Chromosomal deletion syndromes

Chromosomal deletion syndromes are the result of the loss of a part of a chromosome. In this case, there is a tendency to develop severe congenital malformations and significant delays in mental and physical development. Chromosomal deletion syndromes are rarely suspected prenatally, but can sometimes be diagnosed during this period if, for some reason (not related to these syndromes), karyotyping is performed. Postnatally, chromosomal deletion syndromes are suspected based on the child's appearance, clinical manifestations and are confirmed by examining the karyotype, as well as other methods of genetic analysis.

5p-Deletion (cri du chat syndrome)

Deletion of the terminal region of the short arm of chromosome 5 (5p) is characterized by a high-pitched, mewing cry, very similar to a kitten's cry, which is observed immediately after birth for several weeks and then disappears. The child is born with low birth weight, hypotonia, microcephaly, a round face with wide-set eyes, slanted (downward) palpebral slits (with or without epicanthus), strabismus, and a nose with a wide base. The ears are low-set, abnormally shaped, may have growths in front of them, and the external auditory canal is often narrowed. Syndactyly, hypertelorism, and heart defects are common. Mental and physical development are severely delayed. Many patients live more than 20 years, but are severely disabled.

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4p-Deletion (Wolff-Hirschhorn syndrome)

Deletion of the short arm of chromosome 4 (4p) results in profound mental retardation. Manifestations may also include a broad or beaked nose, midline scalp defects, ptosis, colobomas, cleft palate, delayed bone maturation, and, in boys, hypospadias and cryptorchidism. Many patients die in the first year of life; relatively few survive beyond 20 years, but are profoundly disabled and prone to infections and epilepsy.

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Linked gene syndromes

These syndromes involve microscopic and submicroscopic deletions of linked genes in specific regions of multiple chromosomes; small chromosome duplications also occur. The effects of duplications are usually milder than those of deletions. Almost all cases are sporadic; however, when mild, as with some 22q11.21 deletions, patients may transmit the syndrome. Numerous syndromes with widely varying manifestations have been identified. Deletions and duplications are often detected using fluorescent probes and other techniques. Sometimes deletions and duplications cannot be detected by cytogenetic methods, but their presence can be confirmed using DNA probes complementary to the missing region.

Telomeric deletions

These are small and often submicroscopic deletions that can occur at either end of a chromosome. Phenotypic changes may be minimal. Telomere deletions may account for a large percentage of non-specific mental retardation, in which the patient has mildly dysmorphic features.