Four DAZ genes in two clusters found in the AZFc region of the human Y chromosome.

The DAZ genes are candidate fertility factors that lie within the human Y chromosome's AZFc region, whose deletion is a common cause of spermatogenic failure. The number of DAZ genes has been difficult to determine, in part because the nucleotide sequences of the DAZ genes are nearly identical. Here, fluorescence in situ hybridization and characterization of BAC clones revealed four full-length DAZ genes on the human Y chromosome. They exist in two clusters, each comprising an inverted pair of DAZ genes (3' <-- 5'::5' --> 3'). Analysis of genomic sequences and testicular transcripts suggested that three or four DAZ genes are translated. Each gene contains at least seven tandem copies of a previously described, 2.4-kb repeat unit that encodes 24 amino acids. In addition, two DAZ genes contain tandem copies of a 10.8-kb repeat unit that encodes the RNA-binding domain, which appears to be multimerized in some DAZ proteins. Combining our present results with previous studies, we can reconstruct several steps in the evolution of the DAZ genes on the Y chromosome. In the ancestral Y-chromosomal DAZ gene, amplification of both intragenic repeats began before the human and cynomolgus (Old World) monkey lineages diverged. During subsequent evolution, an inverted duplication of this modified gene occurred. Finally, the resulting two-gene cluster was duplicated, generating the two-cluster/four-gene arrangement found on modern human Y chromosomes.

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.

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.

Structure and function of ribosomal protein S4 genes on the human and mouse sex chromosomes.

The human sex-linked genes RPS4X and RPS4Y encode distinct isoforms of ribosomal protein S4. Insufficient expression of S4 may play a role in the development of Turner syndrome, the complex human phenotype associated with monosomy X. In mice, the S4 protein is encoded by an X-linked gene, Rps4, and is identical to human S4X; there is no mouse Y homolog. We report here the organization of the human RPS4X and RPS4Y and mouse Rps4 genes. Each gene comprises seven exons; the positions of introns are conserved. The 5' flanking sequences of human RPS4X and mouse Rps4 are very similar, while RPS4Y diverges shortly upstream of the transcription start site. In chickens, S4 is encoded by a single gene that is not sex linked. The chicken protein differs from human S4X by four amino acid substitutions, all within a region encoded by a single exon. Three of the four substitutions are also present in human S4Y, suggesting that the chicken S4 gene may have arisen by recombination between S4X- and S4Y-like sequences. Using isoform-specific antisera, we determined that human S4X and S4Y are both present in translationally active ribosomes. S4Y is about 10 to 15% as abundant as S4X in ribosomes from normal male placental tissue and 46,XY cultured cells. In 49,XYYYY cells, S4Y is about half as abundant as S4X. In 49,XXXXY cells, S4Y is barely detectable. These results bear on the hypothesized role of S4 deficiency in Turner syndrome.