Sex, strain, and species differences affect recombination across an evolutionarily conserved segment of mouse chromosome 16

A region of substantial genetic homology exists between human chromosome 21 (HSA21) and mouse chromosome 16 (MMU16). Analysis of 520 backcross animals has been used to establish gene order in the homologous segment. D21S16h and Mx are shown to represent the known proximal and distal limits of homology between the chromosomes, while Gap43, whose human homolog is on HSA3, is the next proximal marker on MMU16 that has been mapped in the human genome. Recombination frequencies (RFs) in four intervals defined by five loci in the HSA21-homologous region of MMU16 were analyzed in up to 895 progeny of eight different backcrosses. Two of the eight crosses were made with F1 males and six with F1 females. The average RF of 0.249 in 265 backcross progeny of F1 males was significantly higher than the 0.106 average recombination in 320 progeny of F1 females in the interval from D21S16h to Ets-2. This is in contrast to HSA21, which shows higher RFs in female meiosis in the corresponding region. Considerable variation in RF was observed between crosses involving different strains, both in absolute and in relative sizes of the intervals measured. The highest RFs occurred in a cross between the laboratory strain C57BL/6 and MOLD/Rk, an inbred strain derived from Mus musculus molossinus. RFs on this cross were nearly fivefold higher than those reported previously for an interspecific cross between C57BL/6 and Mus spretus.     

Mapping of Col3a1 and Col6a3 to proximal murine chromosome 1 identifies conserved linkage of structural protein genes between murine chromosome 1 and human chromosome 2q

We have investigated the degree of synteny between the long arm (q) of human chromosome 2 and the proximal portion of mouse chromosome 1. To define the limits of synteny, we have determined whether mouse homologs of seven human genes mapping to chromosome 2q cosegregated with anchor loci on mouse chromosome 1. The loci investigated were NEB/Neb, ELN/Eln, COL3A1/Col3a1, CRYG/Len-2, FN1/Fn-1, VIL/Vil, and COL6A3/Col6a3. Ren-1,2 and Acrg were included as two proximal mouse chromosome 1 anchor loci. The segregation of restriction fragment length polymorphisms at these loci was analyzed in the progeny of Mus spretus x C57BL/6J hybrids backcrossed to the C57BL/6J inbred strain. We found that five of the structural protein loci and the two anchor loci form a linkage group on proximal murine chromosome 1. The proposed gene order of this group of linked markers is centromere - Col3a1 - Len-2-Fn-1-Vil-Acrg-Col6a3-Ren1,2. Neb and Eln are linked neither to each other nor to any other marker on proximal mouse chromosome 1. Therefore, the mouse loci Col3a1 and Col6a3 are identified as flanking markers of the linkage group of structural protein loci. The estimated genetic map distances are Col3a1-13.3 cM-Len-2-3.4 cM-Fn-1-3.8 cM-Vil-9.6 cM-Acrg-2.1 cM-Col6a3-18.3 cM-Ren1,2. The available map information for human chromosome 2q markers and mouse chromosome 1 markers presented here tentatively identifies Col3a1 and Col6a3 as the border markers that define the limits of the syntenic chromosome segment. The order of mouse genes on chromosome 1 and their human homologs on chromosome 2q also appears to be conserved, suggesting that mapping of murine genes on the conserved segment may be useful to predict gene order in man.     

Sex-of-offspring-specific transmission ratio distortion on mouse chromosome X

During our study of the DDK syndrome, we observed sex ratio distortion in favor of males among the offspring of F(1) backcrosses between the C57BL/6 and DDK strains. We also observed significant and reproducible transmission ratio distortion in favor of the inheritance of DDK alleles at loci on chromosome X among female offspring but not among male offspring in (C57BL/6 x DDK)F(1) x C57BL/6 and (C57BL/6-Pgk1(a) x DDK)F(1) x C57BL/6 backcrosses. The observed transmission ratio distortion is maximum at DXMit210 in the central region of chromosome X and decreases progressively at proximal and distal loci, in a manner consistent with the predictions of a single distorted locus model. DXMit210 is closely linked to two distortion-controlling loci (Dcsx1 and Dcsx2) described previously in interspecific backcrosses. Our analysis suggests that the female-offspring-specific transmission ratio distortion we observe is likely to be the result of the death of embryos of particular genotypic combinations. In addition, we confirm the previous suggestion that the transmission ratio distortion observed on chromosome X in interspecific backcrosses is also the result of loss of embryos.     

