SNP lies in the center of a short highly conserved sequence and that the blue-eye-associated allele (frequency 78%) breaks up this conserved sequence, part of which forms a consensus binding site for the helicase-like transcription factor (HLTF). We were also able to demonstrate the OCA2 R419Q, rs1800407, coding SNP acts as a penetrance modifier of this new HERC2 SNP for eye color, and somewhat independently, of melanoma risk. We conclude that the conserved region around rs12913832 represents a regulatory region controlling constitutive expression of OCA2 and that the C allele at rs12913832 leads to decreased expression of OCA2, particularly within iris melanocytes, which we postulate to be the ultimate cause of blue eye color.

p21 controls patterning but not homologous recombination in RPE development

p21/WAF1/CIP1/MDA6 is a key cell cycle regulator. Cell cycle regulation is an important part of development, differentiation, DNA repair and apoptosis. Following DNA damage, p53 dependent expression of p21 results in a rapid cell cycle arrest. p21 also appears to be important for the development of melanocytes, promoting their differentiation and melanogenesis. Here, we examine the effect of p21 deficiency on the development of another pigmented tissue, the retinal pigment epithelium. The murine mutation pink-eyed unstable (p(un)) spontaneously reverts to a wild-type allele by homologous recombination. In a retinal pigment epithelium cell this results in pigmentation, which can be observed in the adult eye. The clonal expansion of such cells during development has provided insight into the pattern of retinal pigment epithelium development. In contrast to previous results with Atm, p53 and Gadd45, p(un) reversion events in p21 deficient mice did not show any significant change. These results suggest that p21 does not play any role in maintaining overall genomic stability by regulating homologous recombination frequencies during development. However, the absence of p21 caused a distinct change in the positions of the reversion events within the retinal pigment epithelium. Those events that would normally arrest to produce single cell events continued to proliferate uncovering a cell cycle dysregulation phenotype. It is likely that p21 is involved in controlling the developmental pattern of the retinal pigment. We also found a C57BL/6J specific p21 dependent ocular defect in retinal folding, similar to those reported in the absence of p53.

The rat pink-eyed dilution (p) mutation: an identical intragenic deletion in pink-eye dilute-coat strains and several Wistar-derived albino strains

We identified the rat pink-eyed dilution (p) and pink eye Mishima (p(m)) mutations. The p(m) mutation, which was isolated from a wild rat caught in Mishima Japan in 1961 and is carried in the NIG-III strain, is a splice donor site mutation in intron 5. The p mutation, which was first described in 1914 and is carried in several p/p rats including the RCS and BDV strains, is an intragenic deletion including exons 17 and 18. In addition to RCS and BDV strains, several albino strains, KHR, KMI and WNA, all descendants of albino stock of the Wistar Institute, are homozygous for the p allele. Analyses revealed that the colored p strains and the Wistar-derived albino p strains had the same marker haplotype spanning approximately 4 Mb around the P locus. This indicates that these p strains share a common ancestor and the p allele did not arise independently via recurrent mutations. The historical relationship among the p strains suggests that the p deletion had been maintained in stock heterogeneous for the C and P loci and then was inherited independently by the ancestor of the Wistar albino stock and the ancestor of the pink-eyed agouti rats in Europe.

Effect of N-acetyl cysteine on oxidative DNA damage and the frequency of DNA deletions in atm-deficient mice

Ataxia telangiectasia (AT) is a hereditary human disorder resulting in a wide variety of clinical manifestations, including progressive neurodegeneration, immunodeficiency, and high incidence of lymphoid tumors. Cells from patients with AT show genetic instability, hypersensitivity to radiation, and a continuous state of oxidative stress. Oxidative stress and genetic instability, including DNA deletions, are involved in carcinogenesis. We examined the effect of dietary supplementation with the thiol-containing antioxidant N-acetyl-l-cysteine (NAC) on levels of oxidative DNA damage and the frequency of DNA deletions in Atm-deficient (AT-mutated) mice. We confirmed that Atm-deficient mice display an increased frequency of DNA deletions (Bishop et al., Cancer Res 2000;60:395). Furthermore, we found that Atm-deficient mice have significantly increased levels of 8-OH deoxyguanosine, an indication of oxidative DNA damage. Dietary supplementation with NAC significantly reduced 8-OH deoxyguanosine level and the frequency of DNA deletions in Atm-deficient mice. These levels were similar to the levels in wild-type mice. Our findings demonstrate that NAC counteracts genetic instability and suggest that genetic instability may be a consequence of oxidative stress in Atm-deficient mice.

