A high-resolution map around the locus Om on mouse Chromosome 11

Locating a modifier gene of Ovum mutant through crosses between DDK and C57BL/6J inbred strains in mice

A striking infertile phenotype has been discovered in the DDK strain of mouse. The DDK females are usually infertile when crossed with males of other inbred strains, whereas DDK males exhibit normal fertility in reciprocal crosses. This phenomenon is caused by mutation in the ovum (Om) locus on chromosome 11 and known as the DDK syndrome. Previously, some research groups reported that the embryonic mortality deviated from the semilethal rate in backcrosses between heterozygous (Om/+) females and males of other strains. This embryonic mortality exhibited an aggravated trend with increasing background genes of other strains. These results indicated that some modifier genes of Om were present in other strains. In the present study, a population of N₂2 (Om/+) females from the backcrosses between C57BL/6J (B6) and F₁ (B6♀ × DDK♂) was used to map potential modifier genes of Om. Quantitative trait locus showed that a major locus, namely Amom1 (aggravate modifier gene of Om 1), was located at the middle part of chromosome 9 in mice. The Amom1 could increase the expressivity of Om gene, thereby aggravating embryonic lethality when heterozygous (Om/+) females mated with males of B6 strain. Further, the 1.5 LOD-drop analysis indicated that the confidence interval was between 37.54 and 44.46 cM, ~6.92 cM. Amom1 is the first modifier gene of Om in the B6 background.

Embryologic, cytobiologic and genetic interpretations of DDK syndrome in mice

DDK syndrome is known as embryonic death at the morula-blastocyst stage in female mice of the DDK strain mated with males from other strains (alien males). The embryonic death is interpreted to be caused by incompatibility between oocyte factors and the product from male pronucleus, both of which are under the control of alleles at the same locus on Chromosome 11. This review explains the hypothesis proposing that the embryonic death may be caused primarily by failure in de novo regeneration of centrosomes containing centrioles in the trophectodermal cells. Centrioles disintegrate during gametogenesis in mice, and new centrioles are formed after the cleavage stage during which cell division proceeds with the microtubule organizing center having no centrioles. The failure in de novo regeneration of the centrosomes may arrest cell division and consequently result in embryonic death. Another aspect of DDK syndrome is distortion of the second polar body extrusion in the semi-incompatible cross. In the heterozygous (DDK/alien) oocytes fertilized with alien spermatozoa, DDK allele is more frequently retained in the oocyte nucleus, and alien allele tends to be carried into the polar body. This distortion may possibly be caused by derangement in the spindle system. Therefore, both aspects of DDK syndrome can be regarded as being derived from the abnormality in the centrosome-spindle system according to this hypothesis.

Reduced adiposity in ob/ob mice following total body irradiation and bone marrow transplantation

OBJECTIVE

The objective of this study was to assess long-term metabolic consequences of total body irradiation (TBI) and bone marrow transplantation. Severe obesity develops due to both hypertrophy and hyperplasia of adipocytes. We hypothesized that TBI would arrest adipose tissue growth and would affect insulin resistance (IR).

RESEARCH METHODS AND PROCEDURES

We exposed 2-month-old female ob/ob mice to 8 Grays of TBI followed by bone marrow transplantation and tested the animals for body weight (BW) gain, body composition, blood glucose, and insulin sensitivity.

RESULTS

Two months after TBI, irradiated mice stopped gaining BW, whereas non-treated mice continued to grow. At the age of 9.5 months, body mass of irradiated mice was 60.6 +/- 1.4 grams, which was only 61% of that in non-treated ob/ob controls (99.4 +/- 1.6 grams). Body composition measurements by DXA showed that decreased BW was primarily due to an impaired fat accumulation. This could not result from the production of leptin by bone marrow-derived adipocyte progenitors because inhibition of the obese phenotype was identical in recipients of both B6 and ob/ob bone marrow. Inability of the irradiated mice to accumulate fat was associated with hepatomegaly, lower levels of monocyte chemoattractant protein-1 expression in adipose tissue, and increased IR.

DISCUSSION

Our data argue in favor of the hypothesis that inability of adipose tissue to expand may increase IR. This mouse model may be valuable for studies of late-onset radiation-induced IR in humans.

