Prenatal diagnosis, fetal pathology, and cytogenetic analysis of mosaic trisomy 14.

While true mosaicism occurs in only 0.25 per cent of genetic amniocenteses, nearly 2.5 per cent of amniotic fluid cell cultures contain a second cell line. In the common practice of prenatal diagnosis, an aberrant cell line confined to a single colony is usually disregarded. We present a case of mosaic trisomy 14 which was not detected on initial chromosome analysis. At birth, multiple malformations were apparent. Newborn cytogenetic studies revealed mosaicism [46,XX/46,XX,-14, +i(14q)] with an isochromosome 14 in 37 per cent of lymphocytes. Additional cells from the initial amniotic fluid culture were analysed post-delivery and the isochromosome 14 identified in only one of 12 total colonies. This case illustrates two important lessons in prenatal diagnosis. First, amniotic fluid cell cultures may not accurately reflect the relative distribution of the normal and abnormal cell lines within a mosaic fetus. Second, while it is generally reasonable to disregard mosaicism confined to a single colony, this policy will, on rare occasion, result in diagnostic error. This should be taken into consideration, particularly when dealing with autosomal trisomies potentially compatible with livebirth.

Tissue-specific mosaicism for the stability of a ring 13 chromosome.

A ring 13 chromosome was identified in an infant whose chromosomes were studied because of microcephaly. The ring chromosome was studied over a 3-year-period in lymphocytes, and in both short- and long-term fibroblast cultures. Lymphocyte cultures revealed a consistently stable ring 13 chromosome with minimal loss of genetic material (the distal portion of band q34). Fibroblast cultures contained a ring chromosome a quarter of the size of the original ring and this chromosome was unstable in short- and long-term cultures. The patient's mild dysmorphic features and moderate mental retardation correlate with a stable ring chromosome in which only a small amount of genetic material has been lost rather than with the unstable small ring 13 chromosome observed in fibroblast cultures. The observation of drastic tissue specific differences in ring sizes and stability makes phenotypic karyotypic correlations with ring chromosome patients even more difficult and counselling in cases of prenatal diagnosis questionable.

Duplication (12q) syndrome in female cousins, resulting from maternal (11;12) (p15.5;q24.2) translocations.

We have studied female cousins with partial duplication of 12q. The cousins' mothers (who are sisters) and the maternal grandmother and great grandmother carried a balanced translocation between chromosomes 11 and 12. We have compared our patients with eight other reported cases of partial duplication of the same chromosome segment (12q24----12qter). Placement of the extra material seems to have little effect on the anomalies present; (only two other cases involved chromosome 11). We propose that our patients provide further evidence that duplication of 12q leads to a clinically identifiable syndrome.