Dinucleotide repeat polymorphism proximal to the spinal muscular atrophy region at D5S681

The molecular basis of spinal muscular atrophy (SMA) in South African black patients

SMA is an autosomal recessive disorder that results in symmetrical muscle weakness and wasting due to degeneration of the anterior horns of the spinal cord. The gene for SMA, the survival motor neuron (SMN) gene is found on chromosome 5q13, in a region harbouring a 500kb duplication, resulting in two copies (a telomeric and a centromeric) of each of the genes found within the duplication. SMN1 is homozygously deleted in approximately 95% of patients worldwide. Results of the current study show that only 51% (42/92) of South African black SMA patients have homozygous deletions of the SMN1 gene. This frequency is significantly lower than observed in the South African white patient group and in other international populations. The pattern of deletions in the South African black patients is also significantly different. In order to elucidate the molecular basis of SMA in the black population, a dosage assay enabling the detection of SMN1 deletion heterozygotes was independently developed. This assay confirmed SMN1 heterozygosity in at least 70% of black non-deletion SMA patients. However, no second disease-causing mutation or a common chromosomal background for this mutation could be identified in these patients. The frequency of SMA in both the black and white population was also determined using the SMN1 gene dosage assay. Results showed that SMA is more common than previously thought with carrier rates of 1 in 50 and 1 in 23 and a predicted birth incidence of 1 in 3574 and 1 in 1945 in the black population and the white population, respectively. Development and incorporation of the SMN1 dosage assay into the molecular diagnostic service will increase the percentage of cases in which the diagnosis of SMA can be confirmed and allow preclinical and prenatal diagnosis. Further gene characterisation and functional studies would need to be performed in order to further define the molecular basis of SMA in the South African black population.

Prenatal prediction in families with autosomal recessive proximal spinal muscular atrophy (5q11.2-q13.3): molecular genetics and clinical experience in 109 cases

Prenatal prediction in families at risk for autosomal recessive proximal spinal muscular atrophy (SMA) mainly of type I is often requested due to the high incidence and the fetal outcome of the disease. So far, only indirect genotype analysis can be performed in SMA families, since the gene has not yet been identified. We present our experience of 109 prenatal diagnoses obtained in 91 families by use of single- and multi-locus polymorphic microsatellites of the region 5q11.2-q13.3. The marker combinations and specific features of the closest microsatellites are described in detail. From 137 requests for prenatal prediction of SMA between October 1991 and August 1994, 28 families were excluded, mostly because the clinical diagnosis was uncertain or doubtful. Others had to be classified as 'SMA-variants' or showed autosomal dominant transmission of SMA. Of the 109 prenatal diagnoses performed, 29 fetuses were diagnosed to be at high risk (> 99 per cent) of developing the disease, while in seven additional pregnancies no exact prediction could be made due to a recombination event in one parental haplotype. Altogether, recombinations between closely flanking markers were observed in 14 cases. In 35 cases, the parents decided to terminate the pregnancy. Of the remaining pregnancies, 32 could be followed beyond term. All infants were reported to develop normally without signs of SMA. Two children were born with transverse reduction defects of one hand, which was most likely related to early chorionic villus sampling at 9 and 10 weeks' gestation. No further abnormalities could be detected. The limits of indirect genotype analysis and the problems of diagnostic accuracy and heterogeneity of proximal SMA are discussed.