Direct molecular identification of the mouse pink-eyed unstable mutation by genome scanning

African origin of an intragenic deletion of the human P gene in tyrosinase positive oculocutaneous albinism

Oculocutaneous albinism (OCA) is a genetically heterogeneous hypopigmentation disorder. One of the two major autosomal recessive forms involves the tyrosinase gene (OCA1), while the other form (OCA2) has recently been associated with alterations of the P gene on chromosome 15. OCA2 is about twice as common as OCA1 in African and African-American populations. We now describe an interstitial deletion that removes a single exon of the P gene. In a large family from an inbred population of tri-racial origin, all individuals with OCA2 were found to be homozygous for this allele. Moreover, the same mutant P allele was detected in several unrelated African American individuals with OCA2, but not in Caucasians with OCA2. The detection of the same allele in two unrelated Africans with OCA2 indicates an African origin for this allele.

Genome scanning detects genetic alterations in human ovarian carcinoma

Genome scanning, originally used to detect mouse mutations, is a technique which can rapidly identify differences between genomic DNA samples. The procedure is essentially a high resolution Southern analysis using a probe that hybridizes to a medium copy number (1000-2000 copies per haploid genome) repetitive element naturally dispersed throughout the genome. This technique detects genetic changes (primarily large scale genetic changes, e.g., amplifications and deletions) as differences in hybridization band intensity. The use of a probe derived from an endogenous human retroviral-like repetitive sequence, the RTVL-H element, has made genome scanning in humans feasible. In this report, the genome scanning technique was used to evaluate genomic DNA extracted from 14 frozen ovarian tumors. These included 8 high grade serous cystadenocarcinomas, 2 endometrioid carcinomas, one malignant mixed mullerian tumor, 2 Krukenberg tumors, and one tumor where histological classification was unavailable. Band amplifications were identified in 11 cases, with the most prominent amplifications observed in the high grade serous cystadenocarcinomas. In some of the cases, the amplifications involved bands of identical molecular size suggesting that similar underlying changes occurred in different tumors and are potentially associated with specific histological tumor types or clinical behavior. Band deletions were also observed in one endometrioid tumor where blood leukocyte genomic DNA was available from the same patient, allowing a direct comparison.

A genetic linkage map of the mouse: current applications and future prospects

Technological advances have made possible the development of high-resolution genetic linkage maps for the mouse. These maps in turn offer exciting prospects for understanding mammalian genome evolution through comparative mapping, for developing mouse models of human disease, and for identifying the function of all genes in the organism.

From gene to protein: determination of melanin synthesis

Melanin production in mammals is regulated at a variety of levels (tissue, cellular, and subcellular), and many gene loci are involved in the determination of color patterns directed by the melanocyte. Many of the genes involved in these complex processes have now been cloned, and even the simplest mutation can lead to dramatic changes in the phenotype of the individual. Many, if not all, of the pigment related genes have pleiotropic effects on the development and differentiation of the organism, and perhaps because of this, the melanocyte is evolving as an important model for the study of gene regulation and action at the functional level. In view of the importance of pigmentation as a photoprotective barrier and as a cosmetic factor affecting appearance and social acceptance, the importance of these studies seems destined to increase significantly in the future.

Restriction landmark genomic scanning method and its various applications

We have developed a new genome scanning method (restriction landmark genomic scanning (RLGS), based on the new concept of using restriction enzyme sites as landmarks. RLGS employs direct end labeling of the genomic DNA digested with a restriction enzyme and two-dimensional electrophoresis with high-resolution. Its advantages are: (i) high-speed scanning ability, allowing simultaneous scanning of thousands of restriction landmarks; (ii) extension of the scanning field using different kinds of landmarks in an additional series of electrophoresis; (iii) application to any type of organism because of direct-labeling of restriction enzyme sites and no hybridization procedure; and (iv) reflection of the copy number of the restriction landmark by the spot intensity which enables distinction of haploid and diploid genomic DNAs. The RLGS method has various applications because it can be used to scan for physical genomic DNA states, such as amplification, deletion and methylation. The copy number of the locus of a restriction landmark can be estimated by the spot intensity to find either an amplified or deleted region. The methylation state of genomic DNA can also be discovered by use of a methylation-sensitive restriction enzyme sites as a restriction landmark (restriction landmark genomic scanning for screening methylated sites, RLGS-M). This article introduces the basic principle of RLGS and its applications to the analysis of cancer, mouse mutant DNAs and tissue-specific methylation, showing the usefulness of RLGS for a variety of biological fields.

