Smith-Magenis Syndrome

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Smith-Magenis Syndrome

• Presented by Sara Mickelson

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What is SMS?

• Syndrome was first described in 1982 by Ann
  Smith(genetic counselor) and Dr. Ruth Magenis
• Occurs in 1 in 25,000 people
• Arises from a spontaneous heterozygous deletion
  of part of chromosome 17p11.2 through non-allelic
  homologous recombination
• No correlation between the size of the deletion
  and the severity of the phenotype
• People with chromosome 17p11.2 duplicated have a
  mild phenotype

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Phenotypes

• Wide range of phenotypes
• Mental retardation (IQ 20-78)
• Behavioral abnormalities
• Aggressive and Self-inflicted injuries
• Self-hugging, polyembolokoilamania
• Sleep disturbances
• Melatonin imbalance resembles jet lag
• Delayed speech and motor development
• Distinct physical characteristics

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Journal of Medical Genetics 36
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Cloning the SMS region

• Common deletion region is 4Mb as detected by
  pulsed-field gel electrophoresis and FISH
• Somatic rodenthuman hybrid cell lines with the
  chromosome 17 deletion determined that the
  critical region is 1.1Mb
• 16 BACs and 2 PACs were used to assemble the
  contig transcription map
• Known genes and ESTs were hybridized to
  EcoRI-digested BACs to map the initial contig

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Cloning cont.

• Gene order was determined by human genome project
  sequence analysis and Southern hybridization for
  the presence or absence of a certain gene or EST
• Sequence data analysis also identified three low
  copy repeat sequences
• Somatic cell hybrid analysis revealed that the
  SMS breakpoints occurred within the distal and
  proximal SMS-REPs

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Transcription Map
Figure 1 Lucas et al EJHG 9 (2001)
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Repeated Sequences

• Highly homologous (98) and chromosome 17
  contains several Low Copy Repeats which act as
  substrates for NAHR
• Three low copy repeat sequences proximal
  (256kb), middle(241kb), and distal(176kb)
• Middle SMS-REP is an inverted copy
• Each SMS-REP contains roughly 14 genes
• Critical deletion region occurs between the
  proximal and middle SMS-REPs
• Divergence from a progenitor 40-65mya

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Location of SMS-REPs
Figure 7 Bi et al. 2002
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SMS-REPs cont.

• hotspots for NAHR within SMS-REPs
• 12kb region within the 34kb KER gene cluster
• Contains gt300bps with perfect identity along with
  polymorphic nucleotides
• 2.1kb AT rich inverted repeats flank proximal
  middle but not the distal
• Hairpin formation initiates NAHR event

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Figure 2 Bi et al. 2002
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Figure 5 Bi et al Am.J.Hum.Genet. 73(2003)
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Cloning cont.

• Sequence analysis also determined that the
  deletion region contains 25 genes and 14 ESTs
• Critical region of chromosome 17 contains a high
  average of gene composition compared to the whole
  human genome

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Identifying Candidate Genes

• Looked at genes that are developmentally
  regulated (mental behavioral) and expressed in
  the neural crest (craniofacial heart
  development)
• Dosage sensitive
• Transcription factors?
• Effect development

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Candidate Genes

• Used 6 markers from chromosome 17p11-17p12
  regions
• Plasmid clones of 14 ESTs in critical region were
  sequenced and obtained commercially
• Sequence analysis and tissue expression (Northern
  blot) was used to identify 6 possible candidate
  genes within the SMS critical region

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What are the Candidate Genes?

• FLII actin binding severing in fly cell
  adhesion and protein-protein interaction
• LLGLI associated with cytoskeleton and serine
  kinase
• DRG2 GTP binding protein
• RASD1 ras related protein in GTPases
• NT5M dephosphorylation of T U as a
  mitochondrial deoxyribonucleotidase
• Their roles in SMS are still unknown

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The Candidate Gene RAI1

• Retinoic-acid induced 1 gene is expressed in all
  adult tissues
• Homologous to mouse Rai1 gene that influences
  neuronal differentiation
• Haploinsufficiency accounts for facial,
  otolaryngological, neurological, and behavioral
  abnormalities
• Heart renal defects due to other genes within
  chromosome 17p11.2 since gt90 of people with SMS
  have part of the critical region deleted

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Lucas et al EJHG 9 (2001)
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RAI1 cont.

