Mechanisms of ring chromosome formation in 11 cases of human ring chromosome 21.

We studied the mechanism of ring chromosome 21 (r(21)) formation in 13 patients (11 unique r(21)s), consisting of 7 from five families with familial r(21) and 6 with de novo r(21). The copy number of chromosome 21 sequences in the rings of these patients was determined by quantitative dosage analyses for 13 loci on 21q. Nine of 11 r(21)s, including the 5 familial r(21)s, showed no evidence for duplication of 21q sequences but did show molecular evidence of partial deletion of 21q. These data were consistent with the breakage and reunion of short- and long-arm regions to form the r(21), resulting in deletion of varying amounts of 21q22.1 to 21qter. The data from one individual who had a Down syndrome phenotype were consistent with asymmetric breakage and reunion of 21q sequences from an intermediate isochromosome or Robertsonian translocation chromosome as reported by Wong et al. Another patient, who also exhibited Down syndrome, showed evidence of a third mechanism of ring formation. The likely initial event was breakage and reunion of the short and long arms, resulting in a small r(21), followed by a sister-chromatid exchange resulting in a double-sized and symmetrically dicentric r(21). The phenotype of patients correlated well with the extent of deletion or duplication of chromosome 21 sequences. These data demonstrate three mechanisms of r(21) formation and show that the phenotype of r(21) patients varies with the extent of chromosome 21 monosomy or trisomy.

Fragile site at 12q13 associated with phenotypic abnormalities.

A dysmorphic 3 year old boy with severe psychomotor retardation is described. His karyotype was 45,XY,t(13q;14q)rob, fra (12q13). The relationship between fra(12q13) and the clinical picture is discussed.

X long arm deletion with oligomenorrhoea.

A 35-year-old female patient with oligomenorrhoea had a deletion of the long arm of the X chromosome. The breakpoint at band q23 caused infertility in spite of excessive pituitary stimulation. The aberrant X chromosome was inactivated in all cells analysed.

Isodicentric X chromosome in a woman with characteristics of gonadal dysgenesis.

This paper presents a female patient with primary amenorrhea in whose karyotype an aberrant X chromosome was present. The chromosome resulted from the fusion of two X chromosomes at distal parts of the long arm and from the loss of the segment q24 leads to qter. The clinical and cytogenetic picture is compared with that of patients with the same aberration reported by other authors.

A case of long arm deletion of the X chromosome in a patient with secondary amenorrhea.

This paper presents the clinical and cytogenetic findings in a female patient with secondary amenorrhea and normal phenotype. Some difficulties related to karyotype-phenotype correlation are discussed.

[2 cases of partial trisomy 10p due to a paternal translocation t(10p;18)(p13;q23)]. / Deux cas de trisomie 10p partielle dus à une translocation paternelle t(10;18)(p13;q23

Two brothers trisomic for the distal two thirds of 10p are reported. Trisomy results from the malsegregation of a familial translocation rcp (10;18)(p13;q23) present in the father, a half-brother and the grand-father of the propositi. The phenotype is comparable to that of other 10p trisomic patients reported in the literature.

[Reciprocal translocation in the family of 2 prositas with partial trisomy of the chromosome 7q]. / Translocation réciproque dans la famille de deux proposants avec une trisomie partielle du chromosome 4q

Two patients with partial trisomy 7q are reported. The fathers of both patients are distantly related and carry the same balanced translocation - 46,XY,rep(4;7) (q35;q32).