DAZ family proteins exist throughout male germ cell development and transit from nucleus to cytoplasm at meiosis in humans and mice.

The human DAZ gene family is expressed in germ cells and consists of a cluster of nearly identical DAZ (deleted in azoospermia) genes on the Y chromosome and an autosomal homolog, DAZL (DAZ-like). Only the autosomal gene is found in mice. Y-chromosome deletions that encompass the DAZ genes are a common cause of spermatogenic failure in men, and autosomal homologs of DAZ are essential for testicular germ cell development in mice and Drosophila. Previous studies have reported that mouse DAZL protein is strictly cytoplasmic and that human DAZ protein is restricted to postmeiotic cells. By contrast, we report here that human DAZ and human and mouse DAZL proteins are present in both the nuclei and cytoplasm of fetal gonocytes and in spermatogonial nuclei. The proteins relocate to the cytoplasm during male meiosis. Further observations using human tissues indicate that, unlike DAZ, human DAZL protein persists in spermatids and even spermatozoa. These results, combined with findings in diverse species, suggest that DAZ family proteins function in multiple cellular compartments at multiple points in male germ cell development. They may act during meiosis and much earlier, when spermatogonial stem cell populations are established.

Male infertility: analysis of the markers and genes on the human Y chromosome.

The long arm of the human Y chromosome is required for male fertility. Deletions in three different regions can cause severe spermatogenic defects ranging from non-obstructive azoospermia to oligozoospermia. Use of intracytoplasmic sperm injection (ICSI) may allow Y chromosome defects to be passed from father to son. Thus, numerous reports have stressed the need to offer genetic testing to infertile men who select ICSI and a number of reproductive clinics have begun to do so. The primary objectives of this review were: firstly, to discuss the characteristics of the published set of polymerase chain reaction markers and how these characteristics affect interpretation of Y chromosome deletion analysis and secondly, to summarize the recent literature pertaining to the genes on the Y chromosome.

Azoospermic men with deletion of the DAZ gene cluster are capable of completing spermatogenesis: fertilization, normal embryonic development and pregnancy occur when retrieved testicular spermatozoa are used for intracytoplasmic sperm injection.

Some men with non-obstructive azoospermia harbour fully formed spermatozoa within their testicular tissue that can be used to achieve pregnancy via intracytoplasmic sperm injection (ICSI). Recently, Reijo et al. (1995) provided compelling evidence that the DAZ gene cluster is a strong candidate for one of the elusive azoospermia factors (AZF) located on the long arm of the Y chromosome. The DAZ gene cluster is deleted in 13% of azoospermic men and a small percentage of severely oligozoospermic men. Vertical transmission from father to son of AZF region deletions has also been described. Presumably these fathers were oligozoospermic. This led us to ask whether the azoospermic male with deletions of the AZF/DAZ region can also complete minimal spermatogenesis and whether any spermatozoa found could participate in fertilization, embryo development and pregnancy. Three out of six (50%) of the azoospermic men with AZF/DAZ deletions had spermatozoa identified within their harvested testicular tissue. When these spermatozoa were used for ICSI, fertilization occurred in 36% of injected oocytes. This compared favourably with testicular spermatozoa retrieved from non-obstructive azoospermic men without AZF/DAZ gene deletions. In one case, a twin conception resulted, which represents the first term pregnancy reported using spermatozoa from an AZF/DAZ deleted azoospermic male. Therefore it is necessary to take the possibility of transmission of infertility or sterility to our patients' offspring seriously when utilizing today's reproductive technologies, as spermatogenesis in men with AZF/DAZ deletions is by no means an exceptional occurrence.

The DAZ gene cluster on the human Y chromosome arose from an autosomal gene that was transposed, repeatedly amplified and pruned.

