Genotype-phenotype relationship in 2 SMA III patients with novel mutations in the Tudor domain.
OBJECTIVE
We report the cases of 2 patients with late-onset spinal muscular atrophy (SMA) type III, who were hemizygous for SMN1 deletion and carriers of novel SMN1 intragenic missense mutations, and we investigate the genotype-phenotype relationship.


METHODS
Patients were tested for SMN1 deletions with standard methodology. Sequencing of all exons, exon-intron junctions, and flanking sequences of SMN1 by nested PCR was used to detect intragenic point mutations. SMN1 and SMN2 quantification was undertaken to investigate the genotype-phenotype relationship.


RESULTS
Two novel point mutations were identified in exon 3 of SMN1 (p.Tyr130Cys and p.Tyr130His) in the highly conserved Tudor domain of the Smn protein.


CONCLUSIONS
The genetic basis of SMA in the rare cases of compound heterozygous carriers of SMN1 deletions is complex. Small intragenic SMN1 mutations often lead to severe SMA phenotypes, especially if the point mutations lie in exon 3 that codes for the highly conserved Tudor domain of the Smn protein. Although both our patients were carriers of intragenic SMN1 mutations in the coding region of the Tudor domain, they presented with a mild SMA phenotype despite a low SMN2 copy number. We discuss the possible determinant role of these novel missense mutations in the phenotypic outcome and compensatory mechanisms that may account for the genotype-phenotype discrepancy.
Department of Neurology, Hôpital de la Salpêtrière, Groupement Hospitalier Universitaire Est Pitié-Salpêtrière, Paris, France. matthieu.fraidakis@psl.aphp.fr
M J
MJ
Fraidakis
S
S
Drunat
T
T
Maisonobe
B
B
Gerard
P F
PF
Pradat
V
V
Meininger
F
F
Salachas
D000328:Adult; D018628:Gene Dosage; D056726:Genetic Association Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D017354:Point Mutation; D017384:Sequence Deletion; D014897:Spinal Muscular Atrophies of Childhood / Q000235:genetics*; D055533:Survival of Motor Neuron 1 Protein / Q000235:genetics*
C526471:SMN1 protein, human; D055533:Survival of Motor Neuron 1 Protein