Two quantitative trait loci for prepulse inhibition of startle identified on mouse chromosome 16 using chromosome substitution strains

Prepulse inhibition (PPI) of acoustic startle is a genetically complex quantitative phenotype of considerable medical interest due to its impairment in psychiatric disorders such as schizophrenia. To identify quantitative trait loci (QTL) involved in mouse PPI, we studied mouse chromosome substitution strains (CSS) that each carry a homologous chromosome pair from the A/J inbred strain on a host C57BL/6J inbred strain background. We determined that the chromosome 16 substitution strain has elevated PPI compared to C57BL/6J (P = 1.6 x 10(-11)), indicating that chromosome 16 carries one or more PPI genes. QTL mapping using 87 F(2) intercross progeny identified two significant chromosome 16 loci with LODs of 3.9 and 4.7 (significance threshold LOD is 2.3). The QTL were each highly significant independently and do not appear to interact. Sequence variation between B6 and A/J was used to identify strong candidate genes in the QTL regions, some of which have known neuronal functions. In conclusion, we used mouse CSS to rapidly and efficiently identify two significant QTL for PPI on mouse chromosome 16. The regions contain a limited number of strong biological candidate genes that are potential risk genes for psychiatric disorders in which patients have PPI impairments.     

Use of comparative physical and sequence mapping to annotate mouse chromosome 16 and human chromosome 21

Distal mouse chromosome 16 (MMU16) shares conserved linkage with human chromosome 21 (HSA21), trisomy for which causes Down syndrome (DS). A 4.5-Mb physical map extending from Cbr1 to Tmprss2 on MMU16 provides a minimal tiling path of P1 artificial chromosomes (PACs) for comparative mapping and genomic sequencing. Thirty-four expressed sequences were positioned on the mouse map, including 19 that were not physically mapped previously. This region of the mouse:human comparative map shows a high degree of evolutionary conservation of gene order and content, which differs only by insertion of one gene (in mouse) and a small inversion involving two adjacent genes. "Low-pass" (2.2x) mouse sequence from a portion of the contig was ordered and oriented along 510 kb of finished HSA21 sequence. In combination with 68 kb of unique PAC end sequence, the comparison provided confirmation of genes predicted by comparative mapping, indicated gene predictions that are likely to be incorrect, and identified three candidate genes in mouse and human that were not observed in the initial HSA21 sequence annotation. This comparative map and sequence derived from it are powerful tools for identifying genes and regulatory regions, information that will in turn provide insights into the genetic mechanisms by which trisomy 21 results in DS.     

Localization of the rhodopsin gene to the distal half of mouse chromosome 6

We have assigned the mouse rhodopsin gene, Rho, to chromosome 6 using DNA from a set of mouse-hamster somatic hybrid cell lines and a partial cDNA clone for mouse opsin. This assignment rules out the direct involvement of the rhodopsin gene in the known mouse mutations that produce retinal degeneration, including retinal degeneration slow (rds, chromosome 17), retinal degeneration (rd, chromosome 5), Purkinje cell degeneration (pcd, chromosome 13), and nervous (nr, chromosome 8). Segregation of Rho-specific DNA fragment differences among 50 animals from an interspecific backcross (C57BL/6J X Mus spretus) X C57BL/6J indicates that the Rho locus is 4.0 +/- 2.8 map units distal to the locus for the proto-oncogene Raf-1 and 18.0 +/- 5.4 map units proximal to the locus for the proto-oncogene Kras-2. Linkage to Raf-1 was confirmed using four sets of recombinant inbred strains. The two loci RAF1 and RHO are also syntenic on human chromosome 3, but on opposite arms.     

