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Low sperm counts linked to increased risk of genetic disorders
Genomic imprinting is a process of differential expression of certain genes in the foetus based on whether they are present on the paternal or maternal alleles. Imprinting is mediated by epigenetic modifications, of which the most common is differential methylation of DNA sequences associated with imprinted genes in oocytes and spermatozoa. Major changes in genomic methylation occur following fertilisation, but imprinted alleles are protected from these effects. Imprinting disorders can lead to abnormal foetal development and diseases such as Angelman's and Beckwith-Wiedemann's syndromes. Angelman syndrome results in severe learning difficulties, epilepsy, unsteady gait and an unusually happy disposition. In some cases the syndrome is caused by DNA mutations that disrupt the imprinting process in a region of chromosome 15. Beckwith-Wiedemann syndrome is a condition associated with excessive growth before and after birth; this can cause a range of physical abnormalities, and also confers a greatly increased risk of malignant tumour development. In up to 60% of cases it is thought to result from disruption of imprinting in a region of chromosome 11.
Concern over the risk of imprinting disease in children conceived using assisted reproductive technologies (ARTs) such as in vitro fertilisation has been raised due to an unexpectedly high incidence of these syndromes. This has previously been attributed to the loss of maternal imprinting in the oocyte or embryo caused by ART procedures such as oocyte maturation or embryonic culture. A new study published in the Lancet presents evidence linking genetic imprinting disorders with abnormal spermatogenesis in men with low sperm counts [Marques CJ et al. (2004) Lancet 363, 1700-1702].
The Portugese team analysed spermatozoa from semen samples from 123 men undergoing routine investigations for infertility. Of these, 27 were found to have normal sperm and were used as a control group, whilst the remainder had moderate (46 individuals) or severe (50 individuals) oligozoospermia – that is, low sperm count. Sperm that showed normal morphology and motility were isolated from these samples, and DNA was extracted from them for analysis of methylation status in two oppositely imprinted genes: the paternally expressed MEST gene, and the maternally expressed H19 gene. Normally, for the MEST gene the maternal allele is methylated and inactive whilst the paternal allele is not, and for the H19 gene the paternal allele is methylated and inactive whilst the maternal allele is not.
Analysis of the MEST gene showed that the maternal imprint had been correctly erased in all of the normozoospermic (normal) and oligozoospermic samples. However, analysis of the methylation profiles of H19, which is paternally imprinted in early stages of spermatogenesis, revealed differences. Although all of the normal sperm DNA samples were correctly methylated, 24% of the oligozoospermic samples showed incomplete methylation. These samples represented 17% of the moderate and 30% of the severe oligozoospermic groups, with the degree of incomplete methylation being generally greater in the severe as composed to the moderate groups, in terms of the number of unmethylated sites. The authors conclude that abnormal spermatogenesis resulting in low sperm counts is associated with a rise in defective paternal imprinting of the H19 gene. They propose that infertility treatment may therefore promote transmission of paternal imprinting errors, with potentially detrimental developmental effects for embryos conceived following such treatment.
Comment: Although genetic imprinting disorders are generally rare, this paper presents evidence that a significant proportion of men with low sperm counts may have sperm with imprinting defects. The study is very small; extending the analysis to larger sample populations and additional paternally imprinted genes would strengthen the evidence base, as would long-term follow up of children born to men involved in such a study. Nevertheless, this is potentially an important issue in terms of increased risk of disorders among children born following assisted reproduction, though the absolute risk would remain very small. It may be desirable to evaluate current and potential new technologies used in assisted reproduction in terms of the relative risk of imprinting disorders they may represent, both maternal and paternal.