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January-March 2015 | Vol. 8 | Issue 1 | Page No 3-5
Prader-Willi syndrome due to an unbalanced chromosomal rearrangement
Deepti Saxena and Shubha R Phadke
Department of Medical Genetics, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow
Address for Correspondence Email: shubharaophadke@gmail.com
Prader-Willi syndrome is a genomic imprinting disorder occurring at a frequency of 1/10,000 to 1/20,000. It is characterized by hypotonia, feeding difficulties often associated with failure to thrive during infancy and global developmental delay. Around 1 to 4 years of age, children develop hyperphagia leading to obesity. These individuals have short stature, facial dysmorphism that includes almond shaped eyes and upslanting palpebral fissures, small hands and feet, cognitive impairment and hypogonadism. Other features include behavioural and sleep problems and neuroendocrine abnormalities.1 It is caused due to the loss of paternally transmitted genes at the imprinted locus 15q11-q13. In 75% of the cases, the loss is due to deletion in the paternally derived chromosome 15q11-q13 region, 24% of the cases have maternal unipaternal disomy of chromosome 15, 1% of the cases are due to defects in the imprinting centre and <1% of the cases are due to chromosomal translocation. The diagnosis is suspected clinically and is confirmed by DNA methylation testing. Diagnosis can also be made by molecular cytogenetic methods such as Fluorescent in-situ hybridization (FISH), Multiplex ligation dependent probe amplification (MLPA) and chromosomal microarray in cases that have deletion in the 15q11-q13 region. It is important to elucidate the exact genetic mechanism to provide genetic counselling and recurrence risk in the family. In cases with de novo deletion or uniparental disomy, the recurrence risk is low (<1%), whereas it can be upto 50% in cases with an imprinting centre defect. However, when the deletion is the result of any chromosomal rearrangement, the risk of recurrence depends on the specific rearrangement and the empiric risk is around 15% in cases with an inherited chromosomal translocation.2
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