Rescue of the mouse DDK syndrome by parent-of-origin-dependent modifiers

When females of the DDK inbred mouse strain are mated to males of other strains, 90-100% of the resulting embryos die during early embryonic development. This DDK syndrome lethality results from incompatibility between an ooplasmic DDK factor and a non-DDK paternal gene, which map to closely linked loci on chromosome 11. It has been proposed that the expression of the gene that encodes the ooplasmic factor is subject to allelic exclusion in oocytes. Previous studies have demonstrated the existence of recessive modifiers that increase lethality in the C57BL/6 and BALB/c strains. These modifiers are thought to skew the choice of allele undergoing allelic exclusion in the oocytes of heterozygous females. In the present study, we demonstrate the presence of modifiers in three Mus musculus domesticus wild-derived strains, PERA, PERC, and RBA. These modifiers completely rescued DDK syndrome lethality. We mapped the major locus that is responsible for rescue in PERA and PERC crosses to proximal chromosome 13 and named this locus Rmod1 (Rescue Modifier of the DDK Syndrome 1). Our experiments demonstrate that PERA or PERC alleles at Rmod1 rescue lethality independently of allelic exclusion. In addition, rescue of the lethal phenotype depends on the parental origin of the Rmod1 alleles; transmission through the dam leads to rescue, while transmission through the sire has no effect.

The paternal gene of the DDK syndrome maps to the Schlafen gene cluster on mouse chromosome 11

The DDK syndrome is an early embryonic lethal phenotype observed in crosses between females of the DDK inbred mouse strain and many non-DDK males. Lethality results from an incompatibility between a maternal DDK factor and a non-DDK paternal gene, both of which have been mapped to the Ovum mutant (Om) locus on mouse chromosome 11. Here we define a 465-kb candidate interval for the paternal gene by recombinant progeny testing. To further refine the candidate interval we determined whether males from 17 classical and wild-derived inbred strains are interfertile with DDK females. We conclude that the incompatible paternal allele arose in the Mus musculus domesticus lineage and that incompatible strains should share a common haplotype spanning the paternal gene. We tested for association between paternal allele compatibility/incompatibility and 167 genetic variants located in the candidate interval. Two diallelic SNPs, located in the Schlafen gene cluster, are completely predictive of the polar-lethal phenotype. These SNPs also predict the compatible or incompatible status of males of five additional strains.

Maternal transmission ratio distortion at the mouse Om locus results from meiotic drive at the second meiotic division

We have observed maternal transmission ratio distortion (TRD) in favor of DDK alleles at the Ovum mutant (Om) locus on mouse chromosome 11 among the offspring of (C57BL/6 x DDK) F(1) females and C57BL/6 males. Although significant lethality occurs in this backcross ( approximately 50%), differences in the level of TRD found in recombinant vs. nonrecombinant chromosomes among offspring argue that TRD is due to nonrandom segregation of chromatids at the second meiotic division, i.e., true meiotic drive. We tested this hypothesis directly, by determining the centromere and Om genotypes of individual chromatids in zygote stage embryos. We found similar levels of TRD in favor of DDK alleles at Om in the female pronucleus and TRD in favor of C57BL/6 alleles at Om in the second polar body. In those embryos for which complete dyads have been reconstructed, TRD was present only in those inheriting heteromorphic dyads. These results demonstrate that meiotic drive occurs at MII and that preferential death of one genotypic class of embryo does not play a large role in the TRD.

Transcript map of the Ovum mutant (Om) locus: isolation by exon trapping of new candidate genes for the DDK syndrome

The DDK syndrome is defined as the embryonic lethality of F1 mouse embryos from crosses between DDK females and males from other strains (named hereafter as non-DDK strains). Genetically controlled by the Ovum mutant (Om) locus, it is due to a deleterious interaction between a maternal factor present in DDK oocytes and the non-DDK paternal pronucleus. Therefore, the DDK syndrome constitutes a unique genetic tool to study the crucial interactions that take place between the parental genomes and the egg cytoplasm during mammalian development. In this paper, we present an extensive analysis performed by exon trapping on the Om region. Twenty-seven trapped sequences were from genes in the databases: beta-adaptin, CCT zeta2, DNA LigaseIII, Notchless, Rad51l3 and Scya1. Twenty-eight other sequences presented similarities with expressed sequence tags and genomic sequences whereas 57 did not. The pattern of expression of 37 of these markers was established. Importantly, five of them are expressed in DDK oocytes and are candidate genes for the maternal factor, and 20 are candidate genes for the paternal factor since they are expressed in testis. This data is an important step towards identifying the genes responsible for the DDK syndrome.

