Connexion

GENATLAS PHENOTYPE

last update : 11/07/2008

Symbol	SMA3
Location	5q13.2
Name	spinal muscular atrophy, type 3
Other name(s)	Kugelberg-Welander syndrome
Corresponding gene	SMN1 , SMN2 , NAIP
Other symbol(s)	KWS
Main clinical features	onset > 18 months of age, walks independently ~25 steps, life expectancy indefinite proximal muscular atrophy, with onset between 2 and 16 years, with atrophy and weakness of proximal limb muscles, primarily in the legs, followed by distal involvement, twitchings (fasciculations) are an important differentiating sign with limb girdle muscular dystrophy ; muscular biopsy and electromyography show the true nature of the process as a lower motor neuron disease
Genetic determination	autosomal recessive
Function/system disorder	neuromuscular
Type	disease

Remark(s)

SMN1 is considered to be the primary disease causing gene ; SMN2 cannot fully compensate lack of SMN1 expression because most normal SMN2 transcripts lack exon 7; the SMN2 copy number influences disease severity

Genotype/Phenotype correlations

phenotypes in SMN1 mutations span a continuum without clear delineation of subtypes, which are classified by age of onset and maximum function achieved ; homozygous absence of SMN1 associated in 62p100 of SMA3 patients (age of onset less than 3 years) with 2 or 3 SMN2 copies, whereas 65p100 of SMA3 patients (age of onset greater than 3 years ) with 4 to 5 SMN2 copies