Beta cell expression of endogenous xenotropic retrovirus distinguishes diabetes-susceptible NOD/Lt from resistant NON/Lt mice

Endogeneous retroviral expression in beta cells is a feature of prediabetes in nonobese diabetic (NOD) mice. The purpose of this study was to characterize the class-specific pattern of retroviral gene expression in NOD/Lt beta cells versus a related, but diabetes-resistant strain, NON/Lt. Electron microscopic comparison of beta cells from both strains indicated low constitutive expression of the intracisternal type A (IAP) retroviral class. However, NOD beta cells, in contrast to NON beta cells, expressed an additional intracisternal retroviral form resembling a type C particle. Antibodies against both IAP and type C were detected in NOD, with the humoral response to type C, but not IAP, preceding decline in beta cell function. RNA was extracted from freshly isolated islets from NOD and NON males. Comparative Northern blot analysis of total type C retroviral gene expression using a gag-pol DNA probe corroborated expression of endogenous type C proviruses in both NOD and NON islet cells and thymus. Use of class-specific retroviral probes identified the class of expressed endogenous retrovirus distinguishing the two inbred strains. The single ecotropic provirus present in both the NOD and NON genome (Emv-30) was not expressed in islets or thymus of either strain. Comparison of endogenous xenotropic provirus content by Southern blot analysis revealed two unique xenotropic loci (Xmv-65, -66) in NOD; 8.4 and 3.0 kb xenotropic envelope (env) RNA transcripts were detected in NOD, but not NON islets and thymus. NON contained three xenotropic loci common to other inbred strains (Xmv-21, -25, and -28). Both strains were partially characterized for content of recombinant (polytropic and modified polytropic) proviruses. IAP RNA expression was common to both NOD and NON islets and hence could not be specifically associated with the unique intracisternal type C particle found in NOD, but not NON beta cells. In conclusion, this study shows that expression of xenotropic type C but not IAP distinguishes retroviral activity in NOD/Lt versus NON/Lt beta cells. The potential pathogenic role of retroviral gene expression in NOD beta cells is discussed.

A gene for the mouse pink-eyed dilution locus and for human type II oculocutaneous albinism

The mouse pink-eyed dilution (p) locus on chromosome 7 is associated with defects of skin, eye and coat pigmentation. Mutations at p cause a reduction of eumelanin (black-brown) pigment and altered morphology of black pigment granules (eumelanosomes), but have little effect on pheomelanin (yellow-red) pigment. We show here that the human complementary DNA DN10, linked to the p locus in mice, identifies the human homologue (P) of the mouse p gene, and appears to encode an integral membrane transporter protein. The expression pattern of this gene in various p mutant mice correlates with the pigmentation phenotype; moreover, an abnormally sized messenger RNA is detected in one mutant, p(un), which reverts to the normal size in p(un) revertants. The human P gene corresponds to the D15S12 locus within the chromosome segment 15q11-q13, which is typically deleted in patients with Prader-Willi and Angelman syndrome (see ref. 5 for review). These disorders are phenotypically distinct, depending on the parent of origin of the deleted chromosome, but both syndromes are often associated with hypopigmentation of the skin, hair and eyes (see ref. 8 for review), and deletion of the P gene may be responsible for this hypopigmentation. In addition, we report a mutation in both copies of the human P gene in one case of tyrosinase-positive (type II) oculocutaneous albinism, recently linked to 15q11-q13 (ref. 9).

High-frequency genetic reversion mediated by a DNA duplication: the mouse pink-eyed unstable mutation

The mouse pink-eyed unstable (p(un)) mutation, affecting coat color, exhibits one of the highest reported reversion frequencies of any mammalian mutation and is associated with a duplication of genomic DNA at the p locus. In this study, genomic clones containing the boundaries of the p(un) duplication were isolated and characterized. The structure of these sequences and their wild-type and revertant counterparts were analyzed by restriction mapping, PCR product analysis, DNA sequence analysis, and pulsed-field gel electrophoresis. DNA from p(un) was distinguished from wild-type and revertant DNA by a head-to-tail tandem duplication of approximately 70 kilobases. No differences were detected between revertant and wild-type DNAs. Thus, the reversion in phenotype of p(un) mice is coupled with the loss of one copy of an approximately 70-kilobase duplicated segment. Testable models are presented to account for p(un) reversion.

