DNA polymerase beta is critical for mouse meiotic synapsis.

We have shown earlier that DNA polymerase beta (Pol beta) localizes to the synaptonemal complex (SC) during Prophase I of meiosis in mice. Pol beta localizes to synapsed axes during zygonema and pachynema, and it associates with the ends of bivalents during late pachynema and diplonema. To test whether these localization patterns reflect a function for Pol beta in recombination and/or synapsis, we used conditional gene targeting to delete the PolB gene from germ cells. We find that Pol beta-deficient spermatocytes are defective in meiotic chromosome synapsis and undergo apoptosis during Prophase I. We also find that SPO11-dependent gammaH2AX persists on meiotic chromatin, indicating that Pol beta is critical for the repair of SPO11-induced double-strand breaks (DSBs). Pol beta-deficient spermatocytes yielded reduced steady-state levels of the SPO11-oligonucleotide complexes that are formed when SPO11 is removed from the ends of DSBs, and cytological experiments revealed that chromosome-associated foci of replication protein A (RPA), RAD51 and DMC1 are less abundant in Pol beta-deficient spermatocyte nuclei. Localization of Pol beta to meiotic chromosomes requires the formation of SPO11-dependent DSBs. Taken together, these findings strongly indicate that Pol beta is required at a very early step in the processing of meiotic DSBs, at or before the removal of SPO11 from DSB ends and the generation of the 3' single-stranded tails necessary for subsequent strand exchange. The chromosome synapsis defects and Prophase I apoptosis of Pol beta-deficient spermatocytes are likely a direct consequence of these recombination defects.

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.

How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals.

The duck-billed platypus is an extraordinary mammal. Its chromosome complement is no less extraordinary, for it includes a system in which ten sex chromosomes form an extensive meiotic chain in males. Such meiotic multiples are unprecedented in vertebrates but occur sporadically in plant and invertebrate species. In this paper, we review the evolution and formation of meiotic multiples in plants and invertebrates to try to gain insights into the origin of the platypus meiotic multiple. We describe the meiotic hurdles that translocated mammalian chromosomes face, which make longer chains disadvantageous in mammals, and we discuss how sex chromosomes and dosage compensation might have affected the evolution of sex-linked meiotic multiples. We conclude that the evolutionary conservation of the chain in monotremes, the structural properties of the translocated chromosomes and the highly accurate segregation at meiosis make the platypus system remarkably different from meiotic multiples in other species. We discuss alternative evolutionary models, which fall broadly into two categories: either the chain is the result of a sequence of translocation events from an ancestral pair of sex chromosomes (Model I) or the entire chain came into being at once by hybridization of two populations with different chromosomal rearrangements sharing monobrachial homology (Model II).

Meiotic recombination and spatial proximity in the etiology of the recurrent t(11;22).

Although balanced translocations are among the most common human chromosomal aberrations, the constitutional t(11;22)(q23;q11) is the only known recurrent non-Robertsonian translocation. Evidence indicates that de novo formation of the t(11;22) occurs during meiosis. To test the hypothesis that spatial proximity of chromosomes 11 and 22 in meiotic prophase oocytes and spermatocytes plays a role in the rearrangement, the positions of the 11q23 and 22q11 translocation breakpoints were examined. Fluorescence in situ hybridization with use of DNA probes for these sites demonstrates that 11q23 is closer to 22q11 in meiosis than to a control at 6q26. Although chromosome 21p11, another control, often lies as close to 11q23 as does 22q11 during meiosis, chromosome 21 rarely rearranges with 11q23, and the DNA sequence of chromosome 21 appears to be less susceptible than 22q11 to double-strand breaks (DSBs). It has been suggested that the rearrangement recurs as a result of the palindromic AT-rich repeats at both 11q23 and 22q11, which extrude hairpin structures that are susceptible to DSBs. To determine whether the DSBs at these sites coincide with normal hotspots of meiotic recombination, immunocytochemical mapping of MLH1, a protein involved in crossing over, was employed. The results indicate that the translocation breakpoints do not coincide with recombination hotspots and therefore are unlikely to be the result of meiotic programmed DSBs, although MRE11 is likely to be involved. Previous analysis indicated that the DSBs appear to be repaired by a mechanism similar to nonhomologous end joining (NHEJ), although NHEJ is normally suppressed during meiosis. Taken together, these studies support the hypothesis that physical proximity between 11q23 and 22q11--but not typical meiotic recombinational activity in meiotic prophase--plays an important role in the generation of the constitutional t(11;22) rearrangement.

Chromosome chains and platypus sex: kinky connections.

