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Hum. Reprod. Advance Access originally published online on March 5, 2007
Human Reproduction 2007 22(6):1585-1596; doi:10.1093/humrep/dem030
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© The Author 2007. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

The expression of polymerase gamma and mitochondrial transcription factor A and the regulation of mitochondrial DNA content in mature human sperm

Alexandra Amaral1,2, João Ramalho-Santos2 and Justin C. St John1,3

1 The Mitochondrial and Reproductive Genetics Group, The Medical School, University of Birmingham, Birmingham, UK 2 Center for Neuroscience and Cell Biology, Department of Zoology, University of Coimbra, Coimbra, Portugal

3 To whom correspondence should be addressed at: The Mitochondrial and Reproductive Genetics Group, The Medical School, University of Birmingham, Birmingham B15 2TT, UK. E-mail: j.stjohn.1{at}bham.ac.uk

BACKGROUND: Human mitochondrial DNA (mtDNA) encodes 13 polypeptides of the electron transfer chain. Its replication is dependent on the nuclear-encoded polymerase gamma (POLG) and mitochondrial transcription factor A (TFAM). For POLG, only the polyglutamine tract, characterized by a series of CAG repeats, has been investigated in human sperm. However, TFAM is associated with the reduction in mtDNA content of testicular sperm. We have determined whether POLG and TFAM have functional roles in post-ejaculatory sperm mtDNA.

METHODS: Sperm samples were categorized as: normals, samples with one or two abnormal sperm parameters and oligoasthenoteratozoospermics (OATs). These were analysed by fluorescent PCR to determine the number of CAG repeats, real-time PCR for mtDNA copy number and immunocytochemistry and western blotting for patterns of expression for POLG, TFAM and the mtDNA-encoded COXI.

RESULTS: Only the OAT group presented with a significantly higher incidence of heterozygosity for CAG repeats, higher mtDNA content and a lower percentage of sperm expressing POLG and TFAM. Paradoxically, good-quality sperm had fewer mtDNA copies but significantly more sperm expressed POLG, TFAM and COXI.

CONCLUSIONS: Our data support the original findings that an association between sperm quality and POLG CAG repeats does exist. However, the biological significance of these variants in male infertility remains unclear, as these do not seem to affect mtDNA maintenance. The reduction in mtDNA content in normal samples likely reflects normal spermiogenesis, whereas increases in POLG and TFAM expression possibly compensate for the low mtDNA content, maintaining mitochondrial homeostasis.

Key words: mitochondria/mitochondrial DNA/mitochondrial transcription factor A/polymerase gamma/sperm

Submitted on November 10, 2006; resubmitted on January 14, 2007; accepted on January 18, 2007.


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