Recombination between the t6 complex and linked loci in the house mouse

Localization of a novel chromosome 7 locus that suppresses development of N-Methyl-N-nitrosourea-induced murine thymic lymphomas

N-Methyl-N-nitrosourea (MNU) is a potent carcinogen that causes the development of murine thymic lymphomas. MNU-induced tumor incidence varies considerably among different inbred mouse strains. In particular, the AKR strain is highly susceptible, whereas the C57L strain is highly resistant to MNU-induced lymphoma formation. Crosses between AKR and C57L mice were established to investigate the genetic basis for the differential susceptibility of these inbred strains. A strong association between MNU-induced lymphoma development and coat color was observed in (AKR x C57)F2 and AKR x (AKR x C57)F1 progeny such that albino mice developed a higher tumor incidence than nonalbino animals. These data suggest that a locus on chromosome 7 influences tumor development. Analysis of four additional polymorphic loci (D7Rp2, Fes, Hbb, and Int-2) on chromosome 7 in AKR x (AKR x C57)F1 backcross mice revealed a significant linkage between high tumor incidence and homozygous inheritance of AKR alleles at the albino (tyrosinase) and Hbb loci. Thus, inheritance of at least one C57L allele at the albino or Hbb loci was associated with protection against MNU-induced lymphoma development. There was no association between tumor incidence and genotype at the D7Rp2, Fes, or Int-2 loci. Taken together, the data suggest that whereas C57L mice contain a dominant tumor suppressor gene on chromosome 7, in the AKR strain both alleles at this locus are defective resulting in enhanced susceptibility to MNU-induced lymphomagenesis.

The ?-subunit of the skeletal muscle sodium channel is encoded proximal to Tk-1 on mouse Chromosome 11

Recent evidence suggests that the human neuromuscular disorders, hyperkalemic periodic paralysis and paramyotonia congenita, are both caused by genetic defects in the alpha-subunit of the adult skeletal muscle sodium channel, which maps near the growth hormone cluster (GH) on Chromosome (Chr) 17q. In view of the extensive homology between this human chromosome and mouse Chr 11, we typed an interspecies backcross to determine whether the murine homolog (Scn4a) of this sodium channel gene mapped within the conserved chromosomal segment. The cytosolic thymidine kinase gene, Tk-1, was also positioned on the genetic map of Chr 11. Both Scn4a and Tk-1 showed clear linkage to mouse Chr 11 loci previously typed in this backcross, yielding the map order: TrJ-(Re, Hox-2, Krt-1)-Scn4a-Tk-1. No mouse mutant that could be considered a model of either hyperkalemic periodic paralysis or paramyotonia congenita has been mapped to the appropriate region of mouse Chr 11. These data incorporate an additional locus into the already considerable degree of homology observed for these human and mouse chromosomes. These data are also consistent with the view that the conserved segment region may extend to the telomere on mouse Chr 11 and on human 17q.

Two closely related kappa variable region pseudogenes pose an evolutionary paradox

Two pseudogenes belonging to the Igk-V1 variable region group have been isolated from BALB/c mice. The genes share greater than 96.5% identity of nucleotide sequence in a 1800 base pair (bp) region surrounding the coding region, but deletions of 221 bp and 84 bp have removed essential sequences from the two genes. As the deletions are different in the two pseudogenes, they must have occurred independently in each gene during or subsequent to the duplication event which gave rise to the genes from a common ancestral gene. Polymerase chain reaction analysis was used to identify the pseudogenes in inbred strains of mice. BALB/c (Igkc) and AKR (Igka), prototype strains representative of the predominant kappa haplotypes, possess both pseudogenes but no intact copy. Only one of the pseudogenes was present in SJL (Igka). Strains C58, c.C58 (Igkd) and NZB (Igkb) possessed an intact version of the gene. This distribution of haplotypes is consistent with a close linkage of the pseudogenes with other Igk-V1 genes on chromosome 6. The translated amino acid sequence of the pseudogenes indicates that prior to their acquiring deletions they encoded typical Igk-V1 variable regions except for an unusual FR2 region, in which the conserved proline at position 44 is replaced by leucine and the normally hydrophobic position 36 was occupied by histidine. Possible mechanisms to explain the occurrence of deletions in both of the pseudogenes in the recent evolution of BALB/c are discussed. One explanation would be that the two genes were already nonfunctional at the time of the duplication so that the subsequent deletions represent neutral events which became fixed in the inbred strains by a process of genetic drift. Alternatively, if the genes were functional at the time of duplication, their rapid loss due to deletion events suggests that negative selection may have acted to eliminate the genes from the V-region repertoire.

