New allele of mouse DNA/RNA helicase senataxin causes meiotic arrest and infertility

In brief

A new allele of the senataxin gene Setxspcar3 causes meiotic arrest of spermatocytes with aberrant DNA damage and accumulation of R-loops.

Abstract

An unbiased screen for discovering novel mouse genes for fertility identified the spcar3, spermatocyte arrest 3, mutant phenotype. The spcar3 mutation identified a new allele of the Setx gene, encoding senataxin, a DNA/RNA helicase that regulates transcription termination by resolving DNA/RNA hybrid R-loop structures. The Setxspcar3 mutant mice exhibit male infertility and female subfertility. Histology of the Setxspcar3 mutant testes revealed the absence of spermatids and mature spermatozoa in the seminiferous tubules. Cytological analysis of chromosome preparations of the Setxspcar3 mutant spermatocytes revealed normal synapsis, but aberrant DNA damage in the autosomes, defective formation of the sex body, and arrest of meiosis in mid-prophase. Additionally, Setxspcar3 testicular cells exhibit abnormal accumulation of R-loops. Transient expression assays identified regions of the senataxin protein required for sub-nuclear localization. Together, these results not only confirm that senataxin is required for normal meiosis and spermatogenesis but also provide a new resource for the determination of its role in maintaining R-loop formation and genome integrity.

Abstract 4611: Identifying drivers of mammary tumorigenesis & elucidating the mechanisms of cancer initiation in DNA replication defective Chaos3 mice

Distinguishing commonly deleted genes as drivers or passengers of human cancers are key to delineating mechanisms of tumor initiation. The “Chaos3” mouse model of spontaneous breast cancer carries a missense mutation in a gene called Mcm4 (minichromosome maintenance 4), causing destabilization of the MCM2-7 replicative helicase, which in turn causes elevated genomic instability. Approximately 80% of female mice homozygous for this mutation in the C3HeB/FeJ strain background develop mammary tumors with an average latency of 12 months. Genomic analysis of these tumors revealed a recurrent subset of genes with copy number alterations such as Arid1a and Nf1. This project aims to: 1) test whether the genes Arid1a and Nf1 act as mammary tumor drivers in the Chaos3 breast cancer model; 2) understand the genome-wide effects of Arid1a loss in mammary tumorigenesis; 3) identifying potential therapeutic targets for Arid1a-deficient mammary tumors; and 4) unravel the mechanism by which destabilization of MCM2-7 replicative helicase leads to cancer onset. To achieve these aims, several experiments such as conditional mutagenesis, CUTandRUN, RNA-seq, and replication timing profiling were conducted to address my aims. Preliminary data indicates that heterozygous loss of Arid1ain Chaos3 mammary tumors produces a distinct transcriptional profile compared to mammary tumors without Arid1a deletion. Additionally, Chaos3 primary cells have markedly altered replication timing patterns in certain regions of the genome that might explain increased susceptibility to mutations. My experiments can validate new candidates like Arid1a as a driver of sporadic breast cancer in humans. In addition, these studies will not only lend insight into how key tumor suppressors are deleted in the Chaos3 model, but also potentially reveal features of genomic regions that are particularly susceptible to genetic or environmental perturbations to DNA replication. The Chaos3 sporadic breast cancer mouse model mimics sporadic human luminal breast cancer and can be used as a tool to better understand the biology and genetics of a common disease that plagues women worldwide.

Citation Format: Marquita Winters, John Schimenti. Identifying drivers of mammary tumorigenesis & elucidating the mechanisms of cancer initiation in DNA replication defective Chaos3 mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4611.