Mammal sex determination depends on an XY chromosome system, a gene for testis development and a means of activating the X chromosome. The duckbill platypus challenges these dogmas.(1,2) Gutzner et al.(1) find no recognizable SRY sequence and question whether the mammalian X was even the original sex chromosome in the platypus. Instead they suggest that the original platypus sex chromosomes were derived from the ZW chromosome system of birds and reptiles. Unraveling the puzzles of sex determination and dosage compensation in the platypus has been complicated by the fact that it has a surplus of sex chromosomes. Rather than a single X and Y chromosome, the male platypus has five Xs and five Ys.

Variation in human meiotic recombination.

As recently as 20 years ago, there was relatively little information about the number and distribution of recombinational events in human meiosis, and we knew virtually nothing about factors affecting patterns of recombination. However, the generation of a variety of linkage-based genetic mapping tools and, more recently, cytological approaches that enable us to directly visualize the recombinational process in meiocytes, have led to an increased understanding of human meiosis. In this review, we discuss the different approaches used to study meiotic recombination in humans, our understanding of factors that affect the number and location of recombinational events, and clinical consequences of variation in the recombinational process.

Male mouse recombination maps for each autosome identified by chromosome painting.

Linkage maps constructed from genetic analysis of gene order and crossover frequency provide few clues to the basis of genomewide distribution of meiotic recombination, such as chromosome structure, that influences meiotic recombination. To bridge this gap, we have generated the first cytological recombination map that identifies individual autosomes in the male mouse. We prepared meiotic chromosome (synaptonemal complex [SC]) spreads from 110 mouse spermatocytes, identified each autosome by multicolor fluorescence in situ hybridization of chromosome-specific DNA libraries, and mapped >2,000 sites of recombination along individual autosomes, using immunolocalization of MLH1, a mismatch repair protein that marks crossover sites. We show that SC length is strongly correlated with crossover frequency and distribution. Although the length of most SCs corresponds to that predicted from their mitotic chromosome length rank, several SCs are longer or shorter than expected, with corresponding increases and decreases in MLH1 frequency. Although all bivalents share certain general recombination features, such as few crossovers near the centromeres and a high rate of distal recombination, individual bivalents have unique patterns of crossover distribution along their length. In addition to SC length, other, as-yet-unidentified, factors influence crossover distribution leading to hot regions on individual chromosomes, with recombination frequencies as much as six times higher than average, as well as cold spots with no recombination. By reprobing the SC spreads with genetically mapped BACs, we demonstrate a robust strategy for integrating genetic linkage and physical contig maps with mitotic and meiotic chromosome structure.

Ataxia telangiectasia mutated expression and activation in the testis.

Ionizing radiation (IR) and consequent induction of DNA double-strand breaks (DSBs) causes activation of the protein ataxia telangiectasia mutated (ATM). Normally, ATM is present as inactive dimers; however, in response to DSBs, the ATM dimer partners cross-phosphorylate each other on serine 1981, and kinase active ATM monomers are subsequently released. We have studied the presence of both nonphosphorylated as well as active serine 1981 phosphorylated ATM (pS1981-ATM) in the mouse testis. In the nonirradiated testis, ATM was present in spermatogonia and spermatocytes until stage VII of the cycle of the seminiferous epithelium, whereas pS1981-ATM was found only to be present in the sex body of pachytene spermatocytes. In response to IR, ATM became activated by pS1981 cross-phosphorylation in spermatogonia and Sertoli cells. Despite the occurrence of endogenous programmed DSBs during the first meiotic prophase and the presence of ATM in both spermatogonia and spermatocytes, pS1981 phosphorylated ATM did not appear in spermatocytes after treatment with IR. These results show that spermatogonial ATM and ATM in the spermatocytes are differentially regulated. In the mitotically dividing spermatogonia, ATM is activated by cross-phosphorylation, whereas during meiosis nonphosphorylated ATM or differently phosphorylated ATM is already active. ATM has been shown to be present at the synapsed axes of the meiotic chromosomes, and in the ATM knock-out mice spermatogenesis stops at pachytene stage IV of the seminiferous epithelium, indicating that indeed nonphosphorylated ATM is functional during meiosis. Additionally, ATM is constitutively phosphorylated in the sex body where its continued presence remains an enigma.

MEN1 tumor-suppressor protein localizes to telomeres during meiosis.

