Hereditary spastic paraplegias (HSPs) are a group of genetically heterogeneous neurodegenerative conditions. They are characterized by progressive spastic paralysis of the legs as a result of selective, lengthdependent degeneration of the axons of the corticospinal tract. Mutations in 3 genes encoding proteins that work together to shape the ER into sheets and tubules -receptor accessory protein 1 (REEP1), atlastin-1 (ATL1), and spastin (SPAST) -have been found to underlie many cases of HSP in Northern Europe and North America. Applying Sanger and exome sequencing, we have now identified 3 mutations in reticulon 2 (RTN2), which encodes a member of the reticulon family of prototypic ER-shaping proteins, in families with spastic paraplegia 12 (SPG12). These autosomal dominant mutations included a complete deletion of RTN2 and a frameshift mutation predicted to produce a highly truncated protein. Wild-type reticulon 2, but not the truncated protein potentially encoded by the frameshift allele, localized to the ER. RTN2 interacted with spastin, and this interaction required a hydrophobic region in spastin that is involved in ER localization and that is predicted to form a curvature-inducing/sensing hairpin loop domain. Our results directly implicate a reticulon protein in axonopathy, show that this protein participates in a network of interactions among HSP proteins involved in ER shaping, and further support the hypothesis that abnormal ER morphogenesis is a pathogenic mechanism in HSP. IntroductionThe ER is a continuous membrane system comprising the nuclear envelope and a dynamic network of proximal sheets and peripheral tubules. Proteins of 2 classes -the reticulons and the REEP/DP1/yop1p family (referred to herein as the REEPs) -are fundamental to the generation of both sheets and tubules. These proteins share a characteristic sequence feature; in the case of the reticulons this feature is termed the reticulon homology domain (RHD) and consists of 2 long hydrophobic stretches separated by a hydrophilic sequence. Each hydrophobic stretch is thought to sit in the ER membrane as a hairpin loop. Such loop domains have been suggested to generate membrane curvature by occupying more space in the outer leaflet of the membrane than the inner in a process termed "hydrophobic wedging" (1-3). These proteins are thus critical for producing

Objective Charcot-Marie-Tooth (CMT) disease comprises a large number of genetically distinct forms of inherited peripheral neuropathies. The relative uniform phenotypes in many patients with CMT make it difficult to decide which of the over 35 known CMT genes are affected in a given patient. Genetic testing decision trees are therefore broadly based on a small number of major subtypes (eg, CMT1, CMT2) and the observed mutation frequency for CMT genes. Since conventional genetic testing is expensive many rare genes are not being tested for at all. Methods Whole-exome sequencing has recently been introduced as a novel and alternative approach. This method is capable of resequencing a nearly complete set of coding exons in an individual. We performed whole-exome sequencing in an undiagnosed family with CMT. Results Within over 24,000 variants detected in 2 exomes of a CMT family, we identified a nonsynonymous GJB1 (Cx32) mutation. This variant had been reported previously as pathogenic in X-linked CMT families. Sanger sequencing confirmed complete cosegregation in the family. Affected individuals had a marked early involvement of the upper distal extremities and displayed a mild reduction of nerve conduction velocities. Interpretation We have shown for the first time in a genetically highly heterogeneous dominant disease that exome sequencing is a valuable method for comprehensive medical diagnosis. Further improvements of exon capture design, next-generation sequencing accuracy, and a constant price decline will soon lead to the adoption of genomic approaches in gene testing of Mendelian disease.

Hereditary spastic paraplegia (HSP) comprises a group of clinically and genetically heterogeneous diseases that affect the upper motor neurons and their axonal projections. Over 40 chromosomal loci have been identified for autosomal dominant, recessive, and X-linked HSP. Mutations in the genes atlastin, spastin and REEP1 are estimated to account for up to 50% of autosomal dominant HSP and currently guide the molecular diagnosis of HSP. Here we report the mutation screening results of 120 HSP patients from North America for spastin, atlastin, and REEP1, with the latter one partially reported previously. We identified mutations in 36.7% of all tested HSP patients and describe 20 novel changes in spastin and atlastin. Our results add to a growing number of HSP disease associated variants and confirm the high prevalence of atlastin, spastin, and REEP1 mutations in the HSP patient population.