ENU-induced mutant allele of Dnah1, ferf1, causes abnormal sperm behavior and fertilization failure in mice

Given attention to both contraception and treatment of infertility, there is a need to identify genes and sequence variants required for mammalian fertility. Recent unbiased mutagenesis strategies have expanded horizons of genetic control of reproduction. Here we show that male mice homozygous for the ethyl-nitroso-urea-induced ferf1 (fertilization failure 1) mutation are infertile, producing apparently normal sperm that does not fertilize oocytes in standard fertilization in vitro fertilization assays. The ferf1 mutation is a single-base change in the Dnah1 gene, encoding an axoneme-associated dynein heavy chain, and previously associated with male infertility in both mice and humans. This missense mutation causes a single-amino-acid change in the DNAH1 protein in ferf1 mutant mice that leads to abnormal sperm clumping, aberrant sperm motility, and the inability of sperm to penetrate the oocyte's zona pellucida; however, the ferf1 mutant sperm is competent to fertilize zona-free oocytes. Taken together, the various mutations affecting the DNAH1 protein in both mouse and human produce a diversity of phenotypes with both subtle and considerable differences. Thus, future identification of the interacting partners of DNAH1 might lead to understanding its unique function among the sperm dyneins.

MEI4 – a central player in the regulation of meiotic DNA double-strand break formation in the mouse

The formation of programmed DNA double-strand breaks (DSBs) at the beginning of meiotic prophase marks the initiation of meiotic recombination. Meiotic DSB formation is catalyzed by SPO11 and their repair takes place on meiotic chromosome axes. The evolutionarily conserved MEI4 protein is required for meiotic DSB formation and is localized on chromosome axes. Here, we show that HORMAD1, one of the meiotic chromosome axis components, is required for MEI4 localization. Importantly, the quantitative correlation between the level of axis-associated MEI4 and DSB formation suggests that axis-associated MEI4 could be a limiting factor for DSB formation. We also show that MEI1, REC8 and RAD21L are important for proper MEI4 localization. These findings on MEI4 dynamics during meiotic prophase suggest that the association of MEI4 to chromosome axes is required for DSB formation, and that the loss of this association upon DSB repair could contribute to turning off meiotic DSB formation.

Meiosis-specific cohesin mediates homolog recognition in mouse spermatocytes

During meiosis, homologous chromosome (homolog) pairing is promoted by several layers of regulation that include dynamic chromosome movement and meiotic recombination. However, the way in which homologs recognize each other remains a fundamental issue in chromosome biology. Here, we show that homolog recognition or association initiates upon entry into meiotic prophase before axis assembly and double-strand break (DSB) formation. This homolog association develops into tight pairing only during or after axis formation. Intriguingly, the ability to recognize homologs is retained in Sun1 knockout spermatocytes, in which telomere-directed chromosome movement is abolished, and this is the case even in Spo11 knockout spermatocytes, in which DSB-dependent DNA homology search is absent. Disruption of meiosis-specific cohesin RAD21L precludes the initial association of homologs as well as the subsequent pairing in spermatocytes. These findings suggest the intriguing possibility that homolog recognition is achieved primarily by searching for homology in the chromosome architecture as defined by meiosis-specific cohesin rather than in the DNA sequence itself.

Meiotic cohesin complexes are essential for the formation of the axial element in mice

Cohesin is a conserved multisubunit protein complex that participates in chromosome segregation, DNA damage repair, chromatin regulation, and synaptonemal complex (SC) formation. Yeast, but not mice, depleted of the cohesin subunit Rec8 are defective in the formation of the axial elements (AEs) of the SC, suggesting that, in mammals, this function is not conserved. In this paper, we show that spermatocytes from mice lacking the two meiosis-specific cohesin subunits RAD21L and REC8 were unable to initiate RAD51- but not DMC1-mediated double-strand break repair, were not able to assemble their AEs, and arrested as early as the leptotene stage of prophase I, demonstrating that cohesin plays an essential role in AE assembly that is conserved from yeast to mammals.

Haploid embryonic stem cells and the dominance of recessive traits

Diploidy, though essential for normal development, is a foil to geneticists. Two recent studies (Elling et al., 2011, this issue of Cell Stem Cell; Leeb and Wutz, 2011, Nature), report the isolation of haploid pluripotent mouse ESCs, thus enabling efficient functional screening for genes involved in diverse cellular and developmental processes.

