TAILIEUCHUNG - Chapter 063. Chromosome Disorders (Part 9)

Microdeletion Syndromes The term contiguous gene syndrome refers to genetic disorders that mimic a combination of single-gene disorders. They result from the deletion of a small number of tightly clustered genes. Because some are too small to be detected cytogenetically, they are termed microdeletions. The application of molecular techniques has led to the identification of at least 18 of these microdeletion syndromes (Table 63-4). Some of the more common ones include the Wilms' tumor–aniridia complex (WAGR), Miller Dieker syndrome (MDS), and velocardiofacial (VCF) syndrome. WAGR is characterized by mental retardation and involvement of multiple organs, including kidney (Wilm's tumor), eye. | Chapter 063. Chromosome Disorders Part 9 Microdeletion Syndromes The term contiguous gene syndrome refers to genetic disorders that mimic a combination of single-gene disorders. They result from the deletion of a small number of tightly clustered genes. Because some are too small to be detected cytogenetically they are termed microdeletions. The application of molecular techniques has led to the identification of at least 18 of these microdeletion syndromes Table 63-4 . Some of the more common ones include the Wilms tumor-aniridia complex WAGR Miller Dieker syndrome MDS and velocardiofacial VCF syndrome. WAGR is characterized by mental retardation and involvement of multiple organs including kidney Wilm s tumor eye aniridia and the genitourinary system. The cytogenetic abnormality involves a deletion of a part of the short arm of chromosome 11 11p13 which typically is detectable on well-banded chromosome preparations. In MDS a disorder characterized by mental retardation dysmorphic faces and lissencephaly the deletion involves chromosome 17 17p13 . Using FISH 17p deletions have been detected in 90 of patients with MDS as well as in 20 of cases of isolated lissencephaly. Table 63-4 Some Commonly Identified Microdeletion and Microduplication Syndromes Syndrome Cytogenet ic Location Principal Features Imprintin g Effects Langer-Giedion syndrome del Sparse hair bulbous nose variable mental retardation No WAGR complex 11p13 del Wilms tumor aniridia genitourinary disorders mental retardation No Beckwith- 11p15 Macrosomia Yes Wiedemann syndrome dup macroglossia omphalocoele occasionally associated with paternal uniparental disomy see text Retinoblastoma del Retinoblasto ma due to homozygous loss of functional RB allele No obvious effect although abnormal RB allele more likely to be paternal Prader-Willi syndrome 15q11-13 del Obesity hypogonadism mental retardation Yes prototypic imprinting disorder see text Angelman 15q11-13 Ataxic gait .

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