The intestinal mast cell response to Trichinella spiralis infection in mast cell-deficient w/wv mice

The intestinal mast cell response and lymphoblast activity, as measured by the incorporation of 3H-thymidine into mesenteric lymph node cells (MLN) of WBB6F1-w/wv(w/wv) mice, their normal congenic littermates (+/+) and C57BL/6J mice, were compared after infection with Trichinella spiralis. Marked and similar blast cell activity and an increase in number of cells were observed in the MLN of infected w/wv and C57BL/6J mice 7 and 15 days P.I. In contrast to C57BL/6J mice, primary T. spiralis intestinal infections were prolonged in w/wv mice and more muscle larvae were recovered from w/wv mice 29 days post-infection. In C57BL/6J mice mucosal mast cell (MMC) numbers increased on day 7 P.I. whereas in w/wv mice these cells did not increase significantly until day 15 post-infection, reaching a peak on day 22. In w/wv mice, the response to secondary infection as determined by an accelerated expulsion of adult worms did not occur until day 11 postchallenge whereas in +/+ and C57BL/6J mice worm expulsion was nearly complete at that time. In both primary and secondary infections, the MMC numbers in w/wv mice were significantly lower than in C57BL/6J or +/+ mice. The results suggest that prolongation of T. spiralis infection in w/wv mice is associated with delayed appearance of mast cells in the intestinal mucosa which may reflect slow generation of the intestinal inflammatory response.     

A gene(s) for all-trans-retinoic acid-induced forelimb defects mapped and confirmed to murine chromosome 11

All-trans-retinoic acid (RA) induces various anatomical limb dysmorphologies in mice dependent on the time of exposure. During early limb development, RA induces forelimb ectrodactyly (digital absence) with varying susceptibilities for different inbred mouse strains; C57BL/6N are highly susceptible while SWV are resistant. To isolate the genetic basis of this defect, a full-genome scan was performed in 406 backcross fetuses of F(1) males to C57BL/6N females. Fetuses were exposed via a maternal injection of 75 mg of RA per kilogram of body weight on gestational day 9.25. The genome-wide analysis revealed significant linkage to a chromosome 11 locus near D11Mit39 with a maximum LOD score of 9.0 and to a chromosome 4 locus near D4Mit170. An epistatic interaction was detected between loci on chromosome 11 (D11Mit39) and chromosome 18 (D18Mit64). Linkage to the chromosome 11 locus (D11Mit39) was confirmed in RA-treated backcross fetuses of F(1) females to C57BL/6N males. Loci associated with bone density/mass in both human and mouse were previously detected in the same region, suggesting a mechanistic linkage with bone homeostasis. The human syntenic region of this locus has been previously linked to Meckel syndrome; the phenotype includes postaxial polydactyly, an ectopic digital defect hypothesized to be induced by a common molecular pathway with ectrodactyly.     

Three loci on mouse chromosome 5 and 10 modulate sex determination in XX Ods/+ mice

In mouse, XY embryos are committed to the male sex determination pathway after the transient expression of the Y-linked Sry gene in the Sertoli cell lineage between 10.5 and 12.5 dpc. In the C57BL/6J strain, male sex determination program can be modulated by some autosomal genes. The C57BL/6J alleles at these autosomal loci can antagonize male sex determination in combination with specific Sry alleles. In this report, the authors have identified an effect of these C57BL/6J specific alleles in combination with a mutated Sox9 allele, Sox9(Ods). Authors report the mapping of three of these genetic loci on mouse chromosome 5 and 10 in a backcross of the Ods mutation to the C57BL/6J background. Our study confirms the importance of the strain C57BL/6J for the investigation of the genetic mechanisms that control sex determination.     

The L-isoaspartyl/D-aspartyl protein methyltransferase gene (PCMT1) maps to human chromosome 6q22.3-6q24 and the syntenic region of mouse chromosome 10.