Genomic mapping of intracisternal A-particle proviral elements

Intracisternal A-particle (IAP) proviral elements are moderately reiterated and widely dispersed in the mouse genome. Oligonucleotide probes have been derived from three distinctive IAP element subfamilies (LS elements) that are transcriptionally active in normal mouse B- and T-cells. In HindIII digests, LS element-specific oligonucleotides each react with a limited number of restriction fragments that represent junctions between proviral and flanking DNA. These fragments have characteristic strain distribution patterns (SDPs) which are polymorphic in the DNAs of different mouse strains. We have established chromosomal assignments for 44 LS proviral loci by comparing their SDPs with those of known genetic markers in the BXD set of RI mouse strains. Some of the loci have also been scored in the CXB RI set. The IAP LS loci can provide a significant number of markers with a recognized genetic organization to the mouse genome map.

Maternal imprinting of the mouse Snrpn gene and conserved linkage homology with the human Prader-Willi syndrome region

Prader-Willi syndrome (PWS) is associated with paternal gene deficiencies in human chromosome 15q11-13, suggesting that PWS is caused by a deficiency in one or more maternally imprinted genes. We have now mapped a gene, Snrpn, encoding a brain-enriched small nuclear ribonucleoprotein (snRNP)-associated polypeptide SmN, to mouse chromosome 7 in a region of homology with human chromosome 15q11-13 and demonstrated that Snrpn is a maternally imprinted gene in mouse. These studies, in combination with the accompanying human mapping studies showing that SNRPN maps in the Prader-Willi critical region, identify SNRPN as a candidate gene involved in PWS and suggest that PWS may be caused, in part, by defects in mRNA processing.

Molecular characterization of the p(un) allele of the mouse pink-eyed dilution locus

The mouse pink-eyed dilution locus, p, located on chromosome 7, mediates coat and eye color. The human correlate of this gene may underlie some forms of tyrosinase-positive oculocutaneous albinism. Mutations at the p locus result in a reduction in pigmentation of the eyes and coat. Although most mutant p alleles (including all spontaneous mutations) affect only pigmentation, several mutant alleles (all radiation induced) are also associated with a variety of other phenotypes. We have focused our attention on the p(un) mutant allele, a spontaneous mutation, exhibiting one of the highest reversion frequencies reported for a mammalian mutation. Using a new technique, genome scanning, we have cloned fragments of genomic DNA from the p locus that are associated with a DNA duplication in p(un) DNA. These fragments can now be used to locate the p gene-encoding sequences and aid in the molecular characterization of complex mutant p alleles.

The mouse pink-eyed dilution gene: association with human Prader-Willi and Angelman syndromes

Complementary DNA clones from the pink-eyed dilution (p) locus of mouse chromosome 7 were isolated from murine melanoma and melanocyte libraries. The transcript from this gene is missing or altered in six independent mutant alleles of the p locus, suggesting that disruption of this gene results in the hypopigmentation phenotype that defines mutant p alleles. Characterization of the human homolog revealed that it is localized to human chromosome 15 at q11.2-q12, a region associated with Prader-Willi and Angelman syndromes, suggesting that altered expression of this gene may be responsible for the hypopigmentation phenotype exhibited by certain individuals with these disorders.

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

Recessive mutant alleles at the pink-eyed dilution (p) locus on mouse chromosome 7 reduce pigmentation of both the coat and eyes. Here we describe the properties and complementation interactions of 10 p alleles, including 6 not previously reported. Several alleles that cause additional phenotypes affecting development, reproduction, and behavior were shown to be deletions by using DNA probes derived from the p region. An alignment of functional and marker-defined units is proposed, giving a linear complementation map that orders at least four functional loci. The characterization of a nested set of deletions around p will facilitate detailed molecular analyses of the genes and developmental functions associated with this part of the mouse genome.