Sequences Associated With Human Iris Pigmentation

To determine whether and how common polymorphisms are associated with natural distributions of iris colors, we surveyed 851 individuals of mainly European descent at 335 SNP loci in 13 pigmentation genes and 419 other SNPs distributed throughout the genome and known or thought to be informative for certain elements of population structure. We identified numerous SNPs, haplotypes, and diplotypes (diploid pairs of haplotypes) within the OCA2, MYO5A, TYRP1, AIM, DCT, and TYR genes and the CYP1A2-15q22-ter, CYP1B1-2p21, CYP2C8-10q23, CYP2C9-10q24, and MAOA-Xp11.4 regions as significantly associated with iris colors. Half of the associated SNPs were located on chromosome 15, which corresponds with results that others have previously obtained from linkage analysis. We identified 5 additional genes (ASIP, MC1R, POMC, and SILV) and one additional region (GSTT2-22q11.23) with haplotype and/or diplotypes, but not individual SNP alleles associated with iris colors. For most of the genes, multilocus gene-wise genotype sequences were more strongly associated with iris colors than were haplotypes or SNP alleles. Diplotypes for these genes explain 15% of iris color variation. Apart from representing the first comprehensive candidate gene study for variable iris pigmentation and constituting a first step toward developing a classification model for the inference of iris color from DNA, our results suggest that cryptic population structure might serve as a leverage tool for complex trait gene mapping if genomes are screened with the appropriate ancestry informative markers.

Mouse models for induced genetic instability at endogenous loci

Exposure to environmental factors and genetic predisposition of an individual may lead individually or in combination to various genetic diseases including cancer. These diseases may be a consequence of genetic instability resulting in large-scale genomic rearrangements, such as DNA deletions, duplications, and translocations. This review focuses on mouse assays detecting genetic instability at endogenous loci. The frequency of DNA deletions by homologous recombination at the pink-eyed unstable (p(un)) locus is elevated in mice with mutations in ATM, Trp53, Gadd45, and WRN genes and after exposure to carcinogens. Other quantitative in vivo assays detecting loss of heterozygosity events, such as the mammalian spot assay, Dlb-1 mouse and Aprt mouse assays, are also reviewed. These in vivo test systems may predict hazardous effects of an environmental agent and/or genetic predisposition to cancer.

Biochemical and recessive visible specific locus responses of C3H/HeH to fractionated, acute radiation

The recessive visible specific locus test has been widely used for many years to investigate the genetic effects of radiation in mice. We devised an electrophoretic-specific locus test so that biochemical mutations leading to alterations in the activity or amount of four enzymes and proteins, as well as charge changes could be detected. We measured the yield of recessive visible and electrophoretic mutations in the same experiment so that a direct comparison of mutation incidence could be made. Dominant visible mutations were also scored. The recessive visible specific locus response of male C3H/HeH to a fractionated dose of 3 + 3 Gy X-irradiation separated by 24 h was similar to that previously reported for the F1 hybrid widely used in mutagenesis studies, and other strains. The response of C3H/HeH was significantly greater for the recessive visible mutations than for the biochemical mutations, supporting the contention that the recessive visible loci are more mutable than others. Mutational analysis of some of the mutants showed that the lesions ranged from a very deletion (30% of chromosome 14 deleted) to a point mutation. The number of loci scored in the electrophoretic test has been reassessed, and it is now considered that six, not four were scored, and this has implications for the calculation of the doubling dose.

