Mouse chromosome 7

Disruption of the murine protein kinase Cbeta gene promotes gallstone formation and alters biliary lipid and hepatic cholesterol metabolism

The protein kinase C (PKC) family of Ca(2+) and/or lipid-activated serine-threonine protein kinases is implicated in the pathogenesis of obesity and insulin resistance. We recently reported that protein kinase Cβ (PKCβ), a calcium-, diacylglycerol-, and phospholipid-dependent kinase, is critical for maintaining whole body triglyceride homeostasis. We now report that PKCβ deficiency has profound effects on murine hepatic cholesterol metabolism, including hypersensitivity to diet-induced gallstone formation. The incidence of gallstones increased from 9% in control mice to 95% in PKCβ(-/-) mice. Gallstone formation in the mutant mice was accompanied by hyposecretion of bile acids with no alteration in fecal bile acid excretion, increased biliary cholesterol saturation and hydrophobicity indices, as well as hepatic p42/44(MAPK) activation, all of which enhance susceptibility to gallstone formation. Lithogenic diet-fed PKCβ(-/-) mice also displayed decreased expression of hepatic cholesterol-7α-hydroxylase (CYP7A1) and sterol 12α-hydroxylase (CYP8b1). Finally, feeding a modified lithogenic diet supplemented with milk fat, instead of cocoa butter, both increased the severity of and shortened the interval for gallstone formation in PKCβ(-/-) mice and was associated with dramatic increases in cholesterol saturation and hydrophobicity indices. Taken together, the findings reveal a hitherto unrecognized role of PKCβ in fine tuning diet-induced cholesterol and bile acid homeostasis, thus identifying PKCβ as a major physiological regulator of both triglyceride and cholesterol homeostasis.

Consistent allelic loss on mouse chromosome 7 distal to tyrosinase in 4-nitroquinoline-1-oxide-induced oral cavity tumors with loss of heterozygosity at Ha-ras-1

We have previously shown that all CBA/J mice exposed to 4-nitroquinoline-1-oxide (4NQO) eventually develop oral cavity squamous cell carcinomas, and two-thirds of these tumors have Ha-ras-1 (Hras1) point mutations at codon 12. Half of the tumors with Hras1 mutations have loss of heterozygosity (LOH) at Hras1. In the study reported here, seven tumors with LOH at Hras1, six heterozygous for Hras1, and six without Hras1 mutations were analyzed to define the extent of LOH on chromosome (Chr) 7. Microsatellite polymorphisms present in CBA/J mice were used as informative allelic markers. Tumors with LOH at Hras1 showed consistent allelic loss at the distal portion of Chr 7. The boundary of allelic loss lay between the tyrosinase and hemoglobin beta chain loci, which are 6 cM apart. None of the tumors that remained heterozygous for Hras1 or had no Hras1 mutations had evidence of chromosomal loss involving Chr 7. Because LOH was only detected in advanced lesions long after exposure to 4NQO had ceased, we presume that the chromosomal alterations by which LOH occurred were independent of the carcinogen exposure. The development of LOH in only half of the tumors with Hras1 point mutations suggests that LOH was not caused by the initial Hras1 point mutation but was a highly selected event during tumorigenesis.

Distinct genetic loci control development of benign and malignant skin tumours in mice

Genetic susceptibility to chemically induced skin cancer in mice is controlled by multiple unlinked genetic loci. Mus spretus mice have dominant resistance genes which confer resistance to interspecific F1 hybrids with susceptible Mus musculus strains. We have mapped three major resistance loci using a combination of Mapmaker/QTL analysis and multiple regression analysis to mouse chromosomes 5 and 7. At least two independent loci on chromosome 7 exert their effects primarily during benign tumour development and have very little influence on tumour progression. On the other hand, probably a single locus on chromosome 5 affects both early and late stages of malignancy. The results indicate that benign and malignant tumours are largely under independent genetic control.

