A mutation in the vesicle-trafficking protein VAPB causes late-onset spinal muscular atrophy and amyotrophic lateral sclerosis

Abstract

Motor neuron diseases (MNDs) are a group of neurodegenerative disorders with involvement of upper and/or lower motor neurons, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), progressive bulbar palsy, and primary lateral sclerosis. Recently, we have mapped a new locus for an atypical form of ALS/MND (atypical amyotrophic lateral sclerosis [ALS8]) at 20q13.3 in a large white Brazilian family. Here, we report the finding of a novel missense mutation in the vesicle-associated membrane protein/synaptobrevin-associated membrane protein B (VAPB) gene in patients from this family. Subsequently, the same mutation was identified in patients from six additional kindreds but with different clinical courses, such as ALS8, late-onset SMA, and typical severe ALS with rapid progression. Although it was not possible to link all these families, haplotype analysis suggests a founder effect. Members of the vesicle-associated proteins are intracellular membrane proteins that can associate with microtubules and that have been shown to have a function in membrane transport. These data suggest that clinically variable MNDs may be caused by a dysfunction in intracellular membrane trafficking.

Figure A1

Pedigrees from families studied. a, Family 2: white family with diagnosis of atypical ALS. b, Family 3: African Brazilian family with diagnosis of atypical ALS. c, Family 4: white family with diagnosis of typical ALS and atypical ALS. d, Family 5: African Brazilian family with diagnosis of mild SMA. e, Family 6: white family with diagnosis of late-onset SMA type Finkel. f, Family 7: African Brazilian family with diagnosis of late-onset SMA type Finkel. It was not possible to link all these subjects to the same pedigree; however, all patients tested have the same P56S mutation in the VAPB gene. An asterisk (*) indicates DNA was available.

Figure 1

Mapping of the VAPB locus and mutation analyses. a, Pedigree from the first family reported with a diagnosis of ALS/MND (an asterisk [*] indicates DNA was available). b, VAPB locus at 20q13.3. Recombinant events reduced the region to 1.5 Mb, between marker D20S430 and the TUBB1 gene.

Figure 2

Analyses of VAPB gene. a, Chromatogram showing the mutation in VAPB coding sequence and normal control. b, Segregation of the P56S mutation in heterozygous patients and normal controls (homozygous). The HaeIII enzyme cuts in several fragments, producing one major allele of 179 bp in normal subjects and one allele of 207 bp in heterozygous patients. c, Partial amino alignment of the VAP homologues genes. Gaps were introduced for optimal alignment. Lengths of each partial protein are indicated. The amino acid sequence FKVKTTAPRXYCVRPNS is highly conserved (blue), and the proline is indicated (red). d, Haplotype analysis of three kindred studied. All the patients have a common haplotype (red), which was not present in normal relatives (green). e, VAPB models. The top part of the β sandwich is made of green β strands, and the bottom part is made of red strands. Wild type induces a kink and splits half of the strand into the top part of the sandwich (green) and the other half of the strand into the bottom part (red). The P56S mutation disrupts the hydrogen bonds between strands on the same sheet of the β sandwich, favoring new hydrogen bonds that move the bottom half to the opposite side of the β sandwich.

Figure 3

GFP expression in hippocampal neurons and HEK293 cell culture. a, Neurons and HEK cells expressing the VAPB wild type (green). b, Using an ER marker (red), we observed a colocalization of the VAPB and ER compartment (yellow), as reported elsewhere (only the colocalization is shown). c, P56S mutant disrupts the subcellular distribution, showing intracellular aggregates. d–e, P56S mutant does not colocalize either with Golgi apparatus (blue) or with the ER (red).