Nonneuronal expression of the GABA(A) beta3 subunit gene is required for normal palate development in mice

Cleft palate is one of the most common birth defects in humans, in which both genetic and environmental factors are involved. In mice, loss of the GABA(A) receptor beta3 subunit gene (Gabrb3) or the targeted mutagenesis of the GABA synthetic enzyme (Gad1) leads to cleft palate. These observations indicate that a GABAergic system is important in normal palate development. To determine what cell types, neuronal or nonneuronal, are critical for GABA signaling in palate development, we used the neuron-specific enolase promoter to express the beta3 subunit in Gabrb3 mutant mice. Expression of this construct was able to rescue the neurological phenotype, but not the cleft palate phenotype. Combined with the previous observation demonstrating that ubiquitous expression of the beta3 subunit rescued the cleft palate phenotype, a nonneuronal GABAergic system is implicated in palate development. Using immunohistochemistry, we detected GABA in the developing palate, initially in the nasal aspect of palatal epithelium of the vertical shelves; later in the medial edge epithelium of the horizontally oriented palatal shelves and in the epithelial seam during fusion. Based on these observations, we propose that GABA, synthesized by the palatal epithelium, acts as a signaling molecule during orientation and fusion of the palate shelves.

Functional annotation of mammalian genomic DNA sequence by chemical mutagenesis: a fine-structure genetic mutation map of a 1- to 2-cM segment of mouse chromosome 7 corresponding to human chromosome 11p14-p15

Eleven independent, recessive, N-ethyl-N-nitrosourea-induced mutations that map to a approximately 1- to 2-cM region of mouse chromosome (Chr) 7 homologous to human Chr 11p14-p15 were recovered from a screen of 1,218 gametes. These mutations were initially identified in a hemizygous state opposite a large p-locus deletion and subsequently were mapped to finer genomic intervals by crosses to a panel of smaller p deletions. The 11 mutations also were classified into seven complementation groups by pairwise crosses. Four complementation groups were defined by seven prenatally lethal mutations, including a group (l7R3) comprised of two alleles of obvious differing severity. Two allelic mutations (at the psrt locus) result in a severe seizure and runting syndrome, but one mutation (at the fit2 locus) results in a more benign runting phenotype. This experiment has added seven loci, defined by phenotypes of presumed point mutations, to the genetic map of a small (1-2 cM) region of mouse Chr 7 and will facilitate the task of functional annotation of DNA sequence and transcription maps both in the mouse and the corresponding human 11p14-p15 homology region.

ENU mutagenesis: analyzing gene function in mice

With the completion of the human genome, sequence analysis of gene function will move into the center of future genome research. One of the key strategies for studying gene function involves the genetic dissection of biological processes in animal models. Mouse mutants are of particular importance for the analysis of disease pathogenesis and transgenic techniques, and gene targeting have become routine tools. Recently, phenotype-driven strategies using chemical mutagenesis have been the target of increasing interest. In this review, the current state of ENU mutagenesis and its application as a systematic tool of genome analysis are examined.

Susceptibility of proliferating cells to benzo[a]pyrene-induced homologous recombination in mice

The pink-eyed unstable mutation, p(un), is the result of a 70 kb tandem duplication within the murine pink-eyed, p, gene. Deletion of one copy of the duplicated region by homologous deletion/recombination occurs spontaneously in embryos and results in pigmented spots in the fur and eye. Such deletion events are inducible by a variety of DNA damaging agents, as we have observed previously with both fur- and eye-spot assays. Here we describe a study of the effect of exposure to benzo[a]pyrene (B[a]P) at different times of development on reversion induction in the eye. Previously we, among others, have reported that the retinal pigment epithelium (RPE) displays a position effect variegation phenotype in the pattern of pink-eyed unstable reversions. Following an acute exposure to B[a]P or X-rays on the tenth day of gestation an increased frequency of reversion events was detected in a distinct region of the adult RPE. Examining exposure at different times of eye development reveals that both B[a]P and X-rays result in an increased frequency of reversion events, though the increase was only significant following B[a]P exposure, similar to our previous report limited to exposure on the tenth day of gestation. Examination of B[a]P-exposed RPE in the present study revealed distinct regions where the induced events lie and that the positions of these regions are found at increasing distances from the optic nerve the later the time of exposure. This position effect directly reflects the previously observed developmental pattern of the RPE, namely that cells in the regions most distal from the optic nerve are proliferating most vigorously. The numbers and positions of RPE cells displaying the transformed (pigmented) phenotype strongly advocate the proposal that dividing cells are at highest risk to deletions induced by carcinogens.

