DIAGNOSIS IS BASED ON EVIDENCE
Diagnosis of this syndrome relies on showing the deletion inside of 17p11.2. Deletion means that a fragment of DNA is missing from the chromosome. The deletion can range from a small portion to a larger portion.
For the human genotype, the loss in genetic material is only visible with a minimum of 4 million pairs. Below this size, it is a micro deletion which can only be detected using special techniques, with hybridization and fluorescence. If a micro deletion occurs during cell division, it can produce genetic disease ; this genetic disease can be detected even without clinical evidence beginning with the standard genotype by examining the shorter length of chromosome 17. But in all cases, it must be confirmed with high resolution techniques.
Currently, the hybridization technique is the best way to detect the syndrome because this technique allows one to probe the specific area where the deletion has occurred. Using directly this technique must be asked for by the clinician.
Most of the time the deletion size is about 5MB but it does not concern the locus of the PMP22 (which is responsible for the motor neuropathy and sensory neuropathy.
Usually the deletion comes spontaneously (de novo) with no family history. However, if the deletion had been detected, it is recommended that the parents undergo genetic testing.
GENETIC RESEARCH CONDUCTED in France 2004
Genetic research was done in the CHRU Jeanne de Flandres in Lille with 30 patients with Smith Magenis syndrome. Dr. Andrieux and his team studied the size of the deletion in chromosome 17 and looked for a correlation between the genotype and phenotype using DNA analysis.
Results :
Smith-Magenis syndrome (SMS) includes psychomotor delays, mood disorders which include sleep disorders, and specific facial characteristics. The syndrome was classified in 1986 and associated with a deletion of 3.7 million per base pair) on chromosome 17p11.2 (the short part of one of the pair of chromosome 17). A critical region of 1 MB is identified as containing 20 expressed genes.
The loss of one of these genes, RAI1 (identified in 2003) is seemingly responsible for the behavioral disorders, the facial characteristics and the auditory problems found in people with SMS. Using a DNA chip covering the 22MB of the short part of the chromosome 17, we could measure the deletions seen on 30 SMS patients with an average resolution of 300 kb (300,000 base pairs). Three of these patients demonstrated a larger deletion, and the data in the literature estimating at around10% of the SMS patients have a larger deletion. The correlation between genotype and phenotype (between the clinical data and biological data) showed that 2 out of these 3 patients had cleft palates, whereas none of the patients with the most standard deletions (90% of the cases) showed this characteristic. Two of the first patients studied by Smith and Magenis had a cleft palate and one of the two also had a larger deletion than the standard deletion.
Today, the correlation study linking genotype to phenotype on patients with deletion smaller than 1.5 of larger than 9Mb did not support the previous conclusion. The frequency of cleft palates in SMS syndrome can be estimated around 10% based on data in the literature. In the 2 patients with the cleft palates, the commonality was 600kb to 1.6Mb which corresponds to 8-17 genes at the junction of 17p11.2-17p12. A study among families with Van der Woude syndrome (cleft palate) suggests that a gene on 17p11.2-11.1 can be responsible with gene VWS on 1q32-41 for the evidence of cleft palate.
UBB plays a role in modifying the cellular proteins by covalently bonding of ubiquitin and is implicated in the cell cycle pathway and the transduction signal. It is strongly expressed during the embryogenesis and the loss if one copy of the gene seems to be the principle cause of the cleft pallet.
Dr Joris Andrieux
Laboratoire de Génétique Médicale
Hôpital Jeanne de Flandres
CHRU de Lille
References :
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