It is widely believed that most or all Y-chromosomal genes were once shared with the X chromosome. The DAZ gene is a candidate for the human Y-chromosomal Azoospermia Factor (AZF). We report multiple copies of DAZ (> 99% identical in DNA sequence) clustered in the AZF region and a functional DAZ homologue (DAZH) on human chromosome 3. The entire gene family appears to be expressed in germ cells. Sequence analysis indicates that the Y-chromosomal DAZ cluster arose during primate evolution by (i) transposing the autosomal gene to the Y, (ii) amplifying and pruning exons within the transposed gene and (iii) amplifying the modified gene. These results challenge prevailing views of sex chromosome evolution, suggesting that acquisition of autosomal fertility genes is an important process in Y chromosome evolution.

Mouse autosomal homolog of DAZ, a candidate male sterility gene in humans, is expressed in male germ cells before and after puberty.

Deletion of the Azoospermia Factor (AZF) region of the human Y chromosome results in spermatogenic failure. While the identity of the critical missing gene has yet to be established, a strong candidate is the putative RNA-binding protein DAZ (Deleted in Azoospermia). Here we describe the mouse homolog of DAZ. Unlike human DAZ, which is Y-linked, in mouse the Dazh (DAZ homolog) gene maps to chromosome 17. Nonetheless, the predicted amino acid sequences of the gene products are quite similar, especially in their RNP/RRM (putative RNA-binding) domains, and both genes are transcribed predominantly in testes; the mouse gene is transcribed at a lower level in ovaries. Dazh transcripts were not detected in testes of mice that lack germ cells. In testes of wildtype mice, Dazh transcription is detectable 1 day after birth (when the only germ cells are prospermatogonia), increases steadily as spermatogonial stem cells appear, plateaus as the first wave of spermatogenic cells enters meiosis (10 days after birth), and is sustained at this level thereafter. This unique pattern of expression suggests that Dazh participates in differentiation, proliferation, or maintenance of germ cell founder populations before, during, and after the pubertal onset of spermatogenesis. Such functions could readily account for the diverse spermatogenic defects observed in human males with AZF deletions.

Severe oligozoospermia resulting from deletions of azoospermia factor gene on Y chromosome.

BACKGROUND: About 13% of cases of non-obstructive azoospermia are caused by deletion of the azoospermia factor (AZF), a gene or gene complex normally located on the long arm of the Y chromosome. Oligozoospermia is far more common than azoospermia, but little is known about genetic causes. We investigated whether severe oligozoospermia is caused by AZF deletions and, if so, whether those deletions are present in mature spermatozoa. METHODS: By PCR, we tested leucocyte DNA, from 35 men who presented at infertility clinics and who had severe oligozoospermia, for the presence of 118 DNA landmarks scattered across the Y chromosome. In the two men in whom Y-chromosome deletions in leucocyte DNA were detected, we also tested leucocyte DNA from the individuals' fathers, and in one man we tested sperm DNA. FINDINGS: In two men with ejaculate sperm counts of 40 000-100 000 per mL, we detected Y-chromosome deletions in leucocyte DNA similar in location to those previously reported in azoospermic individuals. No Y-chromosome deletions were detected in the fathers of the two men. For one of the two men, sperm DNA was tested, and it showed the same Y-chromosome deletion seen in leucocytes. INTERPRETATION: The Y-chromosome deletions in these two men are de-novo mutations, and are therefore the cause of their severe oligozoospermia. Not only is the absence of AZF compatible with spermatogenesis, albeit at reduced rate, but also the resultant sperm bear the mutant Y chromosome. Because intracytoplasmic sperm injection is increasingly used as a means of circumventing oligozoospermia, AZF deletions could be transmitted by this practice, and would probably result in infertile sons. In cases of severe oligozoospermia, it may be appropriate to offer Y-DNA testing and genetic counselling before starting assisted reproductive procedures.

Gonadoblastoma: molecular definition of the susceptibility region on the Y chromosome.

