Molecular characterization of four intra-t mouse recombinants

Recombination in the proximal region of mouse chromosome 17 is greatly reduced in heterozygotes carrying the wild-type and the t complex-type chromosomes. The reason for this is the presence of two non-overlapping inversions in the t complex. Rare crossing-over does, however, occur within the t complex of the t/+ heterozygotes. Here we characterize four such exceptional intra-t recombinants, tTu1 through tTu4. To map the positions of the genetic exchange in these four recombinants, we analyzed them with DNA probes specific for 16 loci distributed over the t complex. The analysis revealed that in three of the four recombinants, an equal crossing-over occurred in the short region between the two inversions, producing chromosomes carrying either the proximal inversion only (tTu1 and tTu4) or the distal inversion only (tTu2). In the fourth recombinant (tTu3), unequal crossing-over occurred within the proximal inversion between loci D17Leh119 and D17Leh66, producing a chromosome in which the region containing loci Tcp-1, T, and D17Tu5 has been duplicated. The duplication of the Brachyury locus leads to the suppression of the tail-shortening effect normally produced by the interaction of the dominant (T) and recessive (tct) alleles at this locus so that the T/tTu3 mice have normal tails.

The D17Tu5 locus in the t complex: implications for the origin of t haplotypes and inbred strains

The mouse x Chinese hamster cell line R4 4-1 contains only one mouse chromosome, the bulk of which corresponds to Mus musculus chromosomes 17 and 18 (MMU17 and MMU18, respectively). A genomic library was prepared from the R4 4-1 DNA, and a mouse clone was isolated from the library, which-with the help of somatic cell hybrids-could be mapped to the MMU17. A locus defined by a 2.7-kb long Bam HI probe from this clone was designated D17Tu5 (Tu for Tübingen). The locus proved to be polymorphic among inbred strains and wild mice. By testing of recombinant inbred strains and partial t haplotypes, the D17Tu5 locus could be mapped to a position between the D17Leh66E and D17Rp17 loci within the t complex. Two alleles were found at this locus, D17Tu5a and D17Tu5b, defined by Taq I restriction fragment length polymorphism. Both alleles are present among inbred strains and wild mice of the species M. domesticus. All complete t haplotypes tested carry the D17Tu5a allele and all tested wild mice of the species M. musculus, with the exception of those bearing t haplotypes, carry the D17Tu5b allele. Additional alleles are found in some populations of wild mice and in other species of the genus Mus. The distribution of the two alleles among the inbred strains correlates well with their known or postulated genealogy. Their distribution between the two species of Mus and among the mice with T haplotypes suggests a relatively recent origin of the t haplotypes.

[Maternal connective tissue disease associated with congenital AV block]

We report 3 patients with congenital A-V block whose mothers had anti-Ro or anti-La antibodies. One of them had systemic lupus with serum anti-Ro antibodies; the other had anti-Ro antibodies and no clinical manifestations; the third had an undifferentiated connective tissue disorder with anti-La antibodies in the serum. The epidemiology of this association is discussed as well as some therapeutic options.

Mapping of the Sod-2 locus into the t complex on mouse chromosome 17

A human DNA probe specific for the superoxide dismutase gene was used to identify the corresponding mouse gene. Under the chosen hybridizing conditions, the probe detected DNA fragments most likely carrying the mouse Sod-2 gene. Mapping studies revealed that the Sod-2 gene resides in the proximal inversion of the t complex on mouse chromosome 17. All complete t haplotypes tested showed restriction fragment length polymorphism which is distinct from that found in all wild-type chromosomes tested. The Sod-2 locus maps in the same region as some of the loci that influence segregation of t chromosomes in male gametes. The possibility that the Sod-2 locus is related to some of the t-complex distorter or responder loci is discussed. The data indicate that the human homolog of the mouse t complex has split into two regions, the distal region remaining on the p arm of human chromosome 6, while the proximal region has been transposed to the telomeric region of this chromosome's q arm.

