Identification and mapping of 26 human testis mRNAs containing CAG/CTG repeats

Impaired spermatogenesis and male fertility defects in CIZ/Nmp4-disrupted mice

CIZ (Cas interacting zinc finger protein), also called Nmp4 (nuclear matrix protein 4), is a nucleo-cytoplasmic shuttling transcription factor that regulates the expression of collagen and matrix metalloproteinases. CIZ/Nmp4 was originally cloned by its binding to p130(Cas), a focal adhesion protein, and was recently shown to suppress BMP2 (bone mophogenetic protein 2) signalling. To explore the physiological role of CIZ/Nmp4, we disrupted CIZ/Nmp4-gene by inserting beta-galactosidase and neomycin resistance genes into the 2nd exon of CIZ/Nmp4-gene, which is utilized by all the sequenced alternative forms. CIZ-/- mice were born and grew to adulthood. Although they tend to be smaller than wild-type mice, no pathological abnormality was observed except in the testis. Histological analysis of the testes revealed variable degrees of spermatogenic cell degeneration within the seminiferous tubules of CIZ-/- mice, resembling the histology of the 'Germinal-cell aplasia with focal spermatogenesis'. Some of the CIZ-/- male mice developed infertility. TUNEL assay on testis sections revealed an increased occurrence of apoptosis of spermatogenic cells in the testes of CIZ-/- mice. CIZ/Nmp4 was co-localized with Smad1 in the testis, suggesting that a disregulation of BMP signalling could cause these phenotypes. These results suggest that CIZ/Nmp4 plays roles in the progress and the maintenance of spermatogenesis.

Sporadic testicular germ cell cancers do not exhibit specific alteration in CAG/CTG repeats containing genes expressed in human testis

CAG/CTG repeat expansions in genomic DNA of testicular tumour cell lines, and germline DNA from members of families predisposed to this malignancy, have been previously described. In order to identify genes possibly concerned by this alteration, we attempted to clone all possible human testis cDNA containing at least five CAG/CTG repeats. Thirty-four different transcripts were identified. By using PCR and non denaturing gel electrophoresis, we determined the size of their repeats, as well as their polymorphisms in a collection of human testicular germ cell tumours and the normal surrounding tissues. For all tested genes, we detected the presence of several species of the same mRNA for each person. Nine genes exhibited specific patterns of expression among different groups of individuals, indicative of polymorphism. None of these polymorphisms was related to human testicular tumours.

A Role for Selection in Regulating the Evolutionary Emergence of Disease-Causing and Other Coding CAG Repeats in Humans and Mice

The evolutionary expansion of CAG repeats in human triplet expansion disease genes is intriguing because of their deleterious phenotype. In the past, this expansion has been suggested to reflect a broad genomewide expansion of repeats, which would imply that mutational and evolutionary processes acting on repeats differ between species. Here, we tested this hypothesis by analyzing repeat- and flanking-sequence evolution in 28 repeat-containing genes that had been sequenced in humans and mice and by considering overall lengths and distributions of CAG repeats in the two species. We found no evidence that these repeats were longer in humans than in mice. We also found no evidence for preferential accumulation of CAG repeats in the human genome relative to mice from an analysis of the lengths of repeats identified in sequence databases. We then investigated whether sequence properties, such as base and amino acid composition and base substitution rates, showed any relationship to repeat evolution. We found that repeat-containing genes were enriched in certain amino acids, presumably as the result of selection, but that this did not reflect underlying biases in base composition. We also found that regions near repeats showed higher nonsynonymous substitution rates than the remainder of the gene and lower nonsynonymous rates in genes that contained a repeat in both the human and the mouse. Higher rates of nonsynonymous mutation in the neighborhood of repeats presumably reflect weaker purifying selection acting in these regions of the proteins, while the very low rate of nonsynonymous mutation in proteins containing a CAG repeat in both species presumably reflects a high level of purifying selection. Based on these observations, we propose that the mutational processes giving rise to polyglutamine repeats in human and murine proteins do not differ. Instead, we propose that the evolution of polyglutamine repeats in proteins results from an interplay between mutational processes and selection.

Human testis expresses a specific poly(A)-binding protein

In testis mRNA stability and translation initiation are extensively under the control of poly(A)-binding proteins (PABP). Here we have cloned a new human testis-specific PABP (PABP3) of 631 amino acids (70.1 kDa) with 92.5% identical residues to the ubiquitous PABP1. A northern blot of multiple human tissues hybridised with PABP3- and PABP1-specific oligonucleotide probes revealed two PABP3 mRNAs (2.1 and 2.5 kb) detected only in testis, whereas PABP1 mRNA (3.2 kb) was present in all tested tissues. In human adult testis, PABP3 mRNA expression was restricted to round spermatids, whereas PABP1 was expressed in these cells as well as in pachytene spermatocytes. PABP3-specific antibodies identified a protein of 70 kDa in human testis extracts. This protein binds poly(A) with a slightly lower affinity as compared to PABP1. The human PABP3 gene is intronless with a transcription start site 61 nt upstream from the initiation codon. A sequence of 256 bp upstream from the transcription start site drives the promoter activity of PABP3 and its tissue-specific expression. The expression of PABP3 might be a way to bypass PABP1 translational repression and to produce the amount of PABP needed for active mRNA translation in spermatids.

CIZ, a zinc finger protein that interacts with p130(cas) and activates the expression of matrix metalloproteinases

p130(cas) (Cas) is a docking protein that contains an SH3 domain and multiple tyrosine residues. p130(cas) is located at focal adhesions, is tyrosine phosphorylated in response to integrin stimulation, and is thought to transmit signals, via c-Crk and other proteins, for the remodeling of actin stress fibers and cell movement. In a search for the ligands of the SH3 domain of p130(cas) by far-Western screening, we cloned a novel protein named CIZ (for Cas-interacting zinc finger protein). CIZ consists of the following: a putative leucine zipper; a serine/threonine-rich region; a proline-rich sequence; five, six, or eight Krüppel-type C(2)H(2) zinc fingers; and the glutamine-alanine repeat. CIZ binds Cas in cells and is located in the nucleus and at focal adhesions. We showed that CIZ is a nucleocytoplasmic shuttling protein, by using the transient interspecies heterokaryon formation assay. In order to search for the targets of CIZ in nucleus, we determined the DNA binding consensus of CIZ as (G/C)AAAAA(A) by cyclic amplification and selection of targets analysis. The consensus-like sequences are found in several promoters of matrix metalloproteinases (MMPs), which are the enzymes used to degrade the extracellular matrix proteins. CIZ binds to a consensus-like sequence in the MMP-1 (collagenase) promoter. Overexpression of CIZ upregulates the transcriptions from MMP-1, MMP-3 (stromelysin), and MMP-7 (matrilysin) promoters, and this transactivation was enhanced in the presence of Cas. Furthermore, the stable overexpression of CIZ promoted the production of MMP-7 in culture medium. In summary, CIZ, a novel zinc finger protein, binds Cas, is a nucleocytoplasmic shuttling protein, and regulates the expression of MMPs.