Regulating RNA polymerase pausing and transcription elongation in embryonic stem cells

Transitions between pluripotent stem cells and differentiated cells are executed by key transcription regulators. Comparative measurements of RNA polymerase distribution over the genome's primary transcription units in different cell states can identify the genes and steps in the transcription cycle that are regulated during such transitions. To identify the complete transcriptional profiles of RNA polymerases with high sensitivity and resolution, as well as the critical regulated steps upon which regulatory factors act, we used genome-wide nuclear run-on (GRO-seq) to map the density and orientation of transcriptionally engaged RNA polymerases in mouse embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs). In both cell types, progression of a promoter-proximal, paused RNA polymerase II (Pol II) into productive elongation is a rate-limiting step in transcription of ∼40% of mRNA-encoding genes. Importantly, quantitative comparisons between cell types reveal that transcription is controlled frequently at paused Pol II's entry into elongation. Furthermore, "bivalent" ESC genes (exhibiting both active and repressive histone modifications) bound by Polycomb group complexes PRC1 (Polycomb-repressive complex 1) and PRC2 show dramatically reduced levels of paused Pol II at promoters relative to an average gene. In contrast, bivalent promoters bound by only PRC2 allow Pol II pausing, but it is confined to extremely 5' proximal regions. Altogether, these findings identify rate-limiting targets for transcription regulation during cell differentiation.

Defective imprint resetting in carriers of Robertsonian translocation Rb (8.12)

Meiotic silencing of unsynapsed chromatin (MSUC) occurs in the germ cells of translocation carriers and may cause meiotic arrest and infertility. We hypothesized that if bypassing meiotic checkpoints MSUC may cause epigenetic defects in sperm. We investigated the meiotic behavior of the Robertsonian translocation Rb (8.12) in mice. The unsynapsed 8 and 12 trivalent was associated with the XY body during early and mid-pachynema in heterozygous Rb (8.12) carriers, suggesting possible silencing of pericentromeric genes, such as the Dnmt3a gene. In wild-type mice, DNMT3A protein showed a dramatic accumulation in the nucleus during the mid-pachytene stage and distinct association with the XY body. In translocation carriers, DNMT3A was less abundant in a proportion of pachytene spermatocytes that also had unsynapsed pericentromeric regions of chromosomes 8 and 12. The same mice had incomplete methylation of the imprinted H19 differentially methylated region (DMR) in sperm. We propose that impaired H19 imprint establishment results from lack of synapsis in chromosomes 8 and 12 probably through transient silencing of a chromosome 8 or 12 gene during pachynema. Furthermore, our findings support the notion that imprint establishment at the H19 locus extends into pachynema.

Mutagenesis of mouse embryonic stem cells with ethylmethanesulfonate

Unraveling the function of the mammalian genome relies heavily on analyses of the laboratory mouse. Because of its powerful genetics and available technologies to manipulate the genome, plus its developmental and physiological similarities to humans, it has become a goal to generate mutations in all mouse genes and analyze the phenotypic consequences. Gene targeting in embryonic stem (ES) cells is the method of choice for making null mutations in known genes of interest. However, forward genetics approaches, in which mutations are produced randomly throughout the genome, has the advantage of producing alleles of varying severity both within known genes, in non-coding regulatory elements, or in other unannotated functional elements. Such forward genetic mutation screens in mice have typically involved treating male mice with N-ethyl-N-nitrosourea (ENU), followed by three generations of breeding to render potential recessive mutations homozygous, at which time phenotype screens can be performed. An alternative strategy for randomly mutagenizing the mouse genome is by chemical treatment of ES cells. This enables the use of multiple alternative chemicals with different mutational spectra, can reduce breeding to two generations, and impart a higher mutational load. Furthermore, ES cell mutagenesis can be used to create banks of clones that can be screened for point mutations in genes of interest, and to conduct forward genetic screens in vitro to detect potential phenotypes prior to generation of mice. In this chapter, we provide a detailed protocol for mutagenizing ES cells with the point mutagen ethylmethanesulfonate (EMS).

