Autosomal Monosomies

As noted in the Introduction, autosomal monosomies are extremely rare in either liveborns or abortuses, reflecting the severity of the genetic imbalance resulting from the loss of an entire chromosome. The only monosomies that have been reported are monosomy 21, mosaic monosomy 22, and a single case reported in an abstract of a possible mosaic monosomy 20 in a 31/2 year old boy with atypical speech/ language delay, behavior problems, microcephaly, and patchy hypopigmentation of the skin (161).

Monosomy 21

Mosaic monosomy 21 was reported in four liveborns in the early literature (162-165). The most prominent features included intrauterine growth retardation, postnatal growth and mental retardation, hypertonia, facial dysmorphism with downward slanting palpebral fissures, large low-set ears, and micrognathia. A more recent report described pathological findings of an electively terminated 20-week female fetus after mosaic monosomy 21 was diagnosed by repeated amniocenteses (166). The above-described facial abnormalities were present in this abortus. In addition, a complex cardiac malformation, malrotation of the bowel, uterus didelphys, small dysmature ovaries, and focal cystic dysplasia of the lung were noted.

Approximately ten cases of apparently nonmosaic monosomy 21 have been reported in liveborns (reviewed in ref. 167,168,169). Some of these cases have subsequently been shown to represent partial monosomy 21 resulting from an undetected subtle translocation (170), explaining the observation that mosaic monosomy 21 is less commonly observed than apparently nonmosaic monosomy

Triploid Karyotype
Fig. 9. Karyotype of a triploid fetus (69,XXX).

21 and indicating that complete monosomy 21 is almost always incompatible with life. Most patients died before 2 years of age, although one male child survived to 11 years. The phenotypic features were similar to those observed in the mosaics and included intrauterine growth retardation, postnatal growth and mental deficiencies, microcephaly, hypertelorism with downward slanting palpebral fissures, large low-set ears, prominent nose, cleft lip/palate, and micrognathia. Abnormal muscle tone, mostly hypertonia, was common. Cardiac anomalies were present in a few cases.

Monosomy 22

Four cases of mosaic monosomy 22 in liveborns have been reported (171-174). All four were male. One was a 34-week premature infant with gastroschisis who died from intracranial hemorrhage shortly after birth. No dysmorphic features were noted, and an autopsy was not performed (173). Two patients had growth and developmental deficiencies, microcephaly, and mild facial dysmorphism. The fourth patient was a 30-week premature infant with facial features of DiGeorge syndrome, hyperto-nicity, limited extension of major joints, and flexion contractures of all fingers.

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