X inactivation in triploidy and trisomy: the search for autosomal transfactors that choose the active X15

Only one X chromosome functions in diploid human cells irrespective of the sex of the individual and the number of X chromosomes. Yet, as we show, more than one X is active in the majority of human triploid cells. Therefore, we suggest that (i) the active X is chosen by repression of its XIST locus, (ii) the repressor is encoded by an autosome and is dosage sensitive, and (iii) the extra dose of this key repressor enables the expression of more than one X in triploid cells. Because autosomal trisomies might help locate the putative dosage sensitive trans-acting factor, we looked for two active X chromosomes in such cells. Previously, we reported that females trisomic for 18 different human autosomes had only one active X and a normal inactive X chromosome. Now we report the effect of triplication of the four autosomes not studied previously; data about these rare trisomies - full or partial - were used to identify autosomal regions relevant to the choice of active X. We find that triplication of the entire chromosomes 5 and 11 and parts of chromosomes 1 and 19 is associated with normal patterns of X inactivation, excluding these as candidate regions. However, females with inherited triplications of 1p21.3-q25.3, 1p31 and 19p13.2-q13.33 were not ascertained. Thus, if a single key dose-sensitive gene induces XIST repression, it could reside in one of these locations. Alternatively, more than one dosage-sensitive autosomal locus is required to form the repressor complex

Endocrine abnormalities in a patient with partial trisomy 4q.

Partial trisomy of the long arm of chromosome 4, usually resulting from a familial segregation of a balanced translocation, has been described in a number of patients. This report describes the genetic and endocrine findings in a 16 year old 46,XY,12q+ mentally retarded male. The banding pattern of the extra chromatin material from this de novo unbalanced translocation shows that the distal segment of the long arm of chromosome 4 is involved. Comparison of the clinical features in this patient with cases of partial trisomy 4q previously reported support the cytogenetic evidence for this translocation involving the distal portion of 4q. Endocrine data suggested an end-organ resistance, characterised by extreme hyperinsulinaemia, primary hypothyroidism, and hypergonadotrophic hypogonadism associated with no signs of autoimmunity. To our knowledge, no endocrine evaluation has been previously reported in patients with partial trisomy 4q

A susceptibility gene for premature ovarian failure (POF) maps to proximal Xq28

Terminal deletions of the long arm of the human X chromosome have been described in women with premature ovarian failure (POF). We report here the molecular characterization of an inherited deletion in two affected women and in their mother. The two daughters presented secondary amenorrhea at 17 or 22 years respectively, while the mother was fertile. She had four children, but she eventually had premature menopause at 43 years of age. The fine molecular analysis of the deletion showed that the three women carried an identical deletion. We conclude that the phenotypic difference within the family must be attributed to genetic or environmental factors and not to the presence of different extent deletions. By comparison with other deletions in the region, we map a susceptibility gene for POF to 4.5 Mb, in the distal part of Xq. © 2004 Nature Publishing Group All rights reserved