Chapter 063. Chromosome Disorders (Part 9)

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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...

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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 Principal Imprintin ic Location Features g Effects Langer-Giedion 8q24.1 Sparse hair, No syndrome (del) bulbous nose, variable mental retardation WAGR complex 11p13 Wilms' No (del) tumor, aniridia, genitourinary disorders, mental retardation Beckwith- 11p15 Macrosomia, Yes,
Wiedemann syndrome (dup) macroglossia, occasionally omphalocoele associated with "paternal uniparental disomy" (see text) Retinoblastoma 13q14.11 Retinoblasto No (del) ma due to obvious effect, homozygous loss of although functional RB allele abnormal RB allele more likely to be paternal Prader-Willi 15q11-13 Obesity, Yes, syndrome (del) hypogonadism, prototypic mental retardation imprinting disorder (see text) Angelman 15q11-13 Ataxic gait With
syndrome (del) Prader-Willi syndrome, prototypic imprinting disorder (see text) α-Thalassemia 16p13.3 α- No and mental retardation (del) Thalassemia and mental retardation, due to deletion of distal 16p, including α-globin locus Smith-Magenis 17p11.2 Brachycephal No syndrome (del) y, midface hypoplasia, mental retardation Miller-Dieker 17p13 Dysmorphic No syndrome (del) facies, lissencephaly
Charcot-Marie- 17p11.2 Progressive No Tooth syndrome type (dup) neuropathy due to 1A microduplication DiGeorge 22q11 Abnormalities No syndrome/velocardiofaci (del) of third and fourth al syndrome branchial arches