A 2-Mb YAC/BAC-based physical map of the ovum mutant (Om) locus region on mouse chromosome 11

The embryonic lethal phenotype observed when DDK females are crossed with males from other strains results from a deleterious interaction between the egg cytoplasm and the paternal pronucleus soon after fertilization. We have previously mapped the Om locus responsible for this phenotype, called the DDK syndrome, to an approximately 2-cM region of chromosome 11. Here, we report the generation of a physical map of 28 yeast and bacterial artificial chromosome clones encompassing the entire genetic interval containing the Om locus. This contig, spanning approximately 2 Mb, was used to map precisely genes and genetic markers of the region. We determined the maximum physical interval for Om to be 1400 kb. In addition, 11 members of the Scya gene family were found to be organized into two clusters at the borders of the Om region. Two other genes (Rad51l3 and Schlafen 2) and one EST (D11Wsu78e) were also mapped in the Om region. This integrated map provides support for the identification of additional candidate genes for the DDK syndrome.

Heritability of the maternal meiotic drive system linked to Om and high-resolution mapping of the Responder locus in mouse

Matings between (C57BL/6 x DDK)F(1) females and C57BL/6 males result in a significant excess of offspring inheriting maternal DDK alleles in the central region of mouse chromosome 11 due to meiotic drive at the second meiotic division. We have shown previously that the locus subject to selection is in the vicinity of D11Mit66, a marker closely linked to the Om locus that controls the preimplantation embryo-lethal phenotype known as the "DDK syndrome." We have also shown that observation of meiotic drive in this system depends upon the genotype of the sire. Here we show that females that are heterozygous at Om retain the meiotic drive phenotype and define a 0.32-cM candidate interval for the Responder locus in this drive system. In addition, analysis of the inheritance of alleles at Om among the offspring of F(1) intercrosses indicates that the effect of the sire is determined by the sperm genotype at Om or a locus linked to Om.

Modification of survival rate of mouse embryos developing in heterozygous females for ovum mutant gene

The DDK syndrome (polar infertility) is caused by an incompatibility system due to the ovum mutant (Om) locus. For brevity, the following gene symbols are used in the present report: DDK allele, Om; C57BL/6Cr allele, +. In this investigation, we first attempted to introduce the Om allele of DDK strain into the genetic background of C57BL/6Cr strain. The attempt resulted in the production of no young at the third generation of successive backcrosses. Secondly, mating experiments were performed with heterozygous (Om/+) females having background genes of C57BL/6Cr and DDK strains in the ratios 1:1(B1D), 3:1(B3D), 7:1(B7D), and 15:1(B15D). The survival rate of the embryos as judged by the percentage number of live fetuses/number of corpora lutea at Day 12 of pregnancy was 41.3 +/- 3.2%, 27.3 +/- 3. 2%, 16.4 +/- 3.3%, and 11.3 +/- 3.2% (mean +/- SEM) in the B1D, B3D, B7D, and B15D females, respectively, when they were mated with C57BL/6Cr males. Furthermore, the increased embryonic mortality in the heterozygous (Om/+) females with more background genes of C57BL/6Cr strain was found to be due to a failure in blastocyst formation, as in the DDK syndrome. The parallelism between the proportion of C57BL/6Cr background genes and embryonic mortality has led to a hypothesis proposing the participation of a modifier gene, namely that a mechanism similar to allelic exclusion may be working in the synthesis of cytoplasmic factor of eggs and that only the Om allele is activated during oogenesis to produce DDK-type cytoplasmic factor in heterozygous (Om/+) females having a modifier gene in the homozygous state.

Structure and characterization of the murine p80 coilin gene, Coil

Cajal bodies (coiled bodies, CBs) are nuclear organelles of unknown function and are characterized by a wide variety of components including various basal transcription and cell cycle proteins, the nucleolar proteins fibrillarin and Nopp140, numerous small nuclear ribonucleoproteins, the survival motor neuron protein complex, and the marker protein, p80 coilin. To gain insight into the role of p80 coilin in CBs, we have cloned the murine gene Coil and have mapped it to the distal portion of chromosome band 11D. The approximately 2.6-kb transcript is detectable in all tissues analyzed, with the highest levels in brain and testis. Sequence analysis shows that, like its human counterpart, the mouse coilin gene is composed of seven exons and spans nearly 30 kb of genomic DNA. The predicted amino acid sequence reveals two conserved N- and C-terminal domains, and comparison with the Xenopus SPH-1 protein reveals that these three genes are indeed orthologous. These results should facilitate gene disruption experiments aimed at creating a genetic model system to study CBs.

