22q13 deletion syndrome

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22q13 Deletion Syndrome (spoken as "twenty two q one three"), also known as Phelan-McDermid Syndrome, is a genetic disorder caused by a microdeletion on chromosome 22. The deletion occurs at the terminal end of the chromosome at the location designated q13.3. This microdeletion is rarely uncovered by typical genetic screening, therefore a fluorescence in situ hybridization, or FISH, test is recommended to confirm the diagnosis.

This genetic disorder is characterized by general hypotonia, absent to delayed speech, and global developmental delays. It has a prevalence estimated at 2.5-10 per million births, though this is thought to be a gross underestimateFact|date=March 2008.

Characteristics

Individuals with a 22q13 deletion can suffer from a range of symptoms, with mild to very serious physical and behavioral characteristics. Possible symptoms are [ [http://www.22q13.org 22q13 Deletion Syndrome / Phelan-McDermid Syndrome] ] :

Physical
* Absent to severely delayed speech: 99%
* Hypotonia (poor muscle tone): 97%
* Normal to accelerated growth: 95%
* Increased tolerance to pain: 86%
* Thin, flaky toenails: 78%
* Large, fleshy hands: 68%
* Prominent, poorly formed ears: 65%
* Pointed chin: 62%
* Dolicocephaly (elongated head): 57%
* Ptosis (droopy eyelids): 57%
* Tendency to overheat and lack of perspiration: 51%

Behavioral
* Chewing on non food items (clothing, bedding, toys):70%
* Teeth grinding: (percent undetermined)
* Tongue thrusting: (percent undetermined)
* Hair pulling: (percent undetermined)
* Aversion to clothes: (percent undetermined)
* Autistic behaviors: (percent undetermined)

Etiology

The deletion affects the terminal region of the long arm of chromosome 22 (the paternal chromosome in 75% of cases), from 22q13.3 to 22qter. Although the deletion is most typically a result of a de novo mutation, there is an inherited form resulting from familial chromosomal translocations involving the 22 chromosome. In the de novo form, the size of the deletion is variable and can go from 130kbp (130,000 base pairs) to 9Mbp (9,000,000 base pairs). While some clinical signs correlate with the size of the deletion, the main traits of the syndrome appear to be independent of the deletion size, and only related to the presence of the Shank3 gene [OMIM|606230|SH3 AND MULTIPLE ANKYRIN REPEAT DOMAINS 3; SHANK3] . The haploinsufficiency of Shank3 is thought to be the responsible for the neurological deficits of the syndrome. [cite journal |author=Wilson HL, Wong AC, Shaw SR, "et al" |title=Molecular characterisation of the 22q13 deletion syndrome supports the role of haploinsufficiency of SHANK3/PROSAP2 in the major neurological symptoms |journal=J. Med. Genet. |volume=40 |issue=8 |pages=575–84 |year=2003 |pmid=12920066 |pmc=1735560 |doi= |url=http://jmg.bmj.com/cgi/pmidlookup?view=long&pmid=12920066]

The proteins encoded by the Shank genes assemble glutamate receptors with their intracellular signaling apparatus and cytoskeleton at the postsynaptic density. They are important for the formation and stabilisation of synapses:
* Experimentally induced expression of Shank3 has been shown to be sufficient to induce functional dendritic spines in aspiny cerebellar neurons. [cite journal |author=Roussignol G, Ango F, Romorini S, "et al" |title=Shank expression is sufficient to induce functional dendritic spine synapses in aspiny neurons |journal=J. Neurosci. |volume=25 |issue=14 |pages=3560–70 |year=2005 |pmid=15814786 |doi=10.1523/JNEUROSCI.4354-04.2005 |url=]
* Neural network activity up- or down regulates large groups of postsynaptic proteins through ubiquitin-mediated protein degradation. Shank proteins were identified as one of the few postsynaptic density proteins that can be degraded by ubiquitination [cite journal |author=Waites CL, Craig AM, Garner CC |title=Mechanisms of vertebrate synaptogenesis |journal=Annu. Rev. Neurosci. |volume=28 |issue= |pages=251–74 |year=2005 |pmid=16022596 |doi=10.1146/annurev.neuro.27.070203.144336 |url=]