A mouse chromosome 17 gene encodes a testes-specific transcript with unusual properties

We have characterized a novel mouse gene (D17Si11) on chromosome 17 that expresses a major transcript observed uniquely in the testes. The D17Si11 locus has been mapped to the central region of chromosome 17 between H-2 and C3. Sequence analysis demonstrates several unusual features of this locus and its transcript: first is the presence of complementary sets of alternating purine and pyrimidine residues within the 3' region of the transcript that could form double-stranded, hairpin-like secondary structures with properties similar to that of Z-DNA; second is the existence of a hypothetical, long open reading frame in the nucleotide strand that is complementary to the testes transcripts. This complementary strand open reading frame is three times the size of the longest potential open reading frame present in the transcript itself. Although a function for D17Si11 has yet to be determined, the gene is relatively nonpolymorphic in mice and appears conserved in mammals.

Evolution of mouse chromosome 17 and the origin of inversions associated with t haplotypes

Mouse t haplotypes are variant forms of chromosome 17 that exist at high frequencies in worldwide populations of several species of house mouse. They are known to differ from wild-type chromosomes with respect to two relative inversions referred to as proximal and distal. An untested assumption has been that these two inversions originated in the chromosomal lineage leading to present-day t haplotypes. To investigate the evolutionary origins of these inversions and the possibility of additional inversions, interspecific crosses were performed between Mus spretus or Mus abbotti and laboratory strains of Mus domesticus that carried wild-type and t haplotypes forms of chromosome 17. The results provide evidence for the existence of two additional nonoverlapping inversions--one between the proximal and distal inversions and one between the centromere and the proximal inversion. These four inversions span nearly the entire region of t haplotype recombination suppression. Considering the distribution of these inversions among the species studied as well as the organization of the D17Leh66 family of DNA elements, we infer that the proximal inversion occurred on the lineage leading to the common ancestor of M. domesticus and M. abbotti, and that the other three inversions occurred on the separate lineage leading to present-day t haplotypes. Alternative models for the evolution of t haplotypes are discussed in light of these findings.

Synteny on mouse chromosome 5 of homologs for human DNA loci linked to the Huntington disease gene

Comparative mapping in man and mouse has revealed frequent conservation of chromosomal segments, offering a potential approach to human disease genes via their murine homologs. Using DNA markers near the Huntington disease gene on the short arm of chromosome 4, we defined a conserved linkage group on mouse chromosome 5. Linkage analyses using recombinant inbred strains, a standard outcross, and an interspecific backcross were used to assign homologs for five human loci, D4S43, D4S62, QDPR, D4S76, and D4S80, to chromosome 5 and to determine their relationships with previously mapped markers for this autosome. The relative order of the conserved loci was preserved in a linkage group that spanned 13% recombination in the interspecific backcross analysis. The most proximal of the conserved markers on the mouse map, D4S43h, showed no recombination with Emv-1, an endogenous ecotropic virus, in 84 outcross progeny and 19 recombinant inbred strains. Hx, a dominant mutation that causes deformities in limb development, maps approximately 2 cM proximal to Emv-1. Since the human D4S43 locus is less than 1 cM proximal to HD near the telomere of chromosome 4, the murine counterpart of the HD gene might lie between Hx and Emv-1 or D4S43h. Cloning of the region between these markers could generate new probes for conserved human sequences in the vicinity of the HD gene or possibly candidates for the murine counterpart of this human disease locus.

Ecotropic virus involvement in spontaneous B-cell lymphomas of CWD/LeAgl mice

The inbred mouse strain CWD/LeAgl, which has a high incidence of spontaneous B-cell lymphomas, expresses both ecotropic MuLV and MCF viruses. Studies indicated that the MCF viruses expressed in CWD tumors were characteristic of nononcogenic MCF viruses and that ecotropic MuLV may be the etiological agent in spontaneous B-cell lymphomagenesis. Somatically acquired proviruses of approximately the same size were detected in several tumor DNAs suggesting that integration of proviral sequences into specific regions of the mouse genome may be an important step in lymphomagenesis of this strain.