Sperm Motility Defects and Infertility in Male Mice with a Mutation in Nsun7, a Member of the Sun Domain-Containing Family of Putative RNA Methyltransferases1

Poor sperm quality is the major cause of infertility in humans. Other than sex-linked factors, the genetic basis for male infertility is poorly defined, largely due to practical difficulties in studying the inheritance of this trait in humans. As an alternative, we have conducted forward genetic screens in mice to generate relevant models. We report on the identification and characterization of a chemically-induced mutation, Ste5Jcs1, which causes affected male mice to be sterile or subfertile. Mutant sperm exhibited depressed progressive motility associated with a rigid flagellar midpiece (but not principal piece) segment, which could not be rescued by treatment with agents that stimulate cAMP or calcium signaling pathways. Overall mutant sperm ultrastructure appeared normal, including the axoneme, although the midpiece mitochondrial sheath showed abnormal electron density patterns. Positional cloning of Ste5Jcs1 led to the identification of a mutation in a novel gene called Nsun7, which encodes a protein with a Sun domain that is homologous to tRNA and rRNA cytosine methyltransferases. Therefore, Ste5Jcs1 mutation uncovers a previously unrecognized biological process in sperm that underscores the functional compartmentalization of the midpiece and principal piece of the flagellum.

Fast forward to new genes in mammalian reproduction

The study of reproductive genetics in mammals has lagged behind that of simpler and more tractable model organisms, such as D. melanogaster, C. elegans and various yeast models. Although much valuable information has been generated using these organisms, they do not model the genetic and biological complexity of mammalian reproduction. Thus, the majority of genes required for gametogenesis in mammals remain unidentified. To expand on the existing knowledge of mammalian reproductive genetics, we have carried out forward genetic screens in mice to identify infertility mutants and the underlying mutant genes. Two different approaches were used: mutagenesis of the germline in whole mice, and mutagenesis of embryonic stem cells. This was followed by two- or three-generation breeding schemes to identify pedigrees segregating infertility mutations, which were then phenotypically characterized, genetically mapped, and in some cases, positionally cloned. This whole-genome approach has generated a wide collection of mutants with defects ranging from problems with germ cell development to abnormal sperm morphology. These models have allowed us to study the genetics, as well as the physiology, of reproduction in mammals. This review focuses on describing some of the genes identified in these screens and the ongoing effort to characterize additional mutants.

Mutations that affect meiosis in male mice influence the dynamics of the mid-preleptotene and bouquet stages

Meiosis pairs and segregates homologous chromosomes and thereby forms haploid germ cells to compensate the genome doubling at fertilization. Homologue pairing in many eukaryotic species depends on formation of DNA double strand breaks (DSBs) during early prophase I when telomeres begin to cluster at the nuclear periphery (bouquet stage). By fluorescence in situ hybridization criteria, we observe that mid-preleptotene and bouquet stage frequencies are altered in male mice deficient for proteins required for recombination, ubiquitin conjugation and telomere length control. The generally low frequencies of mid-preleptotene spermatocytes were significantly increased in male mice lacking recombination proteins SPO11, MEI1, MLH1, KU80, ubiquitin conjugating enzyme HR6B, and in mice with only one copy of the telomere length regulator Terf1. The bouquet stage was significantly enriched in Atm(-/-), Spo11(-/-), Mei1(m1Jcs/m1Jcs), Mlh1(-/-), Terf1(+/-) and Hr6b(-/-) spermatogenesis, but not in mice lacking recombination proteins DMC1 and HOP2, the non-homologous end-joining DNA repair factor KU80 and the ATM downstream effector GADD45a. Mice defective in spermiogenesis (Tnp1(-/-), Gmcl1(-/-), Asm(-/-)) showed wild-type mid-preleptotene and bouquet frequencies. A low frequency of bouquet spermatocytes in Spo11(-/-)Atm(-/-) spermatogenesis suggests that DSBs contribute to the Atm(-/-)-correlated bouquet stage exit defect. Insignificant changes of bouquet frequencies in mice with defects in early stages of DSB repair (Dmc1(-/-), Hop2(-/-)) suggest that there is an ATM-specific influence on bouquet stage duration. Altogether, it appears that several pathways influence telomere dynamics in mammalian meiosis.