In 2006, a group lead by Thomas Bourgeron from the Pasteur Institute in France, found anomalies of the 22q13 locus in five children with diagnosis of autism and Asperger syndrome. While the absence of the Shank3 gene was found in children with the typical characteristics of the Phelan-McDermid syndrome, its duplication was found in one child diagnosed with Asperger syndrome, [ [http://www.cosmosmagazine.com/node/937 Gene linked to autism discovered] ] [cite journal |author=Durand CM, Betancur C, Boeckers TM, "et al" |title=Mutations in the gene encoding the synaptic scaffolding protein SHANK3 are associated with autism spectrum disorders |journal=Nat. Genet. |volume=39 |issue=1 |pages=25–7 |year=2007 |pmid=17173049 |pmc=2082049 |doi=10.1038/ng1933 |url=] a type of high-functioning autism.

Van Bokhoven "et al." (1997) [cite journal |author=van Bokhoven H, Kissing J, Schepens M, "et al" |title=Assignment of WNT7B to human chromosome band 22q13 by in situ hybridization |journal=Cytogenet. Cell Genet. |volume=77 |issue=3-4 |pages=288–9 |year=1997 |pmid=9284940 |doi= |url=] have also assigned the WNT7B gene to 22q13 [OMIM|601967|WINGLESS-TYPE MMTV INTEGRATION SITE FAMILY, MEMBER 7B; WNT7B] . Wnt7b acts through Dvl1 to the regulation of dendritic development. [cite journal |author=Rosso SB, Sussman D, Wynshaw-Boris A, Salinas PC |title=Wnt signaling through Dishevelled, Rac and JNK regulates dendritic development |journal=Nat. Neurosci. |volume=8 |issue=1 |pages=34–42 |year=2005 |pmid=15608632 |doi=10.1038/nn1374 |url=] found that its overexpression resulted in increased dendritic branching in cultured mouse hippocampal neurons. Knockout mice for Dvl1 are viable, fertile and structurally normal, but show reduced social interaction and abnormal sleeping patterns. [cite journal |author=Wilson HL, Wong AC, Shaw SR, "et al" |title=Molecular characterisation of the 22q13 deletion syndrome supports the role of haploinsufficiency of SHANK3/PROSAP2 in the major neurological symptoms |journal=J. Med. Genet. |volume=40 |issue=8 |pages=575–84 |year=2003 |pmid=12920066 |pmc=1735560 |doi= |url=http://jmg.bmj.com/cgi/pmidlookup?view=long&pmid=12920066]

Incidence

The incidence of the 22q13 deletion syndrome is uncertain. The advanced genetic technique essential for diagnosis, fluorescent in situ hybridization (FISH), has only been available since 1998, and currently requires specialized laboratory facilities. Current thinking is that 22q13 deletion syndrome remains largely under-diagnosed, and may be one of the principal causes of idiopathic mental retardation.

See also

* 22q11.2 deletion syndrome

Notes

References

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* [http://www.22q13.org 22q13.org "22q13 deletion syndrome home"]

External links

* [http://www.genetests.org/profiles/22q13.3 GeneReviews: 22q13.3 deletion syndrome]
* [http://www.22q13.org 22q13.org "22q13 deletion syndrome home"] Support group for families of children affected by the 22q13 deletion syndrome.
* [http://www.nt.net/~a815/22q13.htm "Chromosome 22 central"] Description and Full-text links to related articles.
* [http://www.rarechromo.co.uk/forum/DisordersLeaflets.asp "The rare chromosome disorder group support"] . Support group for rare chromosome disorders in the UK.
* [http://www.22q13.org/pdf/april06.pdf Emory University Genetic Challenge] Would you be able to diagnose a 22q13 deletion? Take the genetic challenge!
* [http://www.alliance22.org Alliance 22 (in French)]
* [http://monsite.orange.fr/le-monde-de-camille/ le-monde-de-camille (in French)]