Can segregation distortion influence gametic disequilibrium?

There are a number of reports of gametic disequilibrium between alleles causing segregation distortion (e.g.talleles inM. musculusandSDalleles inD. melanogaster) and linked loci. These observations have resulted in the conclusion by some researchers that segregation distortion may cause gametic disequilibrium. In this manuscript I have shown that (1) segregation distortion cannot generate gametic disequilibriumde novoand (2) because segregation distortion results in an excess of heterozygotes, the rate of decay of disequilibrium is faster than if segregation distortion were absent. Other factors, such as mutation or selection, appear to generate the observed disequilibrium, and extremely low recombination appears important in retarding its decay.

Comparative mapping of DNA markers from the familial Alzheimer disease and Down syndrome regions of human chromosome 21 to mouse chromosomes 16 and 17

Mouse trisomy 16 has been proposed as an animal model of Down syndrome (DS), since this chromosome contains homologues of several loci from the q22 band of human chromosome 21. The recent mapping of the defect causing familial Alzheimer disease (FAD) and the locus encoding the Alzheimer amyloid beta precursor protein (APP) to human chromosome 21 has prompted a more detailed examination of the extent of conservation of this linkage group between the two species. Using anonymous DNA probes and cloned genes from human chromosome 21 in a combination of recombinant inbred and interspecific mouse backcross analyses, we have established that the linkage group shared by mouse chromosome 16 includes not only the critical DS region of human chromosome 21 but also the APP gene and FAD-linked markers. Extending from the anonymous DNA locus D21S52 to ETS2, the linkage map of six loci spans 39% recombination in man but only 6.4% recombination in the mouse. A break in synteny occurs distal to ETS2, with the homologue of the human marker D21S56 mapping to mouse chromosome 17. Conservation of the linkage relationships of markers in the FAD region suggests that the murine homologue of the FAD locus probably maps to chromosome 16 and that detailed comparison of the corresponding region in both species could facilitate identification of the primary defect in this disorder. The break in synteny between the terminal portion of human chromosome 21 and mouse chromosome 16 indicates, however, that mouse trisomy 16 may not represent a complete model of DS.

Extent of the mouse t complex and its inversions shown by in situ hybridization

Probes for loci situated near one end of the proximal (Tcp-1) and distal (Qa-2, 3) inversions of the mouse t complex have been hybridized to chromosomes of mice with and without t complexes and with morphologically distinguishable chromosome 17s. Both the probe for Tcp-1 and that for Qa-2, 3 hybridized to clearly different positions on t and non-t chromosomes, thus making visible the extent of the two inversions. The proximal inversion extends from roughly the junction of bands A1 and A2 to band A3, and the distal inversion from band A3 to band C. Thus, the whole t complex extends from the band A1-A2 junction to band C, and is therefore somewhat larger than previously thought, and occupies about 1.2% of the genome. A probe for complement component 3 (C3-1), genetically known to be several cM distal to the t complex, was found by in situ hybridization to lie in band E1. The proximal part of chromosome 17 is one of the best known parts of the mouse genome, at both the genetic and molecular levels. It may soon be possible to correlate the length of the t complex in terms of chromosomal distance with its physical length in megabases.

Induction of new mutations in a mouse t-haplotype using ethylnitrosourea mutagenesis

N-ethyl-N-nitrosourea (ENU) was used to induce mutations within thetw5-haplotype of the mouse to make possible a further study of gene arrangement int-mutants and to provide potential landmarks for cloning and sequence studies in the region. Two independent mutants were isolated for each of three loci in thet-region, brachyury (T), quaking (qk), and tufted (tf). The newTktalleles produce tailless mice when atctmutation is present intrans. The newqkktalleles are recessive and homozygous lethal. They are viable, male fertile, and cause seizures and quaking when paired with theqkmutation which previously defined the locus. Thetfktmutations are recessive and phenotypically similar to the mutant alleles available in non-tchromosomes. The mutations were induced in thetw5-haplotype at an average per locus frequency of 1 in 1500. Their isolation demonstrates the power of this technique for obtaining the specific mouse mutants that are needed to genetically dissect a complex mammalian system.