A shared chromosome‐21 haplotype among amyotrophic lateral sclerosis families with the A4V SOD1 mutation

To the Editor: Amyotrophic lateral sclerosis (ALS) is a fatal, degenerative disease specifically affecting motor neurons in the brain and in the spinal cord (1, 2). A gene causing some cases of a hereditary form of ALS was genetically mapped to chromosome-21 (3), and the identity of this gene was revealed to be SOD1, the structural gene for Cu, Zn superoxide dismutase (4). Eleven different base-pair substitution mutations at various sites in the SOD1 gene were initially reported to be associated with hereditary ALS, and a total of 70 different SOD1 mutations have now been described (5). One of these, alanine 4 to valine (A4V; GCC! GTC), was observed to be the most frequently occurring SOD1 alteration in the US population, accounting for approximately 40% of all SOD1 mutations present among our ALS family collection (6). Because the A4V mutation is associated with a rapidly progressive clinical course (6), we wished to investigate the molecular basis for the high prevalence of this SOD1 mutation in the US population. Specifically, we wished to determine, in the extreme, whether the commonness of the A4V mutation was due to a single founder individual or due to a high propensity of the codon for alanine 4 to mutate to valine. In order to address this question, we have performed haplotype analysis for four microsatellite DNA markers encompassing the SOD1 gene on chromosome-21 to determine the number and types of different chromosome-21 associated with the A4V SOD1 mutation. We analysed DNA samples from members of 14 multigenerational ALS families positive for the A4V SOD1 mutation (families 45, 102, 103, 104, 114, 127, 156, 160, 176, 189, 221, 224, 233 and 275). All the families analysed are US/ American Caucasian of European ancestry, with a documented history of ALS occurring in two or more members of the known pedigree. The cohort included the following sample sizes available for genotyping: two families (176 and 127), each represented by >30 individuals, including two and three ALS-affected individuals, respectively; family 103, represented by >20 individuals with one ALS-affected individual; family 102, represented by 12 individuals with one ALS-affected individual; family 104, represented by five individuals with one ALSaffected individual; two families (45 and 189), each represented by one ALS-affected individual and one unaffected individual; seven families, each represented by a single, ALS-affected individual (families 114, 156, 160, 221, 224, 233 and 275). Features of the ALS population studied, including diagnostic criteria for ALS, are as previously described (3). Briefly, hereditary ALS is transmitted as an autosomal dominant trait with age-dependent penetrance (achieving 90% by age 70 years) in all A4V families studied. All available genomic DNA samples for members of these families were genotyped. All ALSaffected family members analysed were positive and heterozygous for the A4V SOD1 mutation. All available individuals of each family were typed for alleles of four highly polymorphic microsatellite DNA markers: D21S213 (7), D21S223 (8), D21S224 (8) and D21S63 (9), as previously described (8, 9) (Table 1). A set of control individuals with standard-size alleles was run on each gel for accurate assignment of allele sizes (9). Haplotype assembly was performed by manual phasing of alleles. ALS-affected individuals in 14 ALS families (all A4V families available for analysis) in the collection of Dr Robert H. Brown, Jr. at Massachusetts General Hospital tested positive for the A4V SOD1 mutation. Of these families, four are large families (102, 103, 127 and 176) in which it was possible to perform allele phasing to determine the specific allele associated with the A4V SOD1 mutation. The other 10 families are small; seven are represented by a single ALS-affected individual, and three by five or fewer individuals. Allele Clin Genet 2004: 66: 247–250 Copyright # Blackwell Munksgaard 2004 Printed in Denmark. All rights reserved CLINICAL GENETICS doi: 10.1111/j.1399-0004.2004. 00298.x