Deletions Causing Spinal Muscular Atrophy Do Not Predispose to Amyotrophic Lateral Sclerosis
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ORIGINAL CONTRIBUTION Deletions Causing Spinal Muscular Atrophy Do Not Predispose to Amyotrophic Lateral Sclerosis Jillian S. Parboosingh, MSc; Vincent Meininger, MD; Diane McKenna-Yasek; Robert H. Brown, Jr, MD; Guy A. Rouleau, MD Background: Amyotrophic lateral sclerosis (ALS) is a ing neurodegeneration in spinal muscular atrophy are rapidly progressive, invariably lethal disease resulting present in patients with ALS in whom the copper/zinc from the premature death of motor neurons of the superoxide dismutase gene is not mutated. motor cortex, brainstem, and spinal cord. In approxi- mately 15% of familial ALS cases, the copper/zinc super- Patients and Methods: Patients in whom ALS oxide dismutase gene is mutated; a juvenile form of was diagnosed were screened for mutations in the familial ALS has been linked to chromosome 2. No SMN and NAIP genes by single strand conformation cause has been identified in the remaining familial ALS analysis. cases or in sporadic cases and the selective neurodegen- erative mechanism remains unknown. Deletions in 2 Results: We found 1 patient with an exon 7 deletion in genes on chromosome 5q, SMN (survival motor neuron the SMN gene; review of clinical status confirmed the mo- gene) and NAIP (neuronal apoptosis inhibitory protein lecular diagnosis of spinal muscular atrophy. No muta- gene), have been identified in spinal muscular atrophy, a tions were found in the remaining patients. disease also characterized by the loss of motor neurons. These genes are implicated in the regulation of apopto- Conclusion: The SMN and NAIP gene mutations are spe- sis, a mechanism that may explain the cell loss found in cific for spinal muscular atrophy and do not predispose the brains and spinal cords of patients with ALS. individuals to ALS. Objective: To determine whether the mutations caus- Arch Neurol. 1999;56:710-712 OTOR NEURON loss is progression of muscular weakness to death characteristic of sev- typically 2 to 5 years after disease onset eral neurodegenera- in the fourth or fifth decade of life. tive diseases, includ- Spinal muscular atrophy was mapped ing spinal muscular to chromosome 5q11.2-13.3,4,5 and par- Matrophy (SMA) and amyotrophic lateral tial deletions of genes in this region are re- sclerosis (ALS). Spinal muscular atrophy ported. In 67% of type 1 and 42% of types is a relatively common childhood disor- II and III SMA chromosomes, deletions der resulting from the loss of spinal mo- were identified in the neuronal apoptosis tor neurons. Childhood SMA is subdi- inhibitory protein (NAIP) gene,6 while ex- vided into 3 groups according to clinical ons 7 and 8 of the telomeric survival mo- From the Centre for Research in severity and age at onset; all forms are in- tor neuron (SMN) gene are mutated in 1 Neuroscience, McGill herited recessively. An adult form of SMA 98.6% of patients with autosomal reces- 7 University and the Montreal exists with an age of onset of 17 to 55 years sive childhood-onset SMA and in some General Hospital Research and either recessive or dominant modes patients with adult-onset SMA.8,9 SMN is Institute, Montreal, Quebec of inheritance.2,3 Amyotrophic lateral scle- proposed to be the disease-determining (Ms Parboosingh and rosis is characterized pathologically by the gene. Clinical severity and age at onset Dr Rouleau); Centre S. L. A., premature death of cortical, spinal, and appear to be correlated with SMN gene Hôtel Dieu de Paris, Paris, brainstem motor neurons. In approxi- expression.10 SMN protein is detectable in France (Dr Meininger); and mately 10% of ALS cases, an autosomal patients with deletions in the telomeric Day Neuromuscular Laboratory, Massachusetts dominant mode of inheritance is ob- SMN gene, indicating the presence of General Hospital, Harvard served; familial ALS (FALS) cases cannot protein encoded by the highly homolo- c Medical School, Boston be distinguished clinically or pathologi- gous centromeric copy ( BCD541). The (Ms McKenna-Yasek and cally from sporadic ALS (SALS) cases. loss of the NAIP gene and other as yet Dr Brown). Clinically, the neuronal loss results in rapid unknown genes in the duplicated SMA ARCH NEUROL / VOL 56, JUNE 1999 710 ©1999 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/23/2021 Summary of Single Strand Conformation Analysis PATIENTS AND METHODS of SMN Gene and cBCD541 Exons 7 and 8* c PATIENT DATA Exon SMN BCD541 No. of Cases 7 − + 1/262 Two hundred sixty-two unrelated patients with ALS + + 221/262 were analyzed for the SMA deletion. Sixty-eight of +/p + 22/262 the patients had a family history of ALS. The diag- + − 18/262 nosis of ALS was established by at least one neurolo- 