Reproductive functions

Stimulation of Sertoli cells by FSH is a prerequisite in primate spermatogenesis and intratesticular androgen activity represents an important co-factor with a positive effect on the supporting function of Sertoli cells. Thus, it can be speculated that the CAG repeat polymorphism within the AR gene could have a limited influence on spermatogenesis. Such an effect can be observed as severely impaired spermatogenesis in X-SBMA patients (Arbizu et al. 1983). The investigation of the possible influence of a polyglutamine stretch within the normal length on sperm production requires a sample of carefully selected patients in which significant confounders (obstructive symptoms due to infections, congenital aplasia of the vas deferens [CBAVD], impaired spermatogenesis due to hormone disorders, deletions in one of the azoospermia-associated regions of the Y-chromosome) have been ruled out.

Control groups consisting of healthy fertile males should be homogenous in terms of ethnic origin (see above). It should be considered that within the cohort of fertile controls, sperm densities below 20 Mill / ml might occur (Rajpert-De Meyts et al. 2002). Unfortunately, a fraction of studies on this subject did not strictly exclude patients described above. Therefore, it is not surprising that conflicting results emerged when infertile and fertile men were compared in regard to their number of CAG repeats. Some studies reported higher numbers of CAG triplets in infertile men (Tut et al. 1997; Legius et al. 1999; Dowsing et al. 1999; Yoshida etal. 1999; Yong etal. 2000; Mifsud etal. 2001; Patrizio etal. 2001; Wallerand etal. 2001; Mengual et al. 2003), but some did not (Lundberg Giwercman et al. 1998; Hiort etal. 1999; Dadze S etal. 2000; van Golde etal. 2002; Rajpert-De Meyts etal. 2002). In contrast to studies from Europe, a relationship between sperm production and CAG repeat length is obviously more likely to be found in mixed populations, especially including men of Asian origin.

When only fertile men covering the whole range of normal sperm concentrations were involved in respective evaluations, a shorter CAG repeat tract was associated with higher sperm numbers (von Eckardstein etal. 2001; Rajpert-De Meyts et al. 2002). Nevertheless, a marked variation of sperm density in relation to the AR polymorphism was observed. Hence, spermatogenesis is likely to be influenced by the number of CAG repeats within the normal range, but whether this reaches relevance for individuals remains doubtful. The range of sperm concentrations leading to infertility is most likely reached at CAG repeat numbers that are associated with X-SBMA. Furthermore, it can be assumed that the proportion of men with longer CAG repeats among infertile patients may, in case of strict selection criteria excluding all known causes of infertility, appear higher than in a control population. Genetic counselling concerning inheritabilityofthis modulator of spermatogenesis is of very restricted value, since the CAG polymorphism is located on the X-chromosome and the specific tract length will affect spermatogenesis of the offspring only in one half of the grandsons.

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