Aberrant pH of melanosomes in pink-eyed dilution (p) mutant melanocytes

In past studies, we cloned the mouse p gene and its human homolog P, which is associated with oculocutaneous albinism type 2. Both mouse and human genes are expressed in melanocytes and encode proteins predicted to have 12 membrane-spanning domains with structural homology to known ion transporters. We have also demonstrated that the p protein is localized to the melanosomal membrane and does not function as a tyrosine transporter. In this study, immunohistochemistry and confocal microscopy were used to show that the p protein plays an important role in the generation or maintenance of melanosomal pH. Melanosomes (and their precursor compartments) were defined by antiserum directed against the melanosomal marker tyrosinase related protein 1. Acidic vesicles were identified by 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine incorporation, visualized with anti-dinitrophenol. In C57BL/6+/+ (wild-type) melanocytes, 94.2% of vesicles demonstrated colocalization of tyrosinase related protein 1 and 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine, indicating that almost all melanosomes or their precursors were acidic. By contrast, only 7%-8% of the staining vesicles in p mutant cell lines (pJ/pJ and pcp/p6H) showed colocalization of tyrosinase related protein 1 and 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine. Thus, without a functional p protein, most melanosomes and their precursors are not acidic. As mammalian tyrosinase activity in situ is apparently dependent on low pH, we postulate that in the absence of a low pH environment brought about by ionic transport mediated by the p protein, tyrosinase activity is severely impaired, leading to the minimal production of melanin that is characteristic of p mutants. Additionally (or alternatively), an abnormal pH may also impair the assembly of the normal melanogenic complex.

Sox6 is a candidate gene for p100H myopathy, heart block, and sudden neonatal death

The mouse p locus encodes a gene that functions in normal pigmentation. We have characterized a radiation-induced mutant allele of the mouse p locus that is associated with a failure-to-thrive syndrome, in addition to diminished pigmentation. Mice homozygous for this mutant allele, p(100H), show delayed growth and die within 2 wk after birth. We have discovered that the mutant mice develop progressive atrioventricular heart block and significant ultrastructural changes in both cardiac and skeletal muscle cells. These observations are common characteristics described in human myopathies. The karyotype of p(100H) chromosomes indicated that the mutation is associated with a chromosome 7 inversion. We demonstrate here that the p(100H) chromosomal inversion disrupts both the p gene and the Sox6 gene. Normal Sox6 gene expression has been examined by Northern blot analysis and was found most abundantly expressed in skeletal muscle in adult mouse tissues, suggesting an involvement of Sox6 in muscle maintenance. The p(100H) mutant is thus a useful animal model in the elucidation of myopathies at the molecular level.

Ionizing radiation and genetic risks. X. The potential "disease phenotypes" of radiation-induced genetic damage in humans: perspectives from human molecular biology and radiation genetics

