Mosaic hexasomy 21.

Abstract

SUMMARY Cases in which there are more than three copies of a sex chromosome, and rarely of an autosome, have been reported, but to our knowledge hexasomy has never been described except in tissue undergoing neoplastic change. This report describes a female infant with multiple malformations in whom we found a mosaic hexasomy 21. This was first detected in amniotic fluid cells and subsequently in skin fibroblasts.Case report Spontaneous delivery of this female infant occurred at about 36 weeks' gestation. Birth weight was about 4000 g and the head circumference was 34 cm.Respirations were gasping and irregular and there was acrocyanosis with dusky discolouration of the lips. The anterior fontanelle was large and cranial sutures, including the metopic, were widely open. Despite its circumference, the head appeared small in comparison to the body. There was marked hypertelorism with short upward slanting palpebral fissures. The globes were small and the corneae cloudy. There was a right sided cleft of the lip, complete cleft of the palate, and marked nasal hypoplasia. The ears were low set and rotated, and the lobes were fleshy. The neck was short with redundant soft tissue folds posteriorly. The chest was narrow with hypoplastic nipples. The heart was normal on auscultation. The sternum was of normal length. External genitalia were normal female and the anus was anteriorly placed. The spine appeared normal.The upper limbs were hypotonic and abnormalities were confined to the hands. Both thumbs were triphalangeal and the fingers were tapered with hypoplastic nails. There was camptodactyly of fingers 4 and 5 on the right and clinodactyly of the right second and left fourth fingers. There were simian lines bilaterally. Dermatoglyphs were unremarkable.The lower limbs were hypotonic with prominent heels and a midplantar vertical crease bilaterally.The baby died shortly after birth. Necropsy confirmed the physical observations. The heart was normal on gross examination and the brain on gross and microscopical study. X-ray examination failed to reveal any skeletal abnormalities and confirmed the clinical impression of borderline microcephaly.The mother and father were both aged 36 and in good health. There had been two previous pregnancies, one ending in an early spontaneous abortion and the second with a normal female whose development has been entirely normal. There was no other relevant family history.

Pallister-Killian syndrome: Cytogenetic and biochemical studies

“…Perhaps, the mitotic nondisjunction occurs between the first cleavage division and early embryonal stage. The case misinterpreted as hexasomy 21 mosaicism (Ketupanya et al, 1984) and ours may be the extreme examples concerning the time of mosaic formation.…”

Mosaic complete tetrasomy 21 in a fetus with complete atrioventricular septal defect and minor morphological variations

“…Moreover, four other patients have been reported but were confused with mosaic tetrasomy 12p due to use of banding techniques alone (Gilgenkrantz et al, 1987;Hall, 1985;Nagarsheth & Mootabar, 1997). This may also be the case of a little girl who died shortly after birth in a context of severe malformations without Down syndrome phenotype and in whom a weak mosaic of a hexasomy 21 (with two supernumerary marker chromosomes supposed to be a double 21;21 translocation) on amniotic fluid and skin but not on blood was demonstrated (Ketupånyå, Crandåll, Blanchard, & Rogers, 1984).…”