Mutagenesis as an unbiased approach to identify novel contraceptive targets

To accommodate diverse personal needs in family planning, diverse contraceptive approaches are desirable. This goal requires identification of new contraceptive targets. Phenotype-driven mutagenesis is an unbiased approach to identify novel genes and functions in reproductive processes. The ReproGenomics Program at The Jackson Laboratory is a United States National Institutes of Health resource for production, identification and distribution of mutant mouse models of infertility that can be used for identification of potential targets for contraception. The strategy of this program is whole genome, random ENU mutagenesis, coupled with a phenotype screen for breeding failure as the only phenotype. A three-generation breeding scheme selects recessive mutations affecting reproductive functions. G3 males and females that fail to reproduce by natural mating to wild-type animals undergo secondary phenotype screens to assess gonad and accessory organ histology, hormone production, gamete production and gamete function in fertilization. The genetic transmission of the infertility trait in each family is confirmed and each mutation is genetically mapped to a defined chromosome region, facilitating identification of candidate genes from sequence and expression databases. Genes essential for fertility in both males and females and acting both meiotically and post-meiotically have been identified by this strategy. Phenotypes include male infertility with normal sperm count, but failure in fertilization of oocytes. Phenotype descriptions of each mutation are posted on the program website, . These unique reproductive mutant mouse resources will lead to new discoveries in andrology (and gynecology) research, as well as reproductive medicine. Dissection of gene function in known and newly discovered reproductive pathways will expand our focus to reveal novel targets for contraception.

Forward genetic screens for meiotic and mitotic recombination-defective mutants in mice

The goal of understanding the function of all mammalian genes is best accomplished through mutational analyses. Although the sequence of the mouse genome is now available and many genes have been identified, it is not possible to ascribe functions accurately to these genes in silico. Gene targeting using embryonic stem cells is ideal for analysis of individual genes selected on the basis of sequence features, but it is impractical for identifying novel genes involved in particular biological processes. Phenotype-based random mutagenesis of the genome is well suited for this goal. In the mouse, N-ethyl-N-nitrosourea (ENU) induces point mutations at a high frequency in the mouse germline. In this chapter, we describe methods for detecting and characterizing recombination mutations in mice produced by ENU mutagenesis. Potential meiotic recombination mutants are identified in a hierarchical fashion, by performing a screen for infertility, then gonad histology to determine whether meiotic arrest occurs, and finally by immunohistochemical analysis of meiotic chromosome with a battery of antibody markers. Screening for mutations potentially required for recombinational repair of DNA damage in somatic cells is performed using a flow cytometry-based micronucleus assay. Both strategies have proved effective in identifying desired classes of mutations.

Segregation distortion of mouse t haplotypes the molecular basis emerges

The t haplotype is an ancestral version of proximal mouse chromosome 17 that has evolved mechanisms to persist as an intact genomic variant in mouse populations. t haplotypes contain mutations that affect embryonic development, male fertility and male transmission ratio distortion (TRD). Collectively, these mutations drive the evolutionary success of t haplotypes, a phenomenon that remains one of the longstanding mysteries of mouse genetics. Molecular genetic analysis of TRD has been confounded by inversions that arose to lock together the various elements of this complex trait. Our first molecular glimpse of the TRD mechanism has finally been revealed with the cloning of the t complex responder (Tcr) locus, a chimeric kinase with a genetically cis active effect. Whereas + sperm in a +/t male have impaired flagellar function caused by the deleterious action of trans-active, t-haplotype-encoded 'distorters,' the mutant activity of Tcr counterbalances the distorter effects, maintaining the motility and fertilizing ability of t sperm.