Estimates of genetic risks of radiation exposure of humans are traditionally expressed as expected increases in the frequencies of genetic diseases (single-gene, chromosomal and multifactorial) over and above those of naturally-occurring ones in the population. An important assumption in expressing risks in this manner is that gonadal radiation exposures can cause an increase in the frequency of mutations and that this would result in an increase in the frequency of genetic diseases under study. However, despite compelling evidence for radiation-induced mutations in experimental systems, no increases in the frequencies of genetic diseases of concern or other adverse effects (i.e., those which are not formally classified as genetic diseases), have been found in human studies involving parents who have sustained radiation exposures. The known differences between spontaneous mutations that underlie naturally-occurring single-gene diseases and radiation-induced mutations studied in experimental systems now permit us to address and resolve these issues to some extent. The fact that spontaneous mutations (among which are point mutations and DNA deletions generally restricted to the gene) originate through a number of different mechanisms and that the latter are intimately related to the DNA organization of the genes, are now well-documented. Further, spontaneous mutations include those that cause diseases through loss of function as well as gain of function of genes. In contrast, most radiation-induced mutations studied in experimental systems (although identified through the phenotypes of the marker genes) are predominantly multigene deletions which cause loss of function; the recoverability of an induced deletion in a livebirth seems dependent on whether the gene and the genomic region in which it is located can tolerate heterozygosity for the deletion and yet be compatible with viability. In retrospect, the successful mutation test systems (such as the mouse specific locus test) used in radiation studies have involved genes which are non-essential for survival and are also located in genomic regions, likewise non-essential for survival. In contrast, most of the human genes at which induced mutations have been looked for, do not seem to have these attributes. The inference therefore is that the failure to find induced germline mutations in humans is not due to the resistance of human genes to induced mutations but due to the structural and functional constraints associated with their recoverability in livebirths. Since the risk of inducible genetic diseases in humans is estimated using rates of "recovered" mutations in mice, there is a need to introduce appropriate correction factors to bridge the gap between these rates and the rates at which mutations causing diseases are potentially recoverable in humans. Since the whole genome is the "target" for radiation-induced genetic damage, the failure to find increases in the frequencies of specific single-gene diseases of societal concern does not imply that there are no genetic risks of radiation exposures: the problem lies in delineating the phenotypes of recoverable genetic damage that are recognizable in livebirths. Data from studies of naturally-occurring microdeletion syndromes in humans and those from mouse radiation studies are instructive in this regard. They (i) support the view that growth retardation, mental retardation and multisystem developmental abnormalities are likely to be among the quantitatively more important adverse effects of radiation-induced genetic damage than mutations in a few selected genes and (ii) underscore the need to expand the focus in risk estimation from known genetic diseases (as has been the case thus far) to include these induced adverse developmental effects although most of these are not formally classified as "genetic diseases". (ABSTRACT TRUNCATED)

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

Germline cysts: a conserved phase of germ cell development?

Germ cells in many vertebrate and invertebrate species initiate gametogenesis by forming groups of interconnected cells known as germline cysts. Recent studies using Xenopus, mouse and Drosophila are beginning to uncover the cellular and molecular mechanisms that control germline cyst formation and, in conjunction with morphological evidence, suggest that the process is highly conserved during evolution. This article discusses these recent findings and argues that cysts play an important and general role in germ line development.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Deletion mapping of the head tilt (het) gene in mice: a vestibular mutation causing specific absence of otoliths

Head tilt (het) is a recessive mutation in mice causing vestibular dysfunction. Homozygotes display abnormal responses to position change and linear acceleration and cannot swim. However, they are not deaf. het was mapped to the proximal region of mouse chromosome 17, near the T locus. Here we report anatomical characterization of het mutants and high resolution mapping using a set of chromosome deletions. The defect in het mutants is limited to the utricle and saccule of the inner ear, which completely lack otoliths. The unique specificity of the het mutation provides an opportunity to better understand the development of the vestibular system. Complementation analyses with a collection of embryonic stem (ES)- and germ cell-induced deletions localized het to an interval near the centromere of chromosome 17 that was indivisible by recombination mapping. This approach demonstrates the utility of chromosome deletions as reagents for mapping and characterizing mutations, particularly in situations where recombinational mapping is inadequate.

A very large protein with diverse functional motifs is deficient in rjs (runty, jerky, sterile) mice

Three radiation-induced alleles of the mouse p locus, p6H, p25H, and pbs, cause defects in growth, coordination, fertility, and maternal behavior in addition to p gene-related hypopigmentation. These alleles are associated with disruption of the p gene plus an adjacent gene involved in the disorders listed. We have identified this adjacent gene, previously named rjs (runty jerky sterile), by positional cloning. The rjs cDNA is very large, covering 15,264 nucleotides. The predicted rjs-encoded protein (4,836 amino acids) contains several sequence motifs, including three RCC1 repeats, a structural motif in common with cytochrome b5, and a HECT domain in common with E6-AP ubiquitin ligase. On the basis of sequence homology and conserved synteny, the rjs gene is the single mouse homolog of a previously described five- or six-member human gene family. This family is represented by at least two genes, HSC7541 and KIAA0393, from human chromosome 15q11-q13. HSC7541 and KIAA0393 lie close to, or within, a region commonly deleted in most Prader-Willi syndrome patients. Previous work has suggested that the multiple phenotypes in rjs mice might be due to a common neuroendocrine defect. In addition to this proposed mode of action, alternative functions of the rjs gene are evaluated in light of its known protein homologies.