Using sequence-tagged sites we have performed deletion mapping of the Y chromosome in sex-reversed female patients with a Y chromosome and gonadoblastoma. The GBY gene (gonadoblastoma locus on the Y chromosome) was sublocalized to a small region near the centromere of the Y chromosome. We estimate the size of the GBY critical region to be approximately 1-2 Mb. Our analysis also indicates that copies of two dispersed Y-linked gene families, TSPY (testis-specific protein, Y-encoded) and YRRM (Y-chromosome RNA recognition motif) are present in all patients and that copies of TSPY but not YRRM fall within the GBY critical region as formally defined by deletion mapping. Two tumor samples showed expression of both genes and in one patient this expression was limited to a unilateral gonadoblastoma but absent in the contralateral streak gonad. Although our results do not directly implicate TSPY or YRRM in the etiology of the tumor, they raise the issue of whether there is one GBY gene in the critical region or possibly multiple GBY loci dispersed on the Y chromosome.

Diverse spermatogenic defects in humans caused by Y chromosome deletions encompassing a novel RNA-binding protein gene.

We have detected deletions of portions of the Y chromosome long arm in 12 of 89 men with azoospermia (no sperm in semen). No Y deletions were detected in their male relatives or in 90 other fertile males. The 12 deletions overlap, defining a region likely to contain one or more genes required for spermatogenesis (the Azoospermia Factor, AZF). Deletion of the AZF region is associated with highly variable testicular defects, ranging from complete absence of germ cells to spermatogenic arrest with occasional production of condensed spermatids. We find no evidence of YRRM genes, recently proposed as AZF candidates, in the AZF region. The region contains a single-copy gene, DAZ (Deleted in AZoospermia), which is transcribed in the adult testis and appears to encode an RNA binding protein. The possibility that DAZ is AZF should now be explored.

Systematic mutational analysis of the yeast beta-tubulin gene.

A systematic strategy was used to create a synoptic set of mutations that are distributed throughout the single beta-tubulin gene of Saccharomyces cerevisiae. Clusters of charged amino acids were targeted for mutagenesis and converted to alanine to maximize alterations on the protein's surface and minimize alterations that affect protein folding. Of the 55 mutations we constructed, three confer dominant-lethality, 11 confer recessive-lethality, 10 confer cold-sensitivity, one confers heat-sensitivity, and 27 confer altered resistance to benomyl. Only 11 alleles give no discernible phenotype. In spite of the fact that beta-tubulin is a highly conserved protein, three-fourths of the mutations do not destroy the ability of the protein to support the growth of yeast at 30 degrees C. The lethal substitutions are primarily located in three regions of the protein and presumably identify domains most critical for beta-tubulin function. Interestingly, most of the conditional-lethal alleles produce specific defects in spindle assembly at their restrictive temperature; cytoplasmic microtubules are relatively unaffected. The exceptions are two mutants that contain abnormally long cytoplasmic microtubules. Mutants with specific spindle defects were not observed in our previous collection of beta-tubulin mutants and should be valuable in dissecting spindle function.

Deletion of a single-copy tRNA affects microtubule function in Saccharomyces cerevisiae.

rts1-1 was identified as an extragenic suppressor of tub2-104, a cold-sensitive allele of the sole gene encoding beta-tubulin in the yeast, Saccharomyces cerevisiae. In addition, rts1-1 cells are heat sensitive and resistant to the microtubule-destabilizing drug, benomyl. The rts1-1 mutation is a deletion of approximately 5 kb of genomic DNA on chromosome X that includes one open reading frame and three tRNA genes. Dissection of this region shows that heat sensitivity is due to deletion of the open reading frame (HIT1). Suppression and benomyl resistance are caused by deletion of the gene encoding a tRNA(Arg)AGG (HSX1). Northern analysis of rts1-1 cells indicates that HSX1 is the only gene encoding this tRNA. Deletion of HSX1 does not suppress the tub2-104 mutation by misreading at the AGG codons in TUB2. It also does not suppress by interfering with the protein arginylation that targets certain proteins for degradation. These results leave open the prospect that this tRNA(Arg)AGG plays a novel role in the cell.