MHC polymorphism pre-dating speciation

Two features distinguish the polymorphism of the major histocompatibility complex (MHC) loci from that of other loci: its high diversity and the large genetic distance between MHC alleles. More than 100 alleles exist in natural populations in the mouse at each of the functional class I and class II alleles, all alleles occurring at frequencies that cannot be explained by recurrent mutations. Some of the alleles differ by approximately 70 nucleotides in the coding region alone and some of the products of the allelic genes differ by more than 50 amino acids. It has generally been assumed that these differences accumulated after species inception. Here, we present evidence for an alternative explanation of the origin of MHC polymorphism: a large part of the MHC polymorphism pre-dates speciation and is passed on from species to species. We describe allelic differences that must have arisen before the separation of mice and rats from a common ancestor more than 10 million years ago.

t-Specific DNA polymorphisms among wild mice from Israel and Spain

Lehrach and his coworkers have isolated a series of DNA probes that specifically hybridize with different regions of mouse chromosome 17 within the t complex. The probes display restriction fragment length polymorphisms, RFLPs, which are specific for the t haplotypes in all laboratory mouse strains tested thus far. Some of these probes have been used to test wild mice populations for these t-associated DNA forms. It is demonstrated that populations from Germany, Switzerland, Italy, Greece, Yugoslavia, Australia, Costa Rica, and Venezuela contain chromosomes in which all the tested DNA loci display the t-specific polymorphisms. The frequency of mice carrying these chromosomes is as high as 31%. Wild mice from Israel and Spain, on the other hand, carry chromosomes displaying t-specific DNA forms only at one or two of the probed loci, while the other loci carry the wild-type (+) forms. These chromosomes thus resemble the partial t haplotypes known from the study of laboratory mice. One possible interpretation of these findings is that these DNA polymorphisms contributed to the assembly of the complete t haplotypes and that these haplotypes may have originated in the Middle East.

Mhc class II DNA polymorphisms within and between chromosomal species of the Spalax ehrenbergi superspecies in Israel

Restriction fragment length polymorphisms (RFLPs) of two major histocompatibility class II genes (P alpha 1 and Q beta) were studied in 13 populations of four chromosomal species (2n = 52, 54, 58 and 60) of the mole rat, Spalax ehrenbergi superspecies in Israel. A substantial frequency of allelic fragments was found in both genes for all populations, including a desert isolate. In the P alpha 1 gene, one allelic fragment is a result of a deletion mutation which is diagnostic of the 2n = 52 chromosomal species. All other ten allelic variants are the result of point mutations. All mutations are located in a short region flanking the 3' end of the gene. Based on Mhc polymorphisms we confirm earlier evidence that gene flow does not occur between the older chromosomal species (2n = 52, 54, 58), and that reproductive isolation decreases, progressively from the oldest to the youngest species (2n = 60).

Nucleotide sequence analysis of class II genes borne by mouse t chromosomes

Five class IIH-2genes borne bytchromosomes were partially sequenced:borne by thetTuw10chromosome of EDY589 (H-2w2);andborne by thetTuw8chromosome of CR0437 (H-2w57), as well asandborne by thetTuw7chromosome of CR0435 (H-2w37). These genes are representatives of the three major groups of alleles found associated withtchromosomes. The sequenced part consisted of almost the entire exon 2 and the entire exon 3 coding for the first and the second domain of the β polypeptide chains, respectively. The sequence was compared with the published sequences ofAβandEβalleles borne, by non-tchromosomes. The comparison revealed that thet-associated alleles are no more similar to one another than they are to the corresponding genes present on non-tchromosomes in laboratory mice. This divergence in the nucleotide sequence among the class II genes is interpreted as evidence that thetcomplex is very old.

Evolutionary expansion of Mhc class I loci in the mole-rat, Spalax ehrenbergi

A cosmid genomic library was prepared from a single individual of the rodent Spalax ehrenbergi, the mole rat, captured in Israel. The library was screened with a mouse probe hybridizing with all mouse class I major-histocompatibility-complex (Mhc) genes; the cross-hybridizing clones were isolated; and their restriction maps were prepared using five enzymes. A total of 93 class I-bearing clones could be identified in the library. Forty-five of these clones showed partial overlaps and could be arranged into 14 clusters. Eleven of these clusters could be shown to contain two class I genes each; the remaining clusters, as well as most of the non-overlapping clones, each contained one class I gene. After the elimination of clones with possible cloning artifacts and of clones that may carry allelic forms of a given gene in the heterozygous animal, the total number of class I loci identified in Spalax is approximately 65. The high number of loci probably arose from the duplication of either the entire class I set or the different class I families. The high number of gene copies might represent a means of selecting different functional genes from the family in different mammalian orders. Three of the approximately 65 Spalax class I genes cross-hybridize with a probe specific for the mouse K, D, and L genes; two of these genes are in the same cluster. These three elements might possibly be the functional class I genes of the mole rat.

