Amyotrophic lateral sclerosis (ALS) is a fatal degenerative motor neuron disorder. Ten percent of cases are inherited; most involve unidentified genes. We report here 13 mutations in the fused in sarcoma/translated in liposarcoma ( FUS/TLS ) gene on chromosome 16 that were specific for familial ALS. The FUS/TLS protein binds to RNA, functions in diverse processes, and is normally located predominantly in the nucleus. In contrast, the mutant forms of FUS/TLS accumulated in the cytoplasm of neurons, a pathology that is similar to that of the gene TAR DNA-binding protein 43 ( TDP43 ), whose mutations also cause ALS. Neuronal cytoplasmic protein aggregation and defective RNA metabolism thus appear to be common pathogenic mechanisms involved in ALS and possibly in other neurodegenerative disorders.

Promoterless recombinant adeno-associated virus is used without nucleases to target the human coagulation factor IX gene to the liver-expressed albumin locus in haemophilia B mice, with an on-target integration into ∼0.5% of the albumin alleles in hepatocytes; stable F9 plasma levels at 7–20% of normal were obtained, leading to normal coagulation times in treated factor-IX-deficient mice. Most genome editing applications under consideration for therapeutic use require the presence of a site-specific endonuclease, but the uncontrolled presence of endonucleases in tissues can potentially cause significant adverse effects. Here, Mark Kay and colleagues describe the use of promoterless recombinant adeno-associated virus to target — without the need for nucleases — the human coagulation factor IX gene to the liver-expressed albumin locus in haemophilia B mice. The approach achieves on-target integration into about 0.5% of the albumin alleles in hepatocytes. Stable F9 plasma levels in treated factor IX deficient mice were 7–20% of normal, leading to coagulation times within the normal range. Site-specific gene addition can allow stable transgene expression for gene therapy. When possible, this is preferred over the use of promiscuously integrating vectors, which are sometimes associated with clonal expansion1 and oncogenesis2. Site-specific endonucleases that can induce high rates of targeted genome editing are finding increasing applications in biological discovery and gene therapy3. However, two safety concerns persist: endonuclease-associated adverse effects, both on-target4 and off-target5,6; and oncogene activation caused by promoter integration, even without nucleases7. Here we perform recombinant adeno-associated virus (rAAV)-mediated promoterless gene targeting without nucleases and demonstrate amelioration of the bleeding diathesis in haemophilia B mice. In particular, we target a promoterless human coagulation factor IX (F9) gene to the liver-expressed mouse albumin (Alb) locus. F9 is targeted, along with a preceding 2A-peptide coding sequence, to be integrated just upstream to the Alb stop codon. While F9 is fused to Alb at the DNA and RNA levels, two separate proteins are synthesized by way of ribosomal skipping. Thus, F9 expression is linked to robust hepatic albumin expression without disrupting it. We injected an AAV8-F9 vector into neonatal and adult mice and achieved on-target integration into ∼0.5% of the albumin alleles in hepatocytes. We established that F9 was produced only from on-target integration, and ribosomal skipping was highly efficient. Stable F9 plasma levels at 7–20% of normal were obtained, and treated F9-deficient mice had normal coagulation times. In conclusion, transgene integration as a 2A-fusion to a highly expressed endogenous gene may obviate the requirement for nucleases and/or vector-borne promoters. This method may allow for safe and efficacious gene targeting in both infants and adults by greatly diminishing off-target effects while still providing therapeutic levels of expression from integration.

The completion of the Human Genome Project, together with a better understanding of some of the emerging genetic patterns of human disease, has enabled a thorