Structural chromosome abnormalities in Down syndrome: a study of two families

Are de novo rea(21;21) chromosomes really de novo?

We report a rare case of recurrent trisomy 21 caused by an isochromosome 21q and what is very likely to be maternal germ-line cell mosaicism. Over 90% of cases of rob(21;21) reported in the literature are due to an isochromosome 21q, with a risk of recurrence of more than 10%.

Down syndrome due to unbalanced homologous acrocentric rearrangements and its recurrence in subsequent pregnancies: prenatal diagnosis by amniocentesis

OBJECTIVE

To present our experience of amniocentesis for the prenatal diagnosis of Down syndrome due to unbalanced homologous acrocentric rearrangements and its recurrence in subsequent pregnancies.

CASE REPORT

From January 1987 to September 2009, six cases with rea(21q21q) Down syndrome were diagnosed among 31,194 patients who underwent amniocentesis at Mackay Memorial Hospital, Taipei, Taiwan. Cytogenetic analysis of parental blood lymphocytes was performed in each case, and polymorphic DNA markers were used to investigate the nature of the aberrant chromosome. Three of the six cases were associated with recurrence in subsequent pregnancies. The rea(21q21q) Down syndrome was associated with advanced maternal age in three cases, a previous child with rea(21q21q) Down syndrome in three cases, an abnormal maternal serum screening result in one case, and an abnormal ultrasound finding in one case. All six cases arose de novo. Among the six cases with molecular analysis results, all had isochromosome 21, five of which were determined to be of maternal origin.

CONCLUSION

We found a frequency of 0.019% for rea(21q21q) Down syndrome in patients undergoing amniocentesis. Down syndrome caused by the homologous rearrangement rea(21q21q) can be associated with recurrence. Prenatal diagnosis of rea(21q21q) Down syndrome should include extensive cytogenetic and molecular analyses of the parents and probands.

Under-ascertainment of mosaic carriers of balanced homologous acrocentric translocations and isochromosomes

Acrocentric rearrangements are the most common chromosome abnormalities in humans. Carriers of homologous acrocentric rearrangements (Robertsonian translocations (ROBs) between homologous chromosomes and isochromosomes) are at very high risk of having multiple spontaneous abortions and chromosomally abnormal offspring. Parents of fetuses and children with unbalanced homologous acrocentric rearrangements are rarely found to be carriers or mosaic for the same rearrangement. Even though recurrent miscarriages may indicate a carrier parent, carriers are rarely identified. Comparison of non-chromosome 21 homologous rearrangements to rea(21q21q) culled from the literature revealed a 7-fold decrease in the number of mosaic cases among the parents of non-rea(21q21q) offspring. This under-ascertainment in parents may be due to low level mosaicism confined to the gonads, a true biological difference between chromosome 21 rearrangements and other homologous acrocentric rearrangements, or simply to the lack of rigorous clinical investigation of the parental karyotypes to uncover mosaicism. We recommend that polymorphic marker analysis be applied to apparently de novo acrocentric rearrangements to distinguish those resulting from biparental postzygotic formation from those resulting from meiotic formation; the latter of which may indicate a potential carrier parent. Parental chromosomal constitutions could then be screened in a large number of cells and in more than one tissue type to identify mosaicism. Identification of mosaicism allows for accurate genetic counseling and discussion of reproductive options. However, given that mosaicism may be restricted to the gonads, prenatal testing is likely to be desired by the family whether or not mosaicism is found.

Recurrence rate for de novo 21q21q translocation Down syndrome: a study of 112 families

Recurrence of de novo GqGq Down syndrome (DS) in nine reported families and the low frequency of this chromosome abnormality in the population prompted this multicenter study to examine recurrence rate. Pedigrees and cytogenetic findings were analyzed from 112 families ascertained by a DS child with de novo GqGq translocation. Four of 112 sets of parents had abnormal cytogenetic findings. One mother and two fathers had a low proportion of mosaicism for trisomy 21 in peripheral lymphocytes. In one of these fathers, fibroblast studies confirmed the mosaicism (others not similarly studied). Two of these three families have one additional child each, both being normal. In the fourth family, the mother is heterozygous for a pericentric inversion of chromosome 21. Ninety of the 112 probands had a total of 130 full sibs and 34 half sibs. None of these 164 sibs was affected; the recurrence rate could still be as high as 2% (with 95% confidence based upon calculation of a one-tailed confidence interval). Because of the latter, the nine reports of recurrence in the literature, and the abnormal cytogenetic findings in peripheral lymphocytes in 4/112 sets of parents (3.6%), we think that caution should be exercised when counseling that the recurrence risk of this event is remote.

A cytogenetic study of a population of mentally retarded males with special reference to the marker (X) syndrome

A cytogenetic survey of a population of 274 mentally retarded males on community placement is described. Thirty-five had an aneuploid chromosome constitution and five had the mar(X) syndrome. The range of clinical variation among the mar(X) probands and their affected relatives is described. Family studies were possible for four of the five mar(X) probands and in two families the mar(X) gene was apparently transmitted through a clinically normal male, suggesting that this type of male transmission may be a not uncommon phenomenon.

A rare case of mosaic Down syndrome 46,XY/46,XY, -21, +i(21q)

Detailed studies were carried out on a patient with a rare type of mosaicism which gave rise to an effective 21 trisomy. The clinical signs of Down syndrome were minimal. The cytogenetic interpretation is that the abnormal clone had an isochromosome derived from a maternal No. 21. The normal cell line appears to be replacing the abnormal clone.

Inversions and other unusual heteromorphisms detected by C-banding

Sixteen patients with unusual heteromorphisms involving alterations of the length and/or position of centromeric heterochromatin are described. Family studies showed that the heteromorphisms were present in other relatives and segregated in the expected 1:1 ratio. There was a significantly greater frequency of unusual heteromorphisms among Orientals than in other races studied.