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The ability of adult stem cells to reside in a quiescent state is crucial for preventing premature exhaustion of the stem cell pool. However, the intrinsic epigenetic factors that regulate spermatogonial stem cell quiescence are largely unknown. Here, we investigate in mice how DNA methyltransferase 3-like (DNMT3L), an epigenetic regulator important for interpreting chromatin context and facilitating de novo DNA methylation, sustains the long-term male germ cell pool. We demonstrated that stem cell-enriched THY1+ spermatogonial stem/progenitor cells (SPCs) constituted a DNMT3L-expressing population in postnatal testes. DNMT3L influenced the stability of promyelocytic leukemia zinc finger (PLZF), potentially by downregulating Cdk2/CDK2 expression, which sequestered CDK2-mediated PLZF degradation. Reduced PLZF in Dnmt3l KO THY1+ cells released its antagonist, Sal-like protein 4A (SALL4A), which is associated with overactivated ERK and AKT signaling cascades. Furthermore, DNMT3L was required to suppress the cell proliferation-promoting factor SALL4B in THY1+ SPCs and to prevent premature stem cell exhaustion. Our results indicate that DNMT3L is required to delicately balance the cycling and quiescence of SPCs. These findings reveal a novel role for DNMT3L in modulating postnatal SPC cell fate decisions.

Chromosome anomalies/rearrangements of the Y chromosome seldom threaten life and are quite common. The 4.5 Mb AZFc region on Yq11.2 is one of the most polymorphic regions in the human genome. AZFc partial deletion is almost inevitably associated with impaired fertility while the functional significance of AZFc partial duplications remains controversial. In this report, a large Y chromosome with one centromere and two heterochromatic blocks was identified incidentally in two men. The first case was ascertained through surveillance for recurrent miscarriage. The second case was ascertained through amniocytes of a fetus and his father. FISH and array-based comparative genomic hybridization showed duplications of the entire AZFc region as well as Yq euchromatic region. The duplications in Case 1 and Case 2 spanned 4.8 Mb and 6.2 Mb, respectively, of the Yq euchromatic region. These two cases suggest that complete AZFc duplication could be completely benign. It awaits further investigation whether this is a bona fide chromosomal polymorphism.

Overlapping cDNA clones for a novel human X-linked gene, XE169, have been isolated and characterized. The composite cDNA sequence comprises 5910 bp (or 5901 bp) plus a poly(A) tail, with a 531 bp 5' and 696 bp 3' untranslated regions. The sequence represents a full-length or near full-length cDNA for the gene since Northern blot analysis reveals only a single prominent band approximately 6 kb in size. Alternative splicing generates two distinct transcripts either containing or missing a stretch of nine nucleotides in the XE169 single large open reading frame, which in turn predict two XE169 protein isoforms composed of 1557 and 1560 amino acids, respectively. Southern hybridization analysis of a panel of human-mouse somatic cell hybrids containing various portions of translocated human X chromosomes has assigned XE169 to the proximal half of the X short arm between Xp21.1 and the centromere. XE169 is expressed in multiple human tissues tested and homologous sequences exist on the human Y chromosome and in the genomes of five other eutherian mammals examined. RT-PCR analysis of somatic cell hybrids containing either an active or an inactive human X chromosome on a rodent background demonstrated that XE169 escapes X-inactivation.

Among apparently healthy men who nonetheless produce few or no sperm, about ten percent have microdeletions on their Y chromosome. Most of these de novo mutations occur in the AZFc region on the long arm of the chromosome. The high frequency of deletion is explained by the extraordinary structure of the region, which consists almost entirely of very long repeat units.

Mammalian sex chromosomes are divided into sex-specific and pseudoautosomal regions. Sequences in the pseudoautosomal region recombine between the sex chromosomes; the sex-specific sequences normally do not. The interface between sex-specific and pseudoautosomal sequences is the pseudoautosomal boundary. The boundary is the centromeric limit to recombination in the pseudoautosomal region. In man, an Alu repeat element is found inserted at the boundary on the Y chromosome. In the evolutionary comparison conducted here, the Alu repeat element is found at the Y boundary in great apes, but it is not found there in two Old World monkeys. During the evolution of the Old World monkey and great ape lineages, homology between the sex chromosomes was maintained by recombination in the sequences telomeric to the Alu insertion site. The Alu repeat element did not create the present-day boundary; instead, it inserted at the preexisting boundary after the Old World monkey and great ape lineages diverged.

The distal short arm of the human X chromosome is of interest because it contains genes which escape X chromosome inactivation and because it is subject to frequent deletions in human patients. The steroid sulfatase gene has been particularly well studied as an example of a gene which escapes X inactivation and which is included in a number of these deletion events. For these reasons a physical map of the region around the STS gene would be of interest. We have constructed a rare cutting enzyme map of this area and have determined the position of several nearby markers with respect to STS. We have also oriented the 5' and 3' ends of the STS gene on this map and have determined the centromeric and telomeric portions of the region. Finally, we have shown that this map can be used to locate deletion breakpoints in STS deficient patients.