BALB/c alleles at modifier loci increase the severity of the maternal effect of the "DDK syndrome"

The Om locus was first described in the DDK inbred mouse strain: DDK mice carry a mutation at Om resulting in a parental effect lethality of F(1) embryos. When DDK females are mated with males of other (non-DDK) inbred strains, e.g., BALB/c, they exhibit a low fertility, whereas the reciprocal cross, non-DDK females x DDK males, is fertile (as is the DDK intrastrain cross). The low fertility is due to the death of (DDK x non-DDK)F(1) embryos at the late-morula to blastocyst stage, which is referred to as the "DDK syndrome." The death of these F(1) embryos is caused by an incompatibility between a DDK maternal factor and the non-DDK paternal pronucleus. Previous genetic studies showed that F(1) mice have an intermediate phenotype compared to parental strains: crosses between F(1) females and non-DDK males are semisterile, as are crosses between DDK females and F(1) males. In the present studies, we have examined the properties of mice heterozygous for BALB/c and DDK Om alleles on an essentially BALB/c genetic background. Surprisingly, we found that the females are quasi-sterile when mated with BALB/c males and, thus, present a phenotype similar to DDK females. These results indicate that BALB/c alleles at modifier loci increase the severity of the DDK syndrome.

Male-offspring-specific, haplotype-dependent, nonrandom cosegregation of alleles at loci on two mouse chromosomes

F(1) backcrosses involving the DDK and C57BL/6 inbred mouse strains show transmission ratio distortion at loci on two different chromosomes, 11 and X. Transmission ratio distortion on chromosome X is restricted to female offspring while that on chromosome 11 is present in offspring of both sexes. In this article we investigate whether the inheritance of alleles at loci on one chromosome is independent of inheritance of alleles on the other. A strong nonrandom association between the inheritance of alleles at loci on both chromosomes is found among male offspring, while independent assortment occurs among female offspring. We also provide evidence that the mechanism by which this phenomenon occurs involves preferential cosegregation of nonparental chromatids of both chromosomes at the second meiotic division, after the ova has been fertilized by a C57BL/6 sperm bearing a Y chromosome. These observations confirm the influence of the sperm in the segregation of chromatids during female meiosis, and indicate that a locus or loci on the Y chromosome are involved in this instance of meiotic drive.

A genetic test to determine the origin of maternal transmission ratio distortion. Meiotic drive at the mouse Om locus

We have shown previously that the progeny of crosses between heterozygous females and C57BL/6 males show transmission ratio distortion at the Om locus on mouse chromosome 11. This result has been replicated in several independent experiments. Here we show that the distortion maps to a single locus on chromosome 11, closely linked to Om, and that gene conversion is not implicated in the origin of this phenomenon. To further investigate the origin of the transmission ratio distortion we generated a test using the well-known effect of recombination on maternal meiotic drive. The genetic test presented here discriminates between unequal segregation of alleles during meiosis and lethality, based on the analysis of genotype at both the distorted locus and the centromere of the same chromosome. We used this test to determine the cause of the transmission ratio distortion observed at the Om locus. Our results indicate that transmission ratio distortion at Om is due to unequal segregation of alleles to the polar body at the second meiotic division. Because the presence of segregation distortion at Om also depends on the genotype of the sire, our results confirm that the sperm can influence segregation of maternal chromosomes to the second polar body.

Confirmation of maternal transmission ratio distortion at Om and direct evidence that the maternal and paternal "DDK syndrome" genes are linked

The polar, preimplantation-embryo lethal phenotype known as the "DDK syndrome" in the mouse is the result of the complex interaction of genetic factors and a parental-origin effect. We previously observed a modest degree of transmission-ratio distortion in favor of the inheritance of DDK alleles in the Ovum mutant (Om) region of Chromosome (Chr) 11, among offspring of reciprocal F1-hybrid females and C57BL/6 males. In this study, we confirm that a significant excess of offspring inherit DDK alleles from F1 mothers and demonstrate that the preference for the inheritance of DDK alleles is not a specific bias against the C57BL/6 allele or a simple preference for offspring that are heterozygous at Om. Because none of the previous genetic models for the inheritance of the "DDK syndrome" predicted transmission-ratio distortion through F1 females, we reconsidered the possibility that the genes encoding the maternal and paternal components of this phenotype were not linked. We have examined the fertility phenotype of N2 females and demonstrate that the inter-strain fertility of these females is correlated with their genotype in the Om region. This result establishes, directly, that the genes encoding the maternal and paternal components of the DDK syndrome are genetically linked.