Functional genomics in the mouse: phenotype-based mutagenesis screens

Significant progress has been made in sequencing the genomes of several model organisms, and efforts are now underway to complete the sequencing of the human genome. In parallel with this effort, new approaches are being developed for the elucidation of the functional content of the human genome. The mouse will have an important role in this phase of the genome project as a model system. In this review we discuss and compare classical genetic approaches to gene function-phenotype-based mutagenesis screens aimed at the establishment of a large collection of single gene mutations affecting a wide range of phenotypic traits in the mouse. Whereas large scale genome-wide screens that are directed at the identification of all loci contributing to a specific phenotype may be impractical, region-specific saturation screens that provide mutations within a delimited chromosomal region are a feasible alternative. Region-specific screens in the mouse can be performed in only two generations by combining high-efficiency chemical mutagenesis with deletion complexes generated using embryonic stem (ES) cells. The ability to create and analyze deletion complexes rapidly, as well as to map novel chemically-induced mutations within these complexes, will facilitate systematic functional analysis of the mouse genome and corresponding gene sequences in humans. Furthermore, as the extent of the mouse genome sequencing effort is still uncertain, we underscore a necessity to direct sequencing efforts to those chromosomal regions that are targets for extensive mutagenesis screens.

Utility of C57BL/6J x 129/SvJae embryonic stem cells for generating chromosomal deletions: tolerance to gamma radiation and microsatellite polymorphism

We have previously reported a method for making nested deletion complexes in mice by irradiation of ES cells. The key to this technology is that F1 hybrid ES cells (called v17.2) of the genotype (BALB/cTa x 129/SvJae) retain germline colonizing ability after exposure to levels of ionizing radiation that induce chromosomal deletions. In an effort to identify other genotypes of ES cells that are suitable for this technology, the radiation sensitivity of the cell line v6.4, which is of the genotype (C57BL/6J x 129/SvJae), was investigated. After treatment with a range of radiation exposures, the developmental potential of these cells was assayed by injecting them into blastocysts to generate chimeric mice. These experiments showed that while cell lethality increased as the level of radiation increased, the surviving ES cells retained full totipotency at all exposure levels, up to 400 Rads. Because polymorphism between parental microsatellite alleles in the F1 hybrid ES cells is important for ascertaining the sizes of induced deletions, the 129/SvJ and 129/SvJae allele sizes of 48 microsatellite loci on chromosome (Chr) 17 were determined. This revealed a higher level of polymorphism between 129 and C57BL/6J on Chr 17. The radiation tolerance, high polymorphism between parental strains, and presence of the widely used C57BL/6J strain component make v6.4 ES cells an attractive cell line for generating radiation-induced chromosomal deletions.

Chromosomal deletion complexes in mice by radiation of embryonic stem cells

Chromosomal deletions ("deficiencies') are powerful tools in the genetic analysis of complex genomes. They have been exploited extensively in Drosophila melanogaster, an organism in which deficiencies can be efficiently induced and selected. Spontaneous deletions in humans have facilitated the dissection of phenotypes in contiguous gene syndromes and led to the positional cloning of critical genes. In mice, deletion complexes created by whole animal irradiation experiments have enabled a systematic characterization of functional units along defined chromosomal regions. However, classical mutagenesis in mice is logistically impractical for generating deletion sets on a genome-wide scale. Here, we report a high-throughput method for generating radiation-induced deletion complexes at defined regions in the genome using ES cells. Dozens of deletions of up to several centiMorgans, encompassing a specific locus, can be created in a single experiment and transmitted through the germline. The ability to rapidly create deletion complexes along chromosomes will facilitate systematic functional analyses of the mammalian genome.

Targeted mutagenesis of a candidate t complex responder gene in mouse t haplotypes does not eliminate transmission ratio distortion

Transmission ratio distortion (TRI) associated with mouse t haplotypes causes +/t males to transmit the t-bearing chromosome to nearly all their offspring. Of the several genes involved in this phenomenon, the t complex responder (Tcrt) locus is absolutely essential for TRD to occur. A candidate Tcrt gene called Tcp10bt was previously cloned from the genetically defined Tcrt region. Its location, restricted expression in testis, and a unique postmeiotic alternative splicing pattern supported the idea that Tcp10bt was Tcrt. To test this hypothesis in a functional assay, ES cells were derived from a viable partial t haplotype, and the Tcp10bt gene was mutated by homologous recombination. Mutant mice were mated to appropriate partial t haplotypes to determine whether the targeted chromosome exhibited transmission ratios characteristic of the responder. The results demonstrated that the targeted chromosome retained full responder activity. Hence, Tcp10bt does not appear to be Tcrt. These and other observations necessitate a reevaluation of genetic mapping data and the actual nature of the responder.