8 + + 84/93 gist subspecialized in the assessment and treatment +/SSCP variants + 3/93 of ALS based on the El Escorial World Federation of + − 6/93 Neurology criteria.14 All cases studied were negative for mutations in the SOD1 gene. *Plus sign indicates present; minus sign, absent; p, C/T intron 7 polymorphism; SMN, survival motor neuron; and cBCD541, protein encoded by the highly homologous centromeric copy. MUTATION ANALYSIS DNA was extracted from whole blood specimens and 1234567 used to amplify exon 6 of the NAIP gene by the poly- merase chain reaction (PCR); coamplification of exon 13 was used as a control. Multiplex PCR was per- formed using the conditions and primers previously described.6 Electrophoresis was performed on the PCR ∗ products using a 1% agarose gel for 11⁄2 hours at 100 V; the PCR products were then stained with ethidium † bromide and visualized under UV light. Exons 7 and 8 of the SMN gene were amplified via PCR sulfur 35–adenosine triphosphate incorpo- ration.7 Electrophoresis was performed on the PCR products using a 30.5 mutation detection enhance- † ment acrylamide gel overnight at 4 W and 4°C. The gel was dried and exposed to x-ray film. ∗ region may also play a role in the variable clinical Single strand conformation analysis of exon 7 of the survival motor neuron expression of the disease. ( SMN) gene (asterisk) and protein encoded by the highly homologous The mechanism of neuronal death in ALS is un- centromeric copy (cBCD541) (dagger). Lanes 2, 3, 5, and 6 show the 4-band pattern of normal chromosomes; lane 1, deletion of exon 7 of cBCD541; lane known, but abnormal apoptosis is considered a possi- 4, deletion of exon 7 of the SMN gene in a patient with spinal muscular bility. Mutations in the copper/zinc superoxide dismu- atrophy; and lane 7, polymorphism in exon 7 (exon 7 of the SMN gene and tase (SOD1) gene have been identified in approximately cBCD541 are also present). 15% of FALS cases, although the mechanism by which these mutations lead to motor neuron death is un- light revealed 2 bands of 434 base pairs (bp) (exon 6) known.11 Although mutations in the SOD1 gene have been and 242 bp (exon 13), indicating the absence of homo- identified in some patients with ALS, in most cases the zygous deletions seen in some patients with SMA. cause, genetic or otherwise, remains a mystery. Patients were also screened for deletions or other Reports of the coexistence of SMA and ALS within mutations in exons 7 and 8 of the SMN gene by single families suggest the possibility of a common genetic ba- strand conformation analysis; the Table summarizes these sis.12,13 Therefore, we elected to explore a possible role results. In controls, 4 bands are apparent after PCR for the NAIP and SMN genes in the pathogenesis of FALS amplification and single strand conformation analysis, and SALS. Genomic DNA specimens from 69 patients with revealing the presence of the SMN gene and cBCD541 non–SOD1-mutated FALS and 194 patients with SALS (Figure). Ninety-three percent of the patients in whom were analyzed for the absence of exon 6 in the NAIP gene SMA is diagnosed are reported to have a deletion of exon and for deletions or aberrant single strand conforma- 7intheSMN gene.7 One patient, in whom ALS was clini- tional polymorphism bands in exons 7 and 8 of the SMN cally diagnosed, was identified to have an exon 7 dele- gene. tion in the SMN gene as shown in lane 4 of the Figure; the centromeric copy remains. Review of this patient’s RESULTS clinical history confirmed that symptoms were consis- tent with a diagnosis of SMA. A polymorphism, C to T We analyzed 262 patients with ALS for the NAIP and SMN transition in intron 7, was observed in 22 patients (9 with gene deletions found in patients with SMA. Exon 6 of the FALS and 13 with SALS; Figure, lane 7); however, in each NAIP gene was successfully amplified via PCR, and elec- case, the intact SMN gene and cBCD541 genes were also trophoresis was performed using a 1% agarose gel (coam- observed. Eighteen patients (2 with FALS and 16 with plification of exon 13 provided an internal control). In SALS) lack the centromeric cBCD541 copy as shown in all cases, ethidium bromide staining visualized under UV the Figure, lane 1. Ninety-three patients were screened ARCH NEUROL / VOL 56, JUNE 1999 711 ©1999 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/23/2021 for mutations in exon 8. Deletions in exon 8 of cBCD541 patients with ALS, without whose support these studies could were observed in 6 cases. In 3 more cases, 2 cases of FALS not be done. We also acknowledge support from the Mus- and 1 of SALS, aberrant migration patterns were ob- cular Dystrophy Association, Tucson, Ariz, and from the served; however, in each case the SMN gene remained Medical Research Council of Canada.