Motor neuron degeneration in a 20-week male fetus: spinal muscular atrophy type 0

BACKGROUND

Neuropathological changes in degenerating motor neurons are well documented in the term neonate with spinal muscular atrophy, but not at midgestation.

METHODS

Postmortem neuropathological examination was performed in a 20-week male fetus with a hypoplastic left cardiac anomaly.

RESULTS

Selective degeneration of spinal and hypoglossal motor neurons was an incidental finding. Degenerating motor neurons were not immunoreactive with neuronal nuclear antigen (NeuN) or neuron-specific enolase (NSE), as were the normal motor neurons. Synaptophysin reactivity was reduced around the soma of degenerating normal motor neurons. Ubiquitin and tau were expressed in degenerating motor neurons. Gliosis, inflammation and microglial activation were lacking in the ventral horns of the spinal cord. Laryngeal striated muscle was unaltered for age. No cerebral malformations or hypoxic-ischaemic changes were found.

CONCLUSION

This case represents an early motor neuronal degeneration and corresponds to the recently described "type 0" spinal muscular atrophy. Lack of contractures is attributed to the early fetal age, since most muscular growth occurs in the second half of gestation.

Neuronal death is enhanced and begins during foetal development in type I spinal muscular atrophy spinal cord

Spinal muscular atrophy (SMA) is an autosomal recessive disorder caused by mutations in the survival motor neurone gene (SMN). The degeneration and loss of the anterior horn cells is the major neuropathological finding in SMA, but the mechanism and timing of this abnormal motor neurone death remain unknown. A quantitative study was carried out comparing neuronal death in controls and SMA foetuses and neonates. Between 12 and 15 weeks of gestational age, a significant increase in nuclear DNA vulnerability, as revealed with the method of in situ end-labelling of nuclear DNA fragmentation, was detected in SMA foetuses and was reflected by a decrease in the number of neurones of the anterior horn. Neurones with nuclear DNA vulnerability are no longer detected at the end of the foetal period and the post-natal period. On the other hand, abnormal morphology of motor neurones, mainly early chromatolytic changes, was observed only after birth. Our findings indicate that in type I SMA, the absence or dysfunction of SMN is reflected by an enhanced neuronal death that is already detectable at 12 weeks, the earliest SMA foetal stage analysed. This is associated with a progressive loss of motor neurones towards the neonatal period. Given that a proportion of the remaining SMA motor neurones in the neonatal period appear with pathological findings not detected at earlier stages, it can be hypothesized that type I SMA results in differential age-dependent responses leading to cell death and motor neurone degeneration during development.

Analysis of the mRNA transcripts of the survival motor neuron (SMN) gene in the tissue of an SMA fetus and the peripheral blood mononuclear cells of normals, carriers and SMA patients

Spinal muscular atrophy (SMA) is a disorder characterized by degeneration of the anterior horn cells of the spinal cord. The gene most highly associated with SMA is the survival motor neuron (SMN) gene. In this study, we present an analysis of messenger RNA (mRNA) expression of the SMN gene in peripheral blood mononuclear cells in normal subjects, SMA carriers and patients from 20 SMA families. We found at least 6-8 different transcripts of SMN gene formed by alternative splicing involving exons 3, 5 and 7. We compared transcripts from the different types of SMA and found no definite differences in transcript patterns and amounts. Normal subjects with the telomeric SMN (SMN(T)) gene only had variable splicing resulting in several transcripts, the most dominant being a transcript containing all coding regions. However, SMA patients with the centromeric SMN (SMN(C)) gene only had a higher degree of splice variation and tended to show little or no exon 7. These results demonstrate that SMN(T) and SMN(C) genes participate in alternative splicing phenomena. The different splicing patterns support the view that the SMN(T) gene is responsible for SMA disease. We also analyzed the transcripts from several tissues of an SMA fetus who had a homozygous SMN(T) gene deletion. Different splicing patterns were also found in these tissues, and were similar to the splicing pattern of leukocytes. We compared the major transcripts from exons 4 to 8 of both the SMN(T) and SMN(C) genes and found that the relative proportion varied among normal subjects, SMA carriers and patients. This approach could be used as a novel diagnostic method. We suggest that analyzing the mRNA expression of the SMN gene in peripheral blood mononuclear cells offers an apparently reliable technique for separating SMA patients, carriers, and normal individuals.