Imprinting in Angelman and Prader-Willi syndromes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are caused by deficiencies of gene expression from paternal or maternal chromosome 15q11-q13, respectively. Many advances have occurred during the past year. The gene for necdin was mapped in the PWS candidate region and found to be paternally expressed in mouse and human. The bisulfite method for analysis of methylation was established for genomic sequencing and diagnostics, and the methylation of Snrpn was studied in detail in the mouse. A region near the Snrpn promoter was shown to function as a silencer in Drosophila. Point mutations were found in the gene for E6-AP ubiquitin-protein ligase (UBE3A) identifying it as the AS gene, and tissue-specific imprinting (maternal expression) was shown in the human brain and in hippocampal neurons and Purkinje cells in the mouse.

Mice devoid of gamma-aminobutyrate type A receptor beta3 subunit have epilepsy, cleft palate, and hypersensitive behavior

gamma-Aminobutyric acid type A receptors (GABA(A)-Rs) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system. The beta3 subunit is an essential component of the GABA(A)-R in many brain regions, especially during development, and is implicated in several pathophysiologic processes. We examined mice harboring a beta3 gene inactivated by gene targeting. GABA(A)-R density is approximately halved in brain of beta3-deficient mice, and GABA(A)-R function is severely impaired. Most beta3-deficient mice die as neonates; some neonatal mortality, but not all, is accompanied by cleft palate. beta3-deficient mice that survive are runted until weaning but achieve normal body size by adulthood, although with reduced life span. These mice are fertile but mothers fail to nurture offspring. Brain morphology is grossly normal, but a number of behaviors are abnormal, consistent with the widespread location of the beta3 subunit. The mice are very hyperactive and hyperresponsive to human contact and other sensory stimuli, and often run continuously in tight circles. When held by the tail, they hold all paws in like a ball, which is frequently a sign of neurological impairment. They have difficulty swimming, walking on grids, and fall off platforms and rotarods, although they do not have a jerky gait. beta3-deficient mice display frequent myoclonus and occasional epileptic seizures, documented by electroencephalographic recording. Hyperactivity, lack of coordination, and seizures are consistent with reduced presynaptic inhibition in spinal cord and impaired inhibition in higher cortical centers and/or pleiotropic developmental defects.

Genetic approaches to CNS disorders with particular reference to GABAA-receptor mutations

The tools of molecular biology will bring the field of human genetics into a new era by permitting the analysis of the genetic contribution to disease. Most single gene disorders, inherited in a Mendelian fashion, will be molecularly diagnosed. In addition, the genetic susceptibility of common, complex diseases such a schizophrenia can be clarified, even though the conditions are not inherited as Mendelian characteristics. The mapping of the human genome will increase the rate at which new disease genes are identified and isolated. Finally, the development of genetically engineered animal models will help to dissect the steps involved in physiological and pathophysiological processes and thereby enhance our understanding of complex biological systems.

Complementation of hypopigmentation in p-mutant (pink-eyed dilution) mouse melanocytes by normal human P cDNA, and defective complementation by OCA2 mutant sequences

Mutations in the P gene of humans and the homologous p-locus of mice, respectively, result in the homologous disorders oculocutaneous albinism type 2 (OCA2) and pink-eyed dilution. Although clearly required for melanin biosynthesis, the specific function of the P gene product, a melanosomal transmembrane protein expressed in melanocytes of the skin, hair, and eyes, is not yet known. Here we describe lines of immortal melanocytes and melanoblasts from mice of the null genotype p(cp)/p(25H). These p-null melanocytes were severely hypopigmented, although they and the melanoblasts expressed mRNAs for a number of melanosomal proteins. Proliferation of the p-null melanoblasts was normal. Both diploid and immortal p-null melanocytes grew more slowly than wild-type melanocytes, however, and were unusually susceptible to the antibiotic G418; these abnormalities were corrected by culture in high concentrations of L-tyrosine. Transfection of the p-null melanocytes with full-length normal human P cDNA resulted in complementation of deficient melanin biosynthesis and hypopigmentation. In contrast, transfection with mutant human P cDNAs containing amino acid substitutions (A481T, V443I) found in patients with OCA2 resulted in minimal or partial correction, consistent with the corresponding pigmentation phenotypes in patients with these mutations. These results demonstrate the utility of this model system for distinguishing true OCA2 mutations from nonpathologic polymorphisms and for quantitating the effect of these mutations on P function.

