(See also Overview of Chromosomal Anomalies.)
Microdeletion syndromes are better defined than are microduplication syndromes, and the significance of many microduplications is still unclear. The reciprocal duplications of well-recognized microdeletions such as 22q11.2 and 7q11.23 have been more clearly defined in recent years.
Microdeletion syndromes differ from chromosomal deletion syndromes in that chromosomal deletion syndromes are usually visible on karyotyping because of their larger size (typically > 5 megabases), whereas the abnormalities in microdeletion syndromes involve smaller segments (typically 1 to 3 megabases) and are detectable only with fluorescent probes (fluorescent in situ hybridization) and chromosomal microarray analysis. A given gene segment can be deleted and duplicated (termed a reciprocal duplication). The clinical effects of microscopic reciprocal duplications tend to be similar but less severe than those of deletions involving the same segment. The term contiguous gene syndrome typically refers to a condition that is commonly associated with microdeletions but that can also be associated with microduplications in which genes are clustered together. (See also Next-generation sequencing technologies.)
Most clinically significant microdeletions and microduplications seem to occur sporadically; however, mildly affected parents may be diagnosed when parental testing is done after a child is found to have an abnormality.
Numerous microdeletion syndromes have been identified, with widely varying manifestations (see Table: Examples of Microdeletion Syndromes).
Reciprocal microduplication involving chromosome 17p11.2 is associated with Potocki-Lupski syndrome. Infants with this disorder have hypotonia, feeding problems, failure to thrive, heart defects, developmental delay, and autism.
Examples of Microdeletion Syndromes
Syndrome |
Chromosomal Deletion |
Description |
|
Alagille syndrome |
20p.12 |
Cholestasis, bile duct paucity, cardiac anomalies, pulmonary artery stenosis, butterfly vertebrae, posterior embryotoxon of the eye |
|
Angelman syndrome |
Maternal chromosome at 15q11 |
Seizures, puppet-like ataxia, frequent laughter, hand flapping, severe intellectual disability |
|
DiGeorge syndrome (DiGeorge anomaly, velocardiofacial syndrome, pharyngeal pouch syndrome, thymic aplasia) |
22q11.21 |
Hypoplasia or lack of thymus and parathyroids, cardiac anomalies, cleft palate, intellectual disability, psychiatric problems |
|
Langer-Giedion syndrome (trichorhinophalangeal syndrome type II) |
8q24.1 |
Exostosis, cone epiphyses, sparse hair, bulbous nose, hearing loss, intellectual disability |
|
Miller-Dieker syndrome |
17p13.3 |
Lissencephaly; short, upturned nose; severe growth retardation; seizures; severe intellectual disability |
|
Paternal chromosome at 15q11 |
In infancy: Hypotonia, poor feeding, failure to thrive In childhood and adolescence: Obesity, hypogonadism, small hands and feet, intellectual disability, obsessive-compulsive behaviors |
|
|
Rubinstein-Taybi syndrome |
16p13− |
Broad thumbs and large toes, prominent nose and columella, intellectual disability |
|
Smith-Magenis syndrome |
17p11.2 |
Brachycephaly, midfacial hypoplasia, prognathism, hoarse voice, short stature, intellectual disability |
|
Williams syndrome |
7q11.23 |
Aortic stenosis, intellectual disability, elfin facies, transient hypercalcemia in infants |
