Prospects for the gene therapy of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by a deficiency of functional SMN protein because of mutations in SMN1. A decrease in SMN activity results in motor neuron cell loss in the spinal cord, leading to a weakness of the proximal muscles responsible for crawling, walking, head/neck control and swallowing as well as the involuntary muscles that control breathing and coughing. Thus, patients present with pulmonary manifestations, paralysis and a shortened lifespan. Gene therapy is emerging as a promising therapeutic strategy for SMA given that the molecular basis for this monogenic disorder is well established. Recent advances and findings from preclinical studies in animal models provide optimism that gene therapy might be an effective therapeutic strategy for treating SMA.     

Modeling spinal muscular atrophy in Drosophila

Spinal Muscular Atrophy (SMA), a recessive hereditary neurodegenerative disease in humans, has been linked to mutations in the survival motor neuron (SMN) gene. SMA patients display early onset lethality coupled with motor neuron loss and skeletal muscle atrophy. We used Drosophila, which encodes a single SMN ortholog, survival motor neuron (Smn), to model SMA, since reduction of Smn function leads to defects that mimic the SMA pathology in humans. Here we show that a normal neuromuscular junction (NMJ) structure depends on SMN expression and that SMN concentrates in the post-synaptic NMJ regions. We conducted a screen for genetic modifiers of an Smn phenotype using the Exelixis collection of transposon-induced mutations, which affects approximately 50% of the Drosophila genome. This screen resulted in the recovery of 27 modifiers, thereby expanding the genetic circuitry of Smn to include several genes not previously known to be associated with this locus. Among the identified modifiers was wishful thinking (wit), a type II BMP receptor, which was shown to alter the Smn NMJ phenotype. Further characterization of two additional members of the BMP signaling pathway, Mothers against dpp (Mad) and Daughters against dpp (Dad), also modify the Smn NMJ phenotype. The NMJ defects caused by loss of Smn function can be ameliorated by increasing BMP signals, suggesting that increased BMP activity in SMA patients may help to alleviate symptoms of the disease. These results confirm that our genetic approach is likely to identify bona fide modulators of SMN activity, especially regarding its role at the neuromuscular junction, and as a consequence, may identify putative SMA therapeutic targets.     

Linkage mapping of the spinal muscular atrophy gene

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder resulting in loss of motor neurons. We have performed linkage analysis on a panel of families using nine markers that are closely linked to the SMA gene. The highest lod score was obtained with the marker D5S351 (Z_max = 10.04 at = 0 excluding two unlinked families, and Z_max = 8.77 at = 0.007 with all families). One type III family did not show linkage to the 5q13 markers, and in one type I consanguineous family the affected individual did not show homozygosity except for the marker D5S435. Three recombinants were identified with the closest centromeric marker, D5S435, which position the gene telomeric of this marker. These recombinants will facilitate finer mapping of the location of the SMA gene. Lastly, two families provide strong evidence for a remarkable variability in presentation of the SMA phenotype, with the age at onset in one family varying from 17 months to 13 years.     

Linkage analysis of spinal muscular atrophy.

Linkage data between four markers on chromosome 5 confirm and extend our previous studies that localized the mutation in spinal muscular atrophy to 5q11.2-q13.3. Localization of D5S6 by in situ hybridization refines the mapping of the defective gene to the region 5q12.2-q13. We also report the use of a highly informative PCR-based polymorphism with five alleles. This RFLP will be particularly useful for prenatal diagnosis where only old tissue samples from affected individuals are available. The high heterozygosity of this locus should also assist in identifying recombinants that will refine the genetic mapping of the mutation.     

The molecular bases of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common recessive autosomal disorder characterized by degeneration of motor neurons of the spinal cord. SMA is caused by mutations of the survival of motor neuron gene that encodes a multifunctional protein, and mouse models have been generated. These advances represent starting points towards an understanding of the pathophysiology of this disease and the design of therapeutic strategies in SMA.     

Genetic carrier screening for spinal muscular atrophy and spinal muscular atrophy with respiratory distress 1 in an isolated population in Israel

Spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by progressive muscle weakness. It is caused by a mutation in the survival motor neuron gene 1 (SMN1) gene. SMA with respiratory distress 1 (SMARD1), an uncommon variant of infantile SMA also inherited in an autosomal recessive manner, is caused by mutations in the immunoglobulin mu-binding protein 2 (IGHMBP2) gene. We carried out genetic carrier screening among the residents of an isolated Israeli Arab village with a high frequency of SMA in order to identify carriers of SMA type I and SMARD1. During 2006, 168 women were tested for SMA, of whom 13.1% were found to be carriers. Of 111 women tested for SMARD1, 9.9% were found to be carriers. Prenatal diagnosis was performed in one couple where both spouses were carriers of SMARD1; the fetus was found to be affected, and the pregnancy was terminated. To the best of our knowledge, this is the first example of the establishment of a large-scale carrier-screening program for SMA and SMARD1 in an isolated population. SMA has a carrier frequency of 1:33-1:60 in most populations and should be considered for inclusion in a population-based genetic-screening program.     

Synaptic defects in spinal muscular atrophy animal models

Proximal spinal muscular atrophy, the most frequent genetic cause of childhood lethality, is caused by homozygous loss or mutation of the SMN1 gene on human chromosome 5, which codes for the survival motor neuron (SMN) protein. SMN plays a role in the assembly of small nuclear ribonucleoproteins and, additionally, in synaptic function. SMN deficiency produces defects in motor neuron β-actin mRNA axonal transport, neurofilament dynamics, neurotransmitter release, and synapse maturation. The underlying molecular mechanisms and, in particular, the role of the cytoskeleton on the pathogenesis of this disease are starting to be revealed.     

Prenatal diagnosis of spinal muscular atrophy in Macedonian families

Spinal muscular atrophy (SMA) is the second most common lethal autosomal recessive disorder of childhood, affecting approximately 1 in 6,000-10,000 births, with a carrier frequency of 1 in 40-60. There is no effective cure or treatment for this disease. Thus, the availability of prenatal testing is important. The aim of this study was to establish an efficient and rapid method for prenatal diagnosis of SMA and genetic counseling in families with risk for having a child with SMA. In this paper we present the results from prenatal diagnosis in Macedonian SMA families using direct analysis of fetal DNA. The probands of these families were previously found to be homozygous for a deletion of exons 7 and 8 of SMN1 gene. DNA obtained from chorionic villas samples and amniocytes was analyzed for deletions in SMN gene. SMN exon 7 and 8 deletion analysis was performed by polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP). Of the 12 prenatal diagnoses, DNA analysis showed normal results in eight fetuses. Four of the fetuses were homozygote for a deletion of exons 7 and 8 of SMN1. After genetic counseling, the parents of the eight normal fetuses decided to continue the pregnancy, while in the four families with affected fetuses, the pregnancy was terminated. The results were confirmed after birth.