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Chromosomal Deletion Syndromes
Chromosomal deletion syndromes result from loss of parts of chromosomes. They may cause severe congenital anomalies and significant intellectual and physical disability. Chromosomal deletion syndromes are rarely suspected prenatally but may be incidentally discovered at that time if karyotyping is done for other reasons. Postnatal diagnosis is suspected by clinical appearance and is confirmed by karyotyping, if the deletion is relatively large, or by other cytogenetic techniques such as fluorescent in situ hybridization or microarray analysis.
Chromosomal deletion syndromes typically involve larger deletions, that are visible on karyotyping. Syndromes involving smaller deletions (and additions) that affect one or more contiguous genes on a chromosome and are not visible on karyotyping are considered contiguous gene syndromes (see Contiguous Gene Syndromes).
Deletion of the end of the short arm of chromosome 5 (5p—usually paternal) is characterized by a high-pitched, mewing cry, closely resembling the cry of a kitten, which is heard in the immediate neonatal period, lasts several weeks, and then disappears. Affected neonates are hypotonic and have low birth weight, microcephaly, a round face with wide-set eyes, downward slanting of the palpebral fissures (with or without epicanthal folds), strabismus, and a broad-based nose. The ears are low-set, abnormally shaped, and frequently have narrow external auditory canals and preauricular tags. Syndactyly, hypertelorism, and cardiac anomalies occur often. Mental and physical development is markedly retarded. Many affected children survive into adulthood but have significant disability.
Deletion of the short arm of chromosome 4 (4p) results in profound intellectual disability. Manifestations also may include epilepsy, a broad or beaked nose, midline scalp defects, ptosis and colobomas, cleft palate, delayed bone development, and, in boys, hypospadias and cryptorchidism. Many affected children die during infancy; those who survive into their 20s have severe disability.
These deletions may be visible on karyotyping but are also sometimes small and submicroscopic and may occur at either telomere (the end of a chromosome). Phenotypic changes may be subtle. Subtelomeric deletions may be present in people with nonspecific intellectual disability and mildly dysmorphic features as well in more severely affected people with multiple congenital anomalies.
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