Cortical myoclonus in Angelman syndrome

Angelman syndrome (AS) results from lack of genetic contribution from maternal chromosome 15q11-13. This region encompasses three GABAA receptor subunit genes (beta3, alpha5, and gamma3). The characteristic phenotype of AS is severe mental retardation, ataxic gait, tremulousness, and jerky movements. We studied the movement disorder in 11 AS patients, aged 3 to 28 years. Two patients had paternal uniparental disomy for chromosome 15, 8 had a >3 Mb deletion, and 1 had a microdeletion involving loci D15S10, D15S113, and GABRB3. All patients exhibited quasicontinuous rhythmic myoclonus mainly involving hands and face, accompanied by rhythmic 5- to 10-Hz electroencephalographic (EEG) activity. Electromyographic bursts lasted 35 +/- 13 msec and had a frequency of 11 +/- 2.4 Hz. Burst-locked EEG averaging in 5 patients, generated a premyoclonus transient preceding the burst by 19 +/- 5 msec. A cortical spread pattern of myoclonic cortical activity was observed. Seven patients also demonstrated myoclonic seizures. No giant somatosensory evoked potentials or C-reflex were observed. The silent period following motor evoked potentials was shortened by 70%, indicating motor cortex hyperexcitability. Treatment with piracetam in 5 patients significantly improved myoclonus. We conclude that spontaneous, rhythmic, fast-bursting cortical myoclonus is a prominent feature of AS.

Complementation analyses for 45 mutations encompassing the pink-eyed dilution (p) locus of the mouse

The homozygous and heterozygous phenotypes are described and characterized for 45 new pink-eyed dilution (p) locus mutations, most of them radiation-induced, that affect survival at various stages of mouse development. Cytogenetically detectable aberrations were found in three of the new p mutations (large deletion, inversion, translocation), with band 7C involved in each case. The complementation map developed from the study of 810 types of compound heterozygotes identifies five functional units: jls and jlm (two distinct juvenile-fitness functions, the latter associated with neuromuscular defects), pl-1 and pl-2 (associated with early-postimplantation and preimplantation death, respectively), and nl [neonatal lethality associated with cleft palate (the frequency of rare "escapers" from this defect varied with the genotype)]. Orientation of these units relative to genetic markers is as follows: centromere, Gas-2, pl-1, jls, jlm p, nl (equatable to cp 1 = Gabrb3); pl-2 probably resides in the c-deletion complex. pl-1 does not mask preimplantation lethals between Gas2 and p; and no genes affecting survival are located between p and cp1. The alleles specifying mottling or darker pigment (generically, pm and px, respectively) probably do not represent deletions of p-coding sequences but could be small rearrangements involving proximal regulatory elements.

A new spontaneous allele at the pink-eyed dilution (p) locus discovered in Mus musculus castaneus

Mutant mice characterized by a cream coat and pink eyes were spontaneously discovered among the descendants of Indonesian wild mice (Mus musculus castaneus). This mutant phenotype was controlled by a single autosomal recessive gene that was allelic to the pink-eyed dilution (p) gene. The mutant mouse phenotypically resembled the original p mouse which was the first mutant identified at this locus. Nevertheless, these two alleles differed in origin, a previous report suggesting that the original p allele was derived from Japanese wild mice (M. m. molossinus). Thus the symbol pcas (pink-eyed castaneus) was proposed for the present mutation allele.

