The similarity of phenotypic effects caused by Xp and Xq deletions in the human female: a hypothesis.

We have collected from the literature adult nonmosaic women with the following aberrant X chromosomes: Xp- (52), Xq- (67), idic(Xp-)(10), idic(Xq-)(9), and interstitial deletions (12). Lack of Xp, and especially Xcen-Xp11 (b region), may cause full-blown Turner syndrome. However, individual Turner symptoms, including gonadal dysgenesis, otherwise seem to be randomly distributed with respect to the different Xp and Xq deletions, although breakpoints distal to Xq25 do not give rise to any phenotypic anomalies except in a few cases of secondary amenorrhea or premature menopause. Of the carriers of an Xp- or Xq- chromosome, 65% and 93%, respectively, suffer from ovarian dysgenesis, whereas all idic(Xp-) and idic(Xq-) chromosomes cause primary or secondary amenorrhea. Xq deletions do not induce specific symptoms different from those caused by Xp deletions. Lack of the tip of Xp has led in 46/52 cases to short stature, but 43% of the Xq- carriers are also short. To explain these observations, we propose the following hypothesis. Since deletions of truly inactivated regions do not seem to cause any symptoms, we assume that the b region (Xcen-p11) always stays active in a normal inactive X, but is inactivated in deleted X chromosomes, especially in Xq- chromosomes. In some cases, inactivation may spread to the tip of Xp; this would explain the apparently variable behavior of the Xg and STS genes, and the short stature of some Xq- carriers. Full chromosome pairing seems to be a prerequisite for the viability of oocytes and thus for gonadal development. Deleted X chromosomes necessarily leave a portion of the normal X unpaired and isodicentrics probably interfere with pairing, resulting in atresia of oocytes. The role played by the "critical region" (Xq13-q24) in ovarian development is still unclear.