Polymorphism of unique noncoding DNA sequences in wild and laboratory mice

Two DNA probes, D17Tu1 and D17Tu2, were isolated from a genomic DNA library containing only two mouse chromosomes, one of which is chromosome 17, carrying the major histocompatibility complex (H-2), as well as the t complex genes. The D17Tu1 probe was mapped to the centromeric region of chromosome 17 and the D17Tu2 probe to the S region of the H-2 complex. Neither of the two probes appeared to detect any genes, but both contained unique, nonrepetitive sequences. Typing of DNA obtained from a large panel of mice revealed the presence of four D17Tu1 patterns in inbred mouse strains, one very common, one less common, and two present in one strain each. The two common patterns could not be detected in appreciable frequencies in the European wild mice tested (one of the two patterns was, however, found in Australian wild mice). Conversely, the patterns found frequently in European wild mice are absent in the laboratory mice. We therefore conclude that wild mice from the sampled regions of Europe could not have provided the ancestral stocks from which inbred strains were derived. Only one D17Tu1 pattern was found in all the populations of Mus musculus tested, while eight patterns were found in Mus domesticus, with virtually all the populations being polymorphic. We suggest that this difference reflects different modes in which the two species colonized Europe. The distribution of the D17Tu2 patterns in inbred strains correlates with the distribution of H-2 haplotypes.

Molecular evidence for the rapid propagation of mouse t haplotypes from a single, recent, ancestral chromosome

Mouse t haplotypes are variant forms of chromosome 17 that exist at high frequencies in worldwide populations of two species of commensal mice. To determine both the relationship of t haplotypes to each other and the species within which they exist, 35 representative t haplotypes were analyzed by means of 10 independent molecular probes, including five DNA clones and five polypeptide spots identified by means of two-dimensional gel electrophoresis. All of the tested haplotypes were found to share restriction fragments and polypeptide spots that are absent in mice carrying wild-type forms of chromosome 17. This observation provides the first direct evidence that all of the known t haplotypes are descendents of a single ancestral chromosome. The absence of variation among t haplotypes could mean that this ancestral chromosome existed relatively recently, in which case it would be necessary to postulate introgressions of t haplotypes across species lines to explain their presence in both Mus domesticus and M. musculus. Alternatively, it is possible that the ancestral chromosome existed prior to the split between M. domesticus and M. musculus and that, by chance, our probes fail to detect polymorphisms that exist among the t haplotypes. A further result of our analysis is the characterization of a partial t haplotype in a wild population of Israeli mice.

Major histocompatibility complex gene organization in the mole rat Spalax ehrenbergi: evidence for transfer of function between class II genes

A genomic DNA library prepared from the kidney of the mole rat Spalax ehrenbergi was screened with mouse probes representing major histocompatibility complex genes that encode alpha and beta polypeptide chains of class II molecules (alpha and beta genes). Restriction maps were constructed for the cross-hybridizing clones, and the class II genes borne by these clones were identified. By this procedure, five main regions containing class II genes were established. One region contained four genes and two gene fragments, the second region contained two genes, the third region contained one gene and one gene fragment, and the remaining two regions contained one gene each. Altogether, six beta genes, two alpha genes, and three alpha-gene fragments were identified. Two of the genes (one alpha and one beta) were established as belonging to the DQ subclass, and all other genes were found to be members of the DP subclass. (Subclass designations are based on the human HLA class II genes). No genes belonging to the DR and DO (DZ) subclasses were found in the library. The absence of DR genes in S. ehrenbergi was also indicated when other experimental methods were used. At least some of the DP loci are polymorphic and most likely also functional. Thus, in the evolution of the mole rat, the DR (and probably also the DO) loci have been deleted and their function(s) has been taken over by the DP loci, which have expanded to a great extent. These findings argue for functional interchangeability of the individual subclasses of class II loci.