The pinkeyed-dilution protein and the eumelanin/pheomelanin switch: in support of a unifying hypothesis

The two major types of mammalian melanin are pheomelanin (yellow or red pigment) and eumelanin (black or brown). The agouti (A) and extension (E) loci determine whether follicular melanocytes will deposit pheomelanin or eumelanin within their melanosomes. Mutations at the murine pinkeyed-dilution (P) locus cause a striking reduction in deposition of eumelanic, but not pheomelanic, pigment. The mRNA encoded at the P locus is not expressed in skin that exclusively produces pheomelanic pigment as a result of mutation at the agouti locus. We have suggested, based upon both genetic and biochemical evidence, that three key melanogenic proteins--tyrosinase, tyrosinase-related-protein-1 (TRP-1), and TRP-2, encoded at the albino (C), brown (B), and slaty (Slt) loci, respectively--form a high-molecular-weight "melanogenic complex" within the melanosome. High-molecular-weight forms of tyrosinase, TRP-1 and TRP-2, are absent from eumelanic ocular tissues of p(un)/p(un) mice that fail to produce normal P-locus transcript, even though these mice are genetically normal at the loci that regulate production of the three melanogenic proteins. We have hypothesized that the presence of the p-locus protein is important for the integrity of the melanogenic complex and for the levels of members of the TRP family. We show here that the yellow skins of mice mutant at the agouti or extension loci, as well as the nonyellow skins of pinkeyed-unstable (p(un)/p(un)) mice, demonstrate greatly diminished levels of tyrosinase, TRP-1 and TRP-2, and an absence or markedly decreased proportion of high-molecular-weight forms of melanogenic proteins. We conclude that normal levels of wild-type P-locus protein are necessary for eumelanogenesis and that the absence of this protein may be necessary, but is not sufficient to cause the melanosome to switch to the production of pheomelanin. We discuss the implications of our results in relation to the interacting genetic controls regulating melanogenesis.

Pleiotropy in microdeletion syndromes: neurologic and spermatogenic abnormalities in mice homozygous for the p6H deletion are likely due to dysfunction of a single gene

Variability and complexity of phenotypes observed in microdeletion syndromes can be due to deletion of a single gene whose product participates in several aspects of development or can be due to the deletion of a number of tightly linked genes, each adding its own effect to the syndrome. The p6H deletion in mouse chromosome 7 presents a good model with which to address this question of multigene vs. single-gene pleiotropy. Mice homozygous for the p6H deletion are diluted in pigmentation, are smaller than their littermates, and manifest a nervous jerky-gait phenotype. Male homozygotes are sterile and exhibit profound abnormalities in spermiogenesis. By using N-ethyl-N-nitrosourea (EtNU) mutagenesis and a breeding protocol designed to recover recessive mutations expressed hemizygously opposite a large p-locus deletion, we have generated three noncomplementing mutations that map to the p6H deletion. Each of these EtNU-induced mutations has adverse effects on the size, nervous behavior, and progression of spermiogenesis that characterize p6H deletion homozygotes. Because EtNU is thought to induce primarily intragenic (point) mutations in mouse stem-cell spermatogonia, we propose that the trio of phenotypes (runtiness, nervous jerky gait, and male sterility) expressed in p6H deletion homozygotes is the result of deletion of a single highly pleiotropic gene. We also predict that a homologous single locus, quite possibly tightly linked and distal to the D15S12 (P) locus in human chromosome 15q11-q13, may be associated with similar developmental abnormalities in humans.

A high-resolution map of the brown (b, Tyrp1) deletion complex of mouse chromosome 4

For over 40 years germ-cell mutagenesis experiments have generated many new mutations at the brown (b or Tyrp1) locus on mouse Chromosome (Chr) 4. These mutations, many of which are deletions, were recovered by the specific-locus mutagenesis technique. Previous analysis of a panel of brown deletions, generated at Oak Ridge, has enabled both a preliminary molecular and a functional map around the locus to be generated. We have used a panel of hybrid DNA from 25 Oak Ridge deletions, where the deleted chromosome was heterozygous with a Mus spretus chromosome, to map polymorphic markers including microclones, microsatellites, and cloned DNA markers. We have generated a fine structure map, based on 25 new markers, of an 8.5-cM region surrounding the brown locus. This map will prove useful in future mapping studies of this region and in the isolation of the genes that lie within it.