Biology Reference
In-Depth Information
(1998) reported an unusual inverted duplication of chromosome 15 involv-
ing the distal portion of chromosome 15q, rather than the proximal, from
q25 to qter (Fig. 5.5A). The infant had severe hypotonia, cardiovascular
defects, CNS anomalies, and dysmorphic facies. Severe hearing loss was also
present, as determined by auditory evoked brainstem response. The child
had such severe developmental anomalies that she died at 12 days of life.
2.2.2.2 Inversion 2
Pericentric inversions are the most frequent chromosomal rearrangements
in humans, occurring in approximately 1% of the population (Therman and
Susman 1993). The majority are inherited and clinically insignificant. Peri-
centromeric inversions of chromosome 2 are the second most commonly
recognized inversion of human chromosomes, after pericentromeric
inversion of chromosome 9. Usually, the breakpoints are in p11q13. Kozma
et al. (1996) reported an unusual inversion of chromosome 2, with break-
points in p13q11.2 (Fig.
5.5B). The child had craniofacial anomalies,
significant hypotonia,
developmental delay,
and severe to profound
bilateral hearing loss.
2.2.2.3 Partial Trisomy 6q
Conrad et al. (1998) reported a toddler with a partial trisomy of approxi-
mately the lower third of chromosome 6 (Fig. 5.5C). The child had micro-
cephaly, facial anomalies, a webbed neck, congenital heart disease, renal
hypoplasia, developmental delay, and bilateral hearing loss. The additional
portion of chromosome 6 was translocated to the short arm of chromo-
some 14 and was inherited from the child's father, who had an apparently
balanced translocation between 6q22 and 14p13.
2.2.2.4 A Tandem Duplication and Deletion
Meschede et al. (1998) reported a translocation between two acrocentric
chromosomes, 14 and 21, with essentially the entire chromosome 21 translo-
cated to the telomeric end of 14q, resulting in a small deletion of 14q32.3
(Fig. 5.6). The phenotype included developmental delay, severe hypotonia,
mild facial dysmorphism, growth retardation, hypospadias, palmar creases,
marbled skin and a patent ductus arteriosus. Marked hearing loss required
hearing aids. A few other deletions of the very terminal portion of 14q have
been reported (Meschede et al. 1998). Although this is the only case in
which hearing loss was documented, it is possible that hearing evaluations
were not performed in all other cases.
2.2.3 Chromosomal Rearrangement Syndromes and Hearing Loss
The majority of chromosomal deletions result in partial monosomy for a
particular chromosomal region. Deletions may result from the unbalanced
segregation of a parental reciprocal translocation, or occur
de novo
.