Evolutionary diversification of class II P loci in the Mhc of the mole-rat Spalax ehrenbergi

The class II region of the major histocompatibility complex (Smh) in the mole rat, Spalax ehrenbergi, consists of only two gene families, P and Q, instead of the four families (P, O, Q, and R) found in all other mammals studied to date. The Spalax P family consists of at least four beta and three alpha genes or gene fragments. In DNA-hybridization experiments, two of the beta genes behave as bona fide P-family members in that they hybridize strongly with human DP beta probes and hybridize weakly with probes specific for other class II gene families. The other two beta genes, on the other hand, hybridize weakly with human DP beta probes and nearly as well with human DQ beta probes. To determine the evolutionary relationships among these P-like genes, we have sequenced one of them. The sequence reveals, on the basis of its organization, that the gene clearly belongs to the P family, yet, on the basis of its nucleotide sequence, it is only slightly more similar to human DP than to human DQ genes. These results indicate that in the Spalax the P family of genes split into two subfamilies, PA and PB. For unknown reasons, one of these subfamilies (PB) retained more similarity to the Q gene family than did the other (PA).

Random cloning of genes from mouse chromosome 17

We describe a method for isolating cosmid clones randomly from mouse chromosome 17. A cosmid library was constructed from the mouse-Chinese hamster cell line R4 4-1 that contains a limited amount of mouse DNA (chromosomes 17 and 18 and some other unidentified material) on a Chinese hamster background. The library was screened with the murine repetitive sequence probe pMBA14, which selectively hybridizes with mouse DNA. The mouse-derived cosmid clones thus identified were individually hybridized with DNA from the mouse-Syrian hamster cell line JS17 containing all mouse chromosomes except chromosome 17 on a Syrian hamster background. We deduced that the cosmid clones that contained sequences absent in JS17 were derived from mouse chromosome 17. One of the chromosome 17-derived cosmid clones, 3-4-1 (located proximal to the T122/T66C segment) was found to be highly polymorphic among European wild-mouse populations and may be a useful probe to elucidate the evolution and migration of Mus species. The randomly isolated mouse-derived cosmid clones can also be screened for the presence of functional genes. Using testicular cDNA as a probe, a testis-specific gene was cloned from mouse chromosome 17.

Low H-2 polymorphism in some Israeli wild mouse populations

Two populations of the wild house mouse, Mus domesticus, found living close to each other (one inhabited a chicken coop and the other an open field at the Educational Farm of the Hebrew University of Jerusalem, East Talpiot, Jerusalem) were studied for their H-2 polymorphism. These two populations were selected because they are well characterized in terms of their ecological parameters; they have been under continuous surveillance for several years. Twenty-seven H-2 homozygous lines were produced by mating wild mice from these two populations with laboratory strains. The H-2w homozygotes were then characterized by serological typing with monoclonal and polyclonal antibodies specific for the known allomorphs controlled by the class I H-2K and H-2D loci or the class II H-2A and H-2E loci. They were also used as donors for immunizations and for the selection of antisera defining the H-2 haplotypes carried by these lines. Four new H-2 haplotypes could be identified: H-2w82 (Kw16 Dw82), H-2w83 (Kw83 Dw16), H-2w84 (Kw84 Dw84), and H-2w85 (Kw83 Dw84), the last haplotype being a recombinant derived from H-2w83 and H-2w84. Antisera defining the new haplotypes were then used for a study of the wild populations. This study revealed that the populations contain only the four identified H-2 haplotypes, having three alleles at the H-2K locus (Kw16, Kw83, Kw84) and three alleles at the H-2D locus (Dw16, Dw82, and Dw84). The alleles occur in the populations with a frequency of 0.12-0.54. There were no significant differences in gene frequencies between the two populations, and the allele frequencies remained more or less stable. There was a significant excess of heterozygotes for at least some of the genes, compared with the frequency expected from Hardy-Weinberg equilibrium. The same antisera were also used to type other populations in the vicinity of Jerusalem. In one population, located 30 km west of Jerusalem, the mice failed to react with any of the reagents. In the other two populations, located 15 km west and 40 km northeast of Jerusalem, three of the four H-2 haplotypes found in East Talpiot were present at high frequencies. It appears, therefore, that only three main H-2 haplotypes and two or three minor ones are present in the area around Jerusalem. This study thus provides the first example of a large mainland population in which the H-2 polymorphism is comparable to that of many other non-H-2 loci.