Problems with procedures

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Podiatric malpractice litigation. What to do and what to expect

Many podiatric physicians will never be sued during their careers, but if a suit happens, it can be one of the most stressful times in their lives. After contacting the insurance carrier, the podiatric physician must then wait as the case develops through the legal system. The deposition is when the podiatric physician will be asked questions about the case. It is important to remember to carefully answer the questions asked. Once in the trial stage, the appearance and testimony of the podiatric physician will be important in the jury's eyes. If a decision is not in your favor, you may be able to appeal the case to a higher court. Some cases may not go to trial as they could be settled or arbitrated along the way. By listening to your attorney and following the attorney's advice and recommendations, the legal process will be easier to manage and understand.

Human homologs of two testes-expressed loci on mouse chromosome 17 map to opposite arms of chromosome 6

Our laboratory has recently cloned and characterized two testes-expressed loci--the Tcp-10 gene family cluster and the D17Si11 gene--that map to the proximal portion of mouse chromosome 17. Human homologs of both loci have been identified and cloned. Somatic cell hybrid lines have been used to map the human homolog of D17Si11 to the short arm of chromosome 6 (p11-p21.1) along with homologs of other genes from the (Pim-1)-(Pgk-2) region of the mouse chromosome. The human TCP 10 locus maps to the long arm of chromosome 6 (q21-qter) along with homologs of other genes from the mouse chromosome 17 region between the centromere and Pim-1. The mapping of large portions of the mouse t haplotype to unlinked regions on human chromosome 6 rules out the possibility that a t-haplotype-like chromosome could exist in humans.

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.

A candidate gene family for the mouse t complex responder (Tcr) locus responsible for haploid effects on sperm function

The mouse t complex responder (Tcr) locus plays a central haploid-specific role in the transmission ratio distortion phenotype expressed during germ cell differentiation in t-carrying males. The accumulated data map Tcr to a region of less than 500 kb. Over 400 kb of this region has been cloned and consists entirely of sequences associated with a clustered family of large cross-hybridizing elements of 30 kb to 70 kb in size. We have characterized a gene family within this region that is expressed uniquely in male germ cells with a complex pattern of RNA processing. Antibodies produced against a product of the putative open reading frame recognize a testes-specific polypeptide. Genetic data support the hypothesis that this polypeptide(s) functions to effect the Tcr phenotype.

An unstable family of large DNA elements in the center of the mouse t complex

We have cloned 363 kb (X 10(3) bases) from a novel, locally dispersed family of 11 large DNA elements, called T66 elements, within the center of complete mouse t haplotypes. Homologies among individual members of the T66 family are observed along a repeated unit of at least 75 kb in length. Individual T66 homology units are classified into three subfamilies through hybridization studies with a series of diagnostic subfamily-specific probes. The organization and number of elements in wild-type forms of chromosome 17 are very different from those found within t haplotype forms of this chromosome. The number of T66 elements present within individual chromosomes is highly polymorphic among both inbred strains of mice and among independently derived t haplotypes. Wild-type chromosomes have between five and nine T66 elements distributed between two loci that are separated by a genetic distance of at least three map units, whereas t haplotypes have between 9 and 11 T66 elements within a single cluster. Many of the rare recovered products of recombination between a t haplotype and a wild-type form of chromosome 17 have resulted from recombination within or near the T66 regions present on each chromosome. Molecular and genetic data lead to the speculation that portions of individual T66 homology units could be involved in t haplotype effects on sperm differentiation.