We present 4 recently diagnosed cases of inverted tandem duplication with involvement of the respective terminal band. Based on these 4 cases and review of the literature, the term "inverted terminal duplication" is proposed to designate specifically the type of inverted tandem duplication which involves the terminal band. A modification of the previous hypothesis of mechanism of origin is advanced. It is speculated further that a telomeric deletion of a meiotic chromosome followed by a U-type reunion of the chromatids, considered to be the first steps of the proposed mechanism of origin, may not be a rare gonadal event.
De novo supernumerary small ring chromosomes have mainly been reported in pediatric patients with clinical abnormalities, thus, there may be bias of ascertainment. Reports on prenatally diagnosed cases with postnatal follow-up are rare. With the availability of chromosome specific alpha-satellite centromeric probes, the interest in these previously unidentifiable supernumerary small ring chromosomes has been rekindled [Callen et al.: J Med Genet 27: 155-159, 1990; Callen et al.: Am J Hum Genet 48:769-782, 1991; Callen et al.: Am J Med Genet 43:709-715, 1992]. We report on 2 prenatal diagnosis cases, where a ring was noted in 25 and 60% of the amniocytes, respectively. The initial G- and C-banding in Case 1 allowed an assumption of a chromosome 1 origin of the extra chromosome. This was confirmed by fluorescence in situ hybridization (FISH) studies using the appropriate probes. No similar initial assumption could be made in Case 2; thus, random trials with multiple probes were performed. A chromosome 19 origin in Case 2 was eventually concluded. The large amount of C-band positive material on the extra chromosome and the normal level 2 fetal ultrasound examination suggested a favorable outcome in both cases, but the possibility of mental retardation could not be ruled out. An empiric risk figure with regard to prenatally diagnosed de novo supernumerary small ring chromosomes is not available. Although the decision making processes of the parents were different, they both decided to continue the pregnancy. At age 9 months and 1 1/2 years both children, a girl and a boy, showed normal growth and development.
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