Mechanisms of DiseaseAzoospermia in patients heterozygous for a mutation in SYCP3
Introduction
Chromosomal defects and specific gene mutations result in male infertility, and many idiopathic cases of infertility are thought to have a genetic basis also.1 Although many genes associated with male infertility are known in mice, the translation of this information to people has been slow. Defects of meiosis during spermatogenesis are a significant cause of azoospermia, but the causes remain largely unknown.
Meiosis produces haploid gametes from diploid parental cells. This process is unique to the germ cells of the gonads. Chromosome number is reduced during meiosis because the initial round of DNA replication is followed by two successive cycles of chromosome segregation. A meiosis-specific supramolecular proteinaceous structure, the synaptonemal complex, is present in the meiotic cells of most organisms and has a key role during meiosis.2, 3, 4 In mammals, the synaptonemal complex consists of synaptonemal complex proteins (SYCP or SCP) 1, 2, and 3.5, 6, 7 In mice, all three genes are expressed only in the testis and ovary and are considered to be germ-cell specific.8
sycp3 (or SCP3) is a DNA-binding protein and a structural component of the synaptonemal complex, which mediates the synapsis or homologous pairing of chromosomes during meiosis of the germ cells. Male mice homozygous for the null mutation of the Sycp3 (Scp3) gene are sterile as a result of massive apoptotic cell death in the testis during meiotic prophase.9 In Sycp3-deficient male mice, axial or lateral elements and synaptonemal complexes are not formed. The chromosomes in the mutant spermatocytes do not synapse, and meiosis arrests. By contrast, heterozygous mutant males are fertile. Sycp3-deficient female mice are fertile and produce healthy offspring, although there is an increased risk of intrauterine death owing to the generation of aneuploid oocytes resulting from defective chromosomal segregation.10
Since Sycp3 seems to be crucial for male fertility in mice, we examined whether this protein is similarly required for human spermatogenesis. We identified the human SYCP3 gene on chromosome 12 and screened azoospermic men presenting with meiotic arrest and normal pregnancy-proven fertile controls for mutations in SYCP3.
Section snippets
Isolation of human SYCP3 cDNA and expression analysis
Murine Sycp3 cDNA has been isolated.11 By use of the mouse Sycp3 aminoacid sequence (NM-011517 in GenBank), we identified a region in the human genome sequence homologous to the mouse aminoacid sequence (AC063950 in GenBank). We designed primers homologous to the SYCP3 DNA sequence (SYCP3F1 and SYCP3R1), encompassing the putative gene introns, and carried out PCR with a human testis cDNA library (Clontech, Palo Alto, CA, USA) as a template. Nested PCR was done with the SYCP3F2/SYCP3R2 primer
Results
Human SYCP3 cDNA was sequenced. While this paper was under review, an independent report of the human SYCP3 gene was published.13 The genomic structure of SYCP3 was determined by comparison of the cDNA sequence with the human genomic sequence database. The SYCP3 gene encodes nine exons extending over 10·8 kb and is located on chromosome 12. The human SYCP3 cDNA encodes a 236 aminoacid protein with two coiled-coil-forming regions (figure 1), homologous with the mouse Sycp3 (66% identity overall).
Discussion
Cases of human male infertility are observed quite frequently in the context of genetic syndromes associated with chromosomal abnormalities. However, there are few reports of specific gene defects resulting in disruption of spermatogenesis and infertility in otherwise healthy men.1
Many genetic causes of human male infertility are known, including both chromosomal and gene defects.1 Although the Y chromosome encodes several genes required for spermatogenesis,14, 15, 16 autosomal genes are
GLOSSARY
- azoospermia
- Absence of germ cells (spermatozoa) in the ejaculate.
- sycp3
- Part of the synaptonemal complex, which is thought to mediate chromosome pairing, recombination, and synapsis. The synaptonemal complex is a meiosis-specific tripartite structure consisting of two parallel lateral regions and a central element. SYCP3 is a major component of the lateral elements and chromosome cores.
- synapsis
- Side-by-side pairing and union of homologous chromosomes.
References (16)
Synaptonemal complexes: structure and function
Curr Opin Cell Biol
(1996)- et al.
The murine SCP3 gene is required for synaptonemal complex assembly, chromosome synapsis, and male fertility
Mol Cell
(2000) - et al.
The genomic structure of SYCP3, a meiosis-specific gene encoding a protein of the chromosome core
Biochim Biophys Acta
(2001) - et al.
A new gene family (FAM9) of low-copy repeats in Xp22.3 expressed exclusively in testis: implications for recombinations in this region
Genomics
(2002) - et al.
Genetic dissection of mammalian fertility pathways
Nat Med
(2002) - et al.
The synaptonemal complex in genetic segregation
Annu Rev Genet
(1984) Meiotic chromosomes: it takes two to tango
Genes Dev
(1997)- et al.
A coiled-coil related protein specific for synapsed regions of meiotic prophase chromosomes
EMBO J
(1992)
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