Infantile-onset ascending hereditary spastic paralysis is associated with mutations in the alsin gene

Abstract

We studied 15 patients, from 10 families, who presented with severe spastic paralysis with an infantile onset and an ascending progression. Spastic paraplegia began during the first 2 years of life and extended to upper limbs within the next few years. During the first decade of life, the disease progressed to tetraplegia, anarthria, dysphagia, and slow eye movements. Overall, the disease was compatible with long survival. Signs of lower motor-neuron involvement were never observed, whereas motor-evoked potentials and magnetic resonance imaging demonstrated a primitive, pure degeneration of the upper motor neurons. Genotyping and linkage analyses demonstrated that this infantile-onset ascending hereditary spastic paralysis (IAHSP) is allelic to the condition previously reported as juvenile amyotrophic lateral sclerosis at the ALS2 locus on chromosome 2q33-35 (LOD score 6.66 at recombination fraction 0). We analyzed ALS2, recently found mutated in consanguineous Arabic families presenting either an ALS2 phenotype or juvenile-onset primary lateral sclerosis (JPLS), as a candidate gene. In 4 of the 10 families, we found abnormalities: three deletions and one splice-site mutation. All the mutations lead to a truncated alsin protein. In one case, the mutation affected both the short and the long alsin transcript. In the six remaining families, absence of cDNA ALS2 mutations suggests either mutations in regulatory ALS2 regions or genetic heterogeneity, as already reported in JPLS. Alsin mutations are responsible for a primitive, retrograde degeneration of the upper motor neurons of the pyramidal tracts, leading to a clinical continuum from infantile (IAHSP) to juvenile forms with (ALS2) or without (JPLS) lower motor-neuron involvement. Further analyses will determine whether other hereditary disorders with primitive involvement of the central motor pathways, as pure forms of spastic paraplegia, could be due to alsin dysfunction.

Figure 1

Pedigree and haplotype analysis of 10 families with the IAHSP phenotype. Individuals from 10 families were genotyped with 15 markers of the 2q33-34 region, including the markers of the ALS2 critical region (D2S116, D2S2309, D2S2214, D2S346, D2S2289, D2S72, D2S307, D2S2189, and D2S2237). Haplotypes were generated under the assumption that the smallest number of recombination events was present. Haplotype analysis defined the boundary of the IAHSP locus within the ALS2 critical region, between markers D2S116 and D2S2237. For ALS2, a plus sign (+) denotes the mutated allele and a minus sign (−) the nonmutated allele. Asterisks (*) denote families with an ALS2 mutation.

Figure 2

Multipoint LOD scores for the ALS2 locus. Markers used, with their distance (in cM), are plotted against multipoint LOD scores. A maximum multipoint LOD score of 6.66 was obtained between the markers D2S116 and D2S2237. The grey bar represents the IAHSP locus, and the black bar represents the ALS2 locus. These loci can be superposed to uncover a region between markers D2S116 and D2S2237.

Figure 3

Electrophoregram results of alsin cDNA sequencing obtained from RT-PCR with RNA extracted from EBV-transformed lymphoblasts of one affected patient from each of the following families: 362, 419, 283, and 278. Three homozygous deletions (in families 362, 283, and 278) were found. All resulted in truncated alsin proteins, involving the short variant only in family 278. For family 419, cDNA analysis demonstrated a 12-bp deletion in the long ALS2 transcript, leading to a truncated long alsin protein. Genomic DNA analysis of the affected patient demonstrated a homozygous point mutation (G→T) in the consensus acceptor splice site (CAG) of exon 6.