The nonobese diabetic scid mouse: model for spontaneous thymomagenesis associated with immunodeficiency

Homozygosity for the severe combined immunodeficiency (scid) mutation results in a block in T- and B-lymphocyte development. An unusually high incidence of spontaneous thymic lymphoma development was observed after transfer of this mutation from the C.B-17 congenic strain background onto the diabetes-susceptible nonobese diabetic (NOD) background. Thymomagenesis in the NOD-scid/scid mouse was associated with expression of an NOD mouse-unique endogenous ecotropic murine leukemia provirus locus (Emv-30, mapped to proximal region of chromosome 11) not expressed in the standard substrain NOD/Lt thymus. All tumors exhibited insertions of ecotropic proviruses, whereas only a subset also exhibited proviral integrations of mink cell focus-forming retrovirus. Neither class of retrovirus was associated with consistent integration into genes previously associated with activation of oncogenesis. We propose that the unusual features of T-cell ontogeny characteristic of the NOD inbred strain synergize with the scid-imparted block in thymocyte development, leading to activation of the NOD-unique Emv-30 to initiate thymomagenesis.

Detection of mutations in DNA

Methods for detecting known and unknown mutations are becoming increasingly important as new disease genes are identified and new mutations are found in them. These methods are also expensive and time consuming. Over the past year major efforts have been directed towards developing new assays and making current assays faster and cheaper.

Control of melanocyte proliferation and differentiation in the mouse epidermis

Melanocyte-stimulating hormone plays an important role in the regulation of melanocyte differentiation in the mouse epidermis by inducing tyrosinase activity, melanosome formation, translocation of melanosomes, and increased dendritogenesis. The proliferative activity of differentiating epidermal melanocytes of newborn mice during the healing of skin wounds is regulated by semi-dominant genes, suggesting that the genes are involved in regulating the proliferative activity of epidermal melanocytes during differentiation. From the results of serum-free culture of epidermal cell suspensions from neonatal mouse skin, basic fibroblast growth factor is shown to stimulate the sustained proliferation of melanoblasts in the presence of dibutyryl adenosine 3',5'-cyclic monophosphate and keratinocyte-derived factors. Moreover, each step of melanocyte differentiation is controlled by numerous coat color genes. These genes control melanocyte differentiation by regulating the differentiation of neural crest cells into melanoblasts in embryonic skin, or by regulating the differentiation of neural crest cells into melanoblasts in embryonic skin, or by regulating transcription and/or translation of the tyrosinase gene in the differentiating melanocytes. These results suggest that melanocyte proliferation and differentiation in the mouse epidermis are controlled by both genetic factors and local tissue environment.

Intracisternal A-particle-specific oligonucleotides provide multilocus probes for genetic linkage studies in the mouse

Oligonucleotide probes representing distinct intracisternal A-particle (IAP) subfamilies were derived from the long terminal repeats (LTRs) of transcriptionally active IAP genes in normal mouse cells. These probes were used to examine the distribution of IAP proviral elements in the genomic DNA of several inbred mouse strains. Each oligonucleotide probe identified multiple polymorphisms between the different strains. The distribution of polymorphic restriction fragments among the CXB set of recombinant inbred (RI) strains demonstrates the feasibility of using these probes for chromosome mapping. These and other subset-specific IAP probes can provide a useful series of multilocus markers for genomic mapping and genetic analysis in the mouse.

The mouse pink-eyed dilution locus: a model for aspects of Prader-Willi syndrome, Angelman syndrome, and a form of hypomelanosis of Ito

The region of mouse Chromosome (Chr) 7 containing the mouse pink-eyed dilution locus, p, is syntenic with human chromosome 15q11-q13, a region associated with three human syndromes, Prader-Willi syndrome (PWS), Angelman syndrome (AS), and a form of hypomelanosis of Ito (HI). Because some mutant alleles of p also share a subset of phenotypes with PWS, AS, and HI, the same gene or genes disrupted by p locus mutations are potentially involved in the phenotypes of PWS, AS, and HI.

A genomic scanning method for higher organisms using restriction sites as landmarks

We have developed a powerful genomic scanning method, termed "restriction landmark genomic scanning," that is useful for analysis of the genomic DNA of higher organisms using restriction sites as landmarks. Genomic DNA is radioactively labeled at cleavage sites specific for a rare cleaving restriction enzyme and then size-fractionated in one dimension. The fractionated DNA is further digested with another more frequently occurring enzyme and separated in the second dimension. This procedure gives a two-dimensional pattern with thousands of scattered spots corresponding to sites for the first enzyme, indicating that the genome of mammals can be scanned at approximately 1-megabase intervals. The position and intensity of a spot reflect its locus and the copy number of the corresponding restriction site, respectively, based on the nature of the end-labeling system. Therefore, this method is widely applicable to genome mapping or detection of alterations in a genome.

Molecular basis of mouse developmental mutants

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