• 8kb transcript that is caused by alternative
  splicing to produce an 1863 AA protein
• Contains CAG repeats nuclear localization
  signals
• Sequence similar to transcriptional coactivator
  TCF20
• RAI1 may interact with other DNA-binding proteins
  to exert effects on transcription

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Mutation causes SMS?

• Mutated RAI1 in three individuals with SMS, but
  no deletion of critical region
• Deletion in exon 3 of RAI1 on one allele
• Causes the protein to be truncated due to
  dominant frameshift mutation
• None of the parents carried any of these mutations

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Mouse knockout

• Human chromosome 17p11.2 is syntenic to the 32-34
  cM region of mouse chromosome 11 and the genetic
  order is highly conserved
• Heterozygous knockout of the syntenic deletion
  region in mouse using the Cre-loxP site-specific
  system
• Several SMS phenotypes were observed in mice
• Craniofacial abnormalities, seizures and abnormal
  EEGs, weight differences, and reduced male
  fertility

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Figure 3 Walz et al. Molecular Cell Biology 23
(2003)
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Screening for SMS

• Screening for SMS among patients with mental
  retardation of unknown causes
• SMS is often under diagnosed because of subtle
  and variable expression
• Initially used Southern blotting and dosage
  comparison between markers, SMS deletion specific
  and chromosome X control probes
• Confirmatory testing used FISH and/or PCR
  microsatellitle genotyping
• 1 in 569 were detected to have SMS

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Figure 5 Struthers et al. J Med Genet 39(2002)
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References

• Allanson, Judith, Greenberg, Frank, and Smith,
  Ann. The face of Smith-Magenis syndrome a
  subjective and objective study. Journal of
  Medical Genetics 36394-397, 1999.
• Lucas, R., Vlangos, C., Das, P., Patel, P., and
  Elsea, S. Genomic organization of the 1.5 Mb
  Smith-Magenis syndrome critical interval
  transcription map, genomic contig, and candidate
  gene analysis. European Journal of Human
  Genetics 9892-902, 2001.
• McBride, Gail. Melatonin disrupts sleep in
  Smith-Magenis syndrome. Lancet 354, 1999.
• Park, S., et al. Structure and Evolution of the
  Smtith-Magenis Syndrome repeat Gene clusters,
  SMS-REPs. Genome Research 12(5)729-738, 2002.
• Shaw, C., Weimin, B., and Lupski, J. Genetic
  proof of unequal meiotic crossovers in reciprocal
  deletion and duplication of 17p11.2. American
  Journal of Human Genetics 711072-1081, 2002.
• Slager, Rebecca E., Newton, Tiffany L., Vlangos,
  Christopher N., Finucane, Brenda, and Elsea,
  Sarah H. Mutations in RAI1 associated with
  Smith-Magenis syndrome. Nature Genetics 33(4)
  466-468, 2003. 
• Struthers, J. L., Carson, N., McGill, M.,
  Khalifa, M. M. Molecular screening for
  Smith-Magenis syndrome among patients with mental
  retardation of unknown cause. Journal of Medical
  Genetics 39(59).
• Walz, K., et al. Modeling del(17)(p11.2p11.2)
  and dup (17)(p11.2p11.2) contiguous gene
  syndromes by chromosome engineering in mice
  Pheontypic consequences of gene dosage
  imbalance. Molecular and Cell Biology
  23(10)3646-3655, 2003.
• Weimin, B., Park, S., Shaw, C., Withers, M.,
  Patel, P., and Lupski, J. Reciprocal Crossovers
  and a Positional Preference for Strand Exchange
  in recombination events resulting in deletion or
  duplication of chromosome 17p11.2. American
  Journal of Human Genetics 731302-1315, 2003. 
• Weimin B., Yan, J., Stankiewicz, P., et al.
  Genes in a Refined Smith-Magenis Syndrome
  Critial Deletion Interval of Chromosome 17p11.2
  and the syntenic region of the mouse. Genome
  Research 12(5)71-28, 2002.