We have mapped the genes for the human and mouse L-isoaspartyl/D-aspartyl protein carboxyl methyltransferase (EC 2.1.1.77) using cDNA probes. We determined that the human gene is present in chromosome 6 by Southern blot analysis of DNA from a panel of mouse-human somatic cell hybrids. In situ hybridization studies allowed us to confirm this identification and further localize the human gene (PCMT1) to the 6q22.3-6q24 region. By analyzing the presence of an EcoRI polymorphism in DNA from backcrosses of C57BL/6J and Mus spretus strains of mice, we localized the mouse gene (Pcmt-1) to chromosome 10, at a position 8.2 +/- 3.5 cM proximal to the Myb locus. This region of the mouse chromosome is homologous to the human 6q24 region.     

The multipoint genetic mapping of mouse chromosome 16.

Utilizing a Mus spretus/Mus domesticus (C57BL/10) interspecific backcross, we have constructed a multipoint genetic map of mouse chromosome 16 that extends 43.2 cM from the proximal Prm-1 locus to the distal Ets-2 locus. The genetic map incorporates three new markers: D16Smh6, a random genomic clone; Pgk-1ps1, a phosphoglycerate kinase pseudogene; and the growth-associated protein Gap43. The map position of Gap43 indicates the presence, on mouse chromosome 16, of a significant-size conserved linkage group with human chromosome 3.     

Localization of insulin-2 (Ins-2) and the obesity mutant tubby (tub) to distinct regions of mouse chromosome 7.

A DNA mapping panel derived from an interspecific backcross was used to position the mouse insulin-2 locus (Ins-2) on Chromosome 7, near H19 (0/114 recombinants) and Th (1/114 recombinants). Ins-2 is part of a human-mouse conserved linkage group that includes Th, H19, and Igf-2. Analysis of segregation in the F2 generation from the cross C57BL/6J-tub/tub x CAST/Ei demonstrated that Ins-2 and the obesity mutant tubby (tub) are distinct loci, thus eliminating Ins-2 as a candidate gene for tub. These results also refine the estimated genetic distance between tub and Hbb to 2.4 +/- 1.4 cM. The predicted location for a human homolog of tubby is HSA 11p15.     

cDNA sequence and chromosomal localization of the mouse parvalbumin gene, Pva

In the homozygous condition, the mutation adr (arrested development of righting response) of the mouse causes a myotonia and a drastic reduction of the Ca2+-binding protein parvalbumin (PV) in fast muscles. Using a rat PV probe, a mouse cDNA clone was isolated from a lambda gt11 wild-type fast-muscle library and its nucleotide sequence was determined. The protein coding and the 3' nontranslated regions of the mouse gene show extensive homology with the rat PV gene. The result of Southern blot hybridization is consistent with a single copy gene for parvalbumin. Restriction fragment length polymorphisms (RFLPs) between Mus musculus domesticus (e.g. C57BL/6) and Mus spretus (SPE) were detected with the enzymes Eco RI, Pst I, and Sst I. The restriction fragment patterns of DNA samples from 65 individual offspring of (C57BL/6 x SPE)F1 x C57BL/6 backcrosses were tested with the PV probe and matched, for linkage detection, to pre-existing patterns established with various RFLP probes on the same samples. A co-distribution of PV-RFLPs with Pvt-1 and Mlvi-2, which had been localized on chromosome 15, was detected. Thus, the structural gene for PV, designated Pva, maps to chromosome 15 of the mouse whereas the adr mutation shows no linkage with markers on this chromosome. Gene locus homology between chromosome 15 of the mouse and chromosome 22 of man (which carries the human PV gene) is discussed.     

Mapping of alpha-spectrin on distal mouse chromosome 1

DNAs from different strains of inbred mice and feral Mus spretus were found to exhibit restriction fragment length polymorphisms (RFLP) when hybridized with a probe prepared from a c-DNA clone of the mouse alpha-spectrin (Spna-1) gene. Studies of five recombinant inbred strains and (C57BL/6 X M. spretus) F1 X C57BL/6 backcross mice demonstrated that these RFLPs were allelic and that Spna-1 is closely linked to Ly-9 and Ly-17 on the distal region of chromosome 1.     

