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Molecular screening for Smith-Magenis syndrome among patients with mental retardation of unknown cause
  1. J L Struthers1,
  2. N Carson2,
  3. M McGill2,
  4. M M Khalifa1
  1. 1Division of Medical Genetics, Department of Pediatrics and Pathology, Queen’s University and Kingston General Hospital, Kingston, Ontario, Canada
  2. 2Molecular Genetics Diagnostic Laboratory, Children’s Hospital of Eastern Ontario, Canada.
  1. Correspondence to:
 Dr M M Khalifa, Medical Genetics, Queen’s University, 20 Barrie Street, Kingston, ON, Canada K7L 3J6;

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Smith-Magenis syndrome is a rare, multiple congenital anomaly/mental retardation syndrome (MCA/MR) associated with interstitial deletion of chromosome 17p11.2. Smith et al1 first described this condition in two patients. To date, more than 150 cases have been described. Patients with SMS display a variable expression of subtle dysmorphic features, MR, short stature, brachydactyly, visual and auditory impairment, behavioural problems, sleep disturbance, and cardiac and renal malformations.2,3 Almost all cases are de novo, non-mosaic, and of either maternal or paternal origin.4,5 Although the abnormalities associated with SMS are well described, their subtlety and variable expression make clinical diagnosis often difficult, particularly in neonates and young infants.6,7

The SMS deleted region ranges in size from <1.5 Mb to 9 Mb and the majority of patients have a ∼5 Mb deletion (∼10-11% of chromosome 17).4,8 In general, there is no obvious correlation between the size of the deletion and the severity of the phenotype.8 In the majority of cases, this deletion is visible on careful routine cytogenetic analysis. Despite this, in several cases the deletion has been missed.7,9 Using a FISH probe specific for SMS has enhanced detection of the syndrome, especially in equivocal cases.10–12 Elsea et al13 reported a patient with a typical SMS phenotype and a normal karyotype at 650 band resolution. This patient was subsequently diagnosed with SMS by FISH.12

Current estimates of the incidence of SMS, based on ascertaining cases in genetics centres, are between 1 in 25 0004 and 1 in 50 00014 births. Because the diagnosis of SMS can be missed both clinically and cytogenetically, it is generally believed that this syndrome might be underdiagnosed. In this study, we screened a large population of patients with MR/DD of unknown cause …

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