OBJECTIVETo study the genetic basis in the patients with clinical diagnosis of spinal muscular atrophy(SMA) but without survival motor neuron telomeric copy (SMN-T) deletion; the relationship between the SMN-C (centromeric) copies and the phenotype; and the distribution of SMN-C and SMN-T copies in the SMA patients, the carriers and the controls.

METHODSQuantitative PCR analysis of SMN-T and SMN-C copies were carried out in 45 patients, 25 consanguineous and 33 control individuals. The patients were identified by clinical manifestation and muscular pathology. Two internal standards of SMN-T and cystic fibrosis transmembrane conductance regulator (CFTR) were constructed. Nonradioactive and nonfluorescence-labelling competitive PCR were used. The numbers of SMN-T and SMN-C copies were determined by calculating the ratios of SMN-T/CFTR and SMN-C/CFTR.

RESULTSQuantitation of SMN-T gene copies in SMA patients revealed that nine cases of type I-III were homozygously deleted. Two cases of type III had only one copy and four cases of type III had two copies. SMA IV and other type cases had two copies. Nine cases of consanguineous individuals had one copy, but other 16 had two copies. All of the normal individuals had two copies. Analysis of SMN-C copies showed that SMA I had < or = 2 copies, II-III had < or = 3 copies, SMA IV and others had 0-3 copies, the consanguineous individuals and normal individuals had 0-3 copies.

CONCLUSIONThe number of copies determined by PCR quantitative assay of SMN-T is in accordance with the result of PCR qualitative assay of homozygous deletion. Quantitative assay of the number of copies can find out the cases and the carriers of heterozygous deletion. The SMA phenotype is related to the number of copies of SMN-C; the smaller the number of copies the patient has, the severer the patient's phenotype will be. The pathogenesis of SMA IV and other types of SMA may not relate to SMN gene.