A torrid zone on mouse chromosome 1 containing a cluster of recombinational hotspots

Within the 2.38-Mb Ath1 region of mouse chromosome 1, 42 of 45 genetic crossovers from crosses between C57BL/6J (B6) and either C3H/HeJ (H) or Mus spretus (SPRET) occurred in four zones (A-D); zone A, 100 kb long, contained a cluster of at least four recombination hotspots. F1 sperm assays indicate that within this "torrid zone" the most active hotspot (A3) can initiate recombination on H and SPRET but not B6 chromosomes. The A3 DNA sequence contains a (G/C)TTT repeat, long stretches of A or T, and a cyclic variation in AT content. Recombination was drastically reduced in a cross between B6 and a B6.SPRET Ath1 congenic strain, but was unaffected in a B6 x B6.H Ath1 congenic cross. Similar nonrandom clustering of hotspots has been observed in yeast and the major histocompatibility complexes of human and mouse. To the extent that torrid zones are a general feature of mammalian genomes, they have considerable implications for genetic mapping strategies in both human populations and mouse crosses.     

Mapping of PRM1 to human chromosome 16 and tight linkage of Prm-1 and Prm-2 on mouse chromosome 16

The protamines are small, arginine-rich nuclear proteins that replace histones and transition proteins late in the haploid phase of spermatogenesis in mammals. The two mouse genes encoding protamines--Prm-1 and Prm-2--have been molecularly cloned and mapped to mouse chromosome 16 (MMU 16). A cDNA clone of mouse Prm-1 that hybridized to the corresponding human gene was used to analyze a panel of somatic cell hybrids made between human lymphoblasts and the E36 hamster cell line. The human gene, which we have designated PRM 1, was syntenic with human chromosome 16 (HSA 16) and discordant with all other human chromosomes. Linkage analysis in the mouse was accomplished using the backcross (Czech II x BALB/c Pt) x Czech II to map Prm-1 and Prm-2 to a position near the 5' terminus of MMU 16. No recombination between Prm-1 and Prm-2 was observed among 89 progeny of the Czech II x BALB/c cross or among 94 progeny of the backcross (CBA/J x BALB/cJ) x BALB/cJ, demonstrating that the two loci are separated by less than 1.6 cM on MMU 16. This tight linkage may be of functional significance, as Prm-1 and Prm-2 are among a limited number of genes known to be expressed postmeiotically in male haploid germ cells.     

Genetic mapping of the Mx influenza virus resistance gene within the region of mouse chromosome 16 that is homologous to human chromosome 21.

A total of 318 progeny from four backcrosses involving different laboratory strains and subspecies of Mus musculus were analyzed to map the Mx gene to the region of mouse chromosome 16 (MMU 16) which is homologous to human chromosome 21 (HSA 21). This result suggests that Mx will be found in the region of HSA 21 which has been implicated in Down syndrome when inherited in three copies.     

Autoradiographic studies of the human Y chromosome

An autoradiographic analysis (using continuous labeling with tritiated thymidine) was made on 317 cells from four normal males. The labeling pattern of the Y chromosome was compared to the first and the last chromosomes to complete replication as well as to G21–22. The Y chromosome was never found to be the last chromosome in the cell to complete replication. Instead, it completed DNA synthesis relatively early (usually among the first 10 chromosomes) but had a distinctively heavy label during the earliest stages of late-S. In 51% of those cells with one labeled G+Y chromosome, a G21–22 was labeled and the Y was not.—It was concluded, therefore, that the human Y chromosome is not a late-replicating chromosome but terminates replication earlier than most of the autosomes. In addition, the Y chromosome cannot be distinguished from the G chromosomes on the basis of a consistent and differential labeling pattern.Supported by USPHS Grant GM 15361.