Induction of different genetic changes by different classes of chemical carcinogens during progression of mouse skin tumors

By analysis of skin tumors from F1 hybrid mice we demonstrated that the genetic events that occur during tumor progression depend on the type of chemical carcinogenesis protocol used to induce tumor growth. More than 95% of tumors induced by initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and promotion with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) exhibited mutations in Ha-ras and trisomy of chromosome 7. Carcinomas induced with multiple DMBA treatments had a lower frequency of alterations on chromosome 7 (50%), but only in tumors with Ha-ras mutations, and had a much wider spectrum of alterations, including trisomy, mitotic recombination, deletion, and gene duplication. Carcinomas induced with multiple N-methyl-N'-nitro-N-nitrosoguanidine treatments only rarely exhibited alterations on chromosome 7 (8%), even if they contained mutant Ha-ras. More frequent numerical alterations of chromosome 11 were also seen in TPA-promoted tumors (23%) than in tumors induced by multiple carcinogen treatments (8%). These results show that postinitiation events are nonrandom and fit a model in which promoting agents induce numerical chromosomal alterations but in which mutagens cause more directed mutational events.

CD19 maps to a region of conservation between human chromosome 16 and mouse chromosome 7

CD19 is a B lymphocyte cell surface protein expressed from the earliest stages of B lymphocyte development until their terminal differentiation into plasma cells. In this report the human CD19 gene (hCD19) was localized to band p11.2 on the proximal short arm of chromosome 16 by in situ hybridization to metaphase chromosomes, using hCD19 cDNA as probe. hCD19 gene localization was confirmed by polymerase chain reaction based analysis with hCD19-specific primers, using a panel of human/hamster somatic cell hybrid DNA as templates. The mouse CD19 gene (mCd19) was mapped to bands F3-F4 of chromosome 7 by in situ hybridization to metaphase chromosomes, using a mCD19 cDNA probe. Segregation analysis of nucleotide sequence polymorphisms in interspecific backcross progeny revealed linkage of mCd19 with hemoglobin beta (Hbb), Int-2, and H19, other loci previously mapped to the same region of mouse chromosome 7, confirming the localization of mCd19 to this region. The order of these loci was determined to be centromere--Hbb--mCd19--H19--Int-2--telomere. The genetic distances between the loci examined, calculated from the recombination frequencies, suggested that mCd19 was located centrally between Hbb and H19. This region of mouse chromosome 7 is homologous to the region of human chromosome 16 to which the hCD19 gene maps. Multiple genes with a lymphocyte-related function also map to this conserved region including genes encoding the IL-4 receptor, CD11a, CD11b, CD11c, CD43 (leukosialin), and protein kinase C beta polypeptide.

Localization of Shaw-related K+ channel genes on mouse and human chromosomes

Four related genes, Shaker, Shab, Shaw, and Shal, encode voltage-gated K+ channels in Drosophila. Multigene subfamilies corresponding to each of these Drosophila genes have been identified in rodents and primates; this suggests that the four genes are older than the common ancestor of present-day insects and mammals and that the expansion of each into a family occurred before the divergence of rodents and primates. In order to define these evolutionary relationships more precisely and to facilitate the search for mammalian candidate K+ channel gene mutations, we have mapped members of the Shaw-homologous gene family in humans and mice. Fluorescence in situ hybridization analysis of human metaphase chromosomes mapped KCNC2 (KShIIIA, KV3.2) and KCNC3 (KShIIID, KV3.3) to Chromosome (Chr) 19q13.3-q13.4. Inheritance patterns of DNA restriction fragment length variants in recombinant inbred strains of mice placed the homologous mouse genes on distal Chr 10 near Ms15-8 and Mdm-1. The mouse Kcnc1 (KShIIIB, NGK2-KV4, KV3.1) gene mapped to Chr7 near Tam-1. These results are consistent with the hypothesis that the generation of the mammalian KCNC gene family included both duplication events to generate family members in tandem arrays (KCNC2, KCNC3) and dispersion of family members to unlinked chromosomal sites (KCNC1). The KNCN2 and KCNC3 genes define a new synteny group between humans and mice.

