Two novel microsatellite markers for prenatal prediction of spinal muscular atrophy (SMA)

Best practice guidelines for molecular analysis in spinal muscular atrophy

With a prevalence of approximately 1/10 000, and a carrier frequency of 1/40-1/60 the proximal spinal muscular atrophies (SMAs) are among the most frequent autosomal recessive hereditary disorders. Patients can be classified clinically into four groups: acute, intermediate, mild, and adult (SMA types I, II, III, and IV, respectively). The complexity and instability of the genomic region at chromosome 5q13 harbouring the disease-causing survival motor neuron 1 (SMN1) gene hamper molecular diagnosis in SMA. In addition, affected individuals with SMA-like phenotypes not caused by SMN1, and asymptomatic individuals with two mutant alleles exist. The SMN gene is present in at least one telomeric (SMN1) and one centromeric copy (SMN2) per chromosome in normal (non-carrier) individuals, although chromosomes containing more copies of SMN1 and/or SMN2 exist. Moreover, the two SMN genes (SMN1 and SMN2) are highly homologous and contain only five base-pair differences within their 3' ends. Also, a relatively high de novo frequency is present in SMA. Guidelines for molecular analysis in diagnostic applications, carrier detection, and prenatal analysis using direct and indirect approaches are described. Overviews of materials used in the molecular diagnosis as well as Internet resources are included.

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.

Exclusion of the gene locus for spinal muscular atrophy on chromosome 5q in a family with infantile olivopontocerebellar atrophy (OPCA) and anterior horn cell degeneration

Two sisters with infantile OPCA plus spinal muscular atrophy (SMA) are reported. Both showed severe hypotonia and psychomotor delay from birth, and in addition, nystagmoid eye movements and vision impairment were evident. Cerebellar hypoplasia with cystic dilatation was seen by neuro-imaging methods. Pathoanatomically, a marked cerebellar hypoplasia and neuronal loss in the basal ganglia, brainstem and anterior horns were found in the deceased girl. Linkage studies with polymorphic markers of the region 5q11.2-q13.3 flanking the gene locus for infantile SMA showed identical parental haplotypes in the patients and their older healthy sister. It can be concluded that the gene locus for infantile SMA on chromosome 5q is not responsible for infantile OPCA plus anterior horn cell degeneration in the described family which might apply to this disorder in general.

Spinal muscular atrophy in trizygotic triplets

The clinical, electrophysiological, pathological and genetic findings in trizygotic triplets with spinal muscular atrophy (SMA) are reported. The first child was clinically affected shortly after birth and the third one first showed symptoms at 1 month of age. Electromyography and a muscle biopsy provided evidence of lower motor neuron disease. The second child remains clinically normal, but electromyography showed fibrillation potentials and regular spontaneous motor unit activity at rest. Genetic linkage analysis revealed that the two siblings with typical type 1 SMA had the same chromosome 5q haplotype, and that the second child had a different haplotype. It is considered that in this family there is a link to SMA 5q and there is little possibility that the second child is affected. These data emphasize the need to adhere to strict clinical criteria for the diagnosis of chromosome 5q SMA.

Prenatal diagnosis of Werdnig-Hoffmann disease: DNA analysis of a mummified umbilical cord using closely linked microsatellite markers

We present a case of prenatal diagnosis of Werdnig-Hoffmann disease, the most severe type of spinal muscular atrophy (SMA). DNA obtained from a mummified umbilical cord of a decreased affected brother of the index case was analysed with four closely linked microsatellite markers [EF1/2a and EF13/14 (D5S125), MAP1B, and JK53CA (D5S112)], flanking the SMA gene, on chromosome 5q11.2-13.3. The fetus was diagnosed as homozygous for the deleterious SMA gene.