Transcript SMS

Smith-Magenis
Syndrome
Presented by: Sara Mickelson
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 nonallelic 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
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
Journal of Medical Genetics 36
Cloning the SMS region
• Common deletion region is ~4Mb as detected by
pulsed-field gel electrophoresis and FISH
• Somatic rodent:human 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
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
Transcription Map
Figure 1 Lucas et al EJHG 9 (2001)
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
Location of SMS-REPs
Figure 7 Bi et al. 2002
SMS-REPs cont.
• “hotspots” for NAHR within SMS-REPs
• 12kb region within the ~34kb KER gene
cluster
• Contains >300bps 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
Figure 2 Bi et al. 2002
Figure 5 Bi et al Am.J.Hum.Genet. 73(2003)
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
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
Candidate Genes
• Used 6 markers from chromosome 17p1117p12 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
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
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 >90% of people with
SMS have part of the critical region deleted
Lucas et al EJHG 9 (2001)
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
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
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
Figure 3 Walz et al. Molecular & Cell Biology 23 (2003)
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
Figure 5 Struthers et al. J Med Genet 39(2002)
References
Allanson, Judith, Greenberg, Frank, and Smith, Ann. “The face of Smith-Magenis syndrome: a subjective
and objective study.” Journal of Medical Genetics 36:394-397, 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 9:892-902, 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 71:1072-1081, 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 73:1302-1315, 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.