Allele-specific replication timing of imprinted gene regions

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.

Molecular analysis of viable spontaneous and radiation-induced albino (c)-locus mutations in the mouse

Thirty-one homozygous-viable, radiation-induced or spontaneous mutations at the albino (c) locus in mouse chromosome 7 were analyzed by Southern blot analysis with a tyrosine cDNA clone and with probes derived from the closely linked proviral integration sites Pmv-31 and Emv-23, which flank the tyrosinase gene on the proximal and distal sides, respectively. Thirteen of 27 radiation-induced and one of four spontaneous mutations manifested deletions or rearrangements for the tyrosinase gene. The sizes of four deletions found to break within the tyrosinase gene itself were estimated to be < or = 36 kb, < or = 40 kb, approximately 260 kb, and approximately 480 kb. Two homozygous-viable deletions were found to include flanking proviral loci, suggesting that they could be from 1500-2000 kb in length, if not longer. The existence of these very large, homozygous-viable deletions suggests that the one-to-two megabases including and surrounding the c locus harbor no genes essential for normal viability or fertility, although genes controlling more subtle (or "nonessential") phenotypes are likely to be present. These data thus provide some insight into the molecular structure of a number of viable c-locus mutations, whose nature could not be predicted solely on the basis of genetic analysis, as could be done for either lethal or reduced-pigment c mutations.

A revised map position for the Ha-ras gene on mouse chromosome 7: implications for analysis of genetic alterations in rodent tumors

The mouse Ha-ras gene has previously been mapped to the central region of chromosome 7, 31 cM from the centromere, using an interspecific Mus musculus/Mus spretus backcross (Saunders AM, Seldin MF, Genomics 8:525-535, 1990). However, analysis of mitotic recombinations in mouse skin tumors from intraspecific F1 hybrid mice suggested a more distal location for the Ha-ras gene on chromosome 7 (Bremner R, Balmain A, Cell 61:407-417, 1990). In the study reported here, we demonstrated, by analysis of Ha-ras gene mutations in skin tumors from interspecific M. spretus/M. musculus F1 hybrids, the existence only in M. spretus of a pseudogene or other Ha-ras-related sequence that is probably the sequence originally mapped by Saunders and Seldin. The functional Ha-ras gene maps to the distal region of chromosome 7, and it is this sequence that acquires mutations in chemically induced tumors.

N-ethyl-N-nitrosourea-induced prenatally lethal mutations define at least two complementation groups within the embryonic ectoderm development (eed) locus in mouse chromosome 7

Two loci [l(7)5Rn and l(7)6Rn] defined by N-ethyl-N-nitrosourea (ENU)-induced, prenatally lethal mutations were mapped by means of trans complementation crosses to mice carrying lethal deletions of the albino (c) locus in Chromosome (Chr) 7. Both loci were found to map to the subregion of the Mod-2-sh-1 interval that contains the eed (embryonic ectoderm development) locus, eed has been defined by the inability of embryos homozygous for certain c deletions to develop beyond the early stages of gastrulation. Evidence for at least two loci necessary for normal prenatal development, rather than one locus, that map within the eed interval came from the observation that two prenatally lethal mutations, 3354SB [l(7)5Rn3354SB] and 4234SB [l(7)6Rn4234SB], could complement each other in trans, but could not each be complemented individually by c deletions known to include the eed locus. A somewhat leaky allele of l(7)5Rn [l(7)5Rn1989SB] was also recovered, in which hemizygotes are often stillborn and homozygotes exhibit variable fitness and survival. The mapping of the loci defined by these mutations is likely to be useful for genetic, molecular, and phenotypic characterization of the eed region, and mutations at either locus (or both loci) may contribute to the eed phenotype.