Multiple endocrine neoplasia type 1 is an autosomal dominant cancer predisposition syndrome caused by mutations in the tumor-suppressor gene MEN1. The gene encodes a nuclear protein, menin, with no recognized functional motifs. Menin has been shown negatively to regulate transcriptional activation mediated by JunD, although the significance of this interaction in normal cell physiology and how the absence of menin leads to tumorigenesis are unknown. Menin is highly expressed in testes. We used immunocytochemistry to explore its role in meiosis and found that it localizes exclusively at telomeres. JunD was not found at telomeres in meiotic cells. In view of elevated telomerase activity or abnormal telomere structure in virtually all malignancies, regulation of telomere function would be an appealing role for a tumor suppressor. However, menin does not specifically associate with telomeres in somatic cells, as indicated by lack of co-localization with the known telomeric protein TRF2. Cells overexpressing menin had normal telomerase activity, and tumors with homozygous MEN1 mutations showed no aberrations in telomere length, indicating that menin does not directly regulate telomerase activity. The role of menin at meiotic telomeres appears to be independent of JunD and may not have a counterpart in somatic cells. These results suggest that menin may play different roles in different tissues through interactions with different proteins.

Correlation of meiotic events in testis sections and microspreads of mouse spermatocytes relative to the mid-pachytene checkpoint.

In mouse, asynaptic meiotic mutants arrest at Testis Epithelial Stage IV. This arrest is 4.5 days after homologous chromosomes begin to synapse and approximately 2.5 days after synapsis is usually completed. To correlate cytological events with meiotic progression in testis and to determine which meiotic events are normally completed by Stage IV, we induced spermatogenic arrest by placing mice on a vitamin A deficient diet. Subsequent injection of retinoic acid and a return to a normal diet resulted in resumption of spermatogenesis with all spermatocytes proceeding through meiosis in a highly synchronous cohort. Between Days 11 and 16 post-injection we prepared one testis for immunocytological and the other for histological evaluation, then used antibodies to SCP3 and either RPA, or MLH1 to follow quantitative changes in synapsis and recombination. RPA was found at sites along the synaptonemal complex as soon as homologs synapsed, and most, but not all, RPA disappeared by Stage IV. MLH1 foci appeared between Stage II and IV and remained through Stage VII, the end point of the study. The data suggest that the earliest the mid-pachytene checkpoint can be activated is Epithelial Stage IV, but that activities monitored by the checkpoint may not be completed by this time.

Function of DNA-protein kinase catalytic subunit during the early meiotic prophase without Ku70 and Ku86.

All components of the double-stranded DNA break (DSB) repair complex DNA-dependent protein kinase (DNA-PK), including Ku70, Ku86, and DNA-PK catalytic subunit (DNA-PKcs), were found in the radiosensitive spermatogonia. Although p53 induction was unaffected, spermatogonial apoptosis occurred faster in the irradiated DNA-PKcs-deficient scid testis. This finding suggests that spermatogonial DNA-PK functions in DNA damage repair rather than p53 induction. Despite the fact that early spermatocytes lack the Ku proteins, spontaneous apoptosis of these cells occurred in the scid testis. The majority of these apoptotic spermatocytes were found at stage IV of the cycle of the seminiferous epithelium where a meiotic checkpoint has been suggested to exist. Meiotic synapsis and recombination during the early meiotic prophase induce DSBs, which are apparently less accurately repaired in scid spermatocytes that then fail to pass the meiotic checkpoint. The role for DNA-PKcs during the meiotic prophase differs from that in mitotic cells; it is not influenced by ionizing radiation and is independent of the Ku heterodimer.

Aberrant interchromosomal exchanges are the predominant cause of the 22q11.2 deletion.

Chromosome 22q11.2 deletions are found in almost 90% of patients with DiGeorge/velocardiofacial syndrome (DGS/VCFS). Large, chromosome-specific low copy repeats (LCRs), flanking and within the deletion interval, are presumed to lead to misalignment and aberrant recombination in meiosis resulting in this frequent microdeletion syndrome. We traced the grandparental origin of regions flanking de novo 3 Mb deletions in 20 informative three-generation families. Haplotype reconstruction showed an unexpectedly high number of proximal interchromosomal exchanges between homologs, occurring in 19/20 families. Instead, the normal chromosome 22 in these probands showed interchromosomal exchanges in 2/15 informative meioses, a rate consistent with the genetic distance. Meiotic exchanges, visualized as MLH1 foci, localize to the distal long arm of chromosome 22 in 75% of human spermatocytes tested, also reflecting the genetic map. Additionally, we found no effect of proband gender or parental age on the crossover frequency. Parental origin studies in 65 de novo 3 Mb deletions (including these 20 patients) demonstrated no bias. Unlike Williams syndrome, we found no chromosomal inversions flanked by LCRs in 22 sets of parents of 22q11 deleted patients, or in eight non-deleted patients with a DGS/VCFS phenotype using FISH. Our data are consistent with significant aberrant interchromosomal exchange events during meiosis I in the proximal region of the affected chromosome 22 as the likely etiology for the deletion. This type of exchange occurs more often than is described for deletions of chromosomes 7q11, 15q11, 17p11 and 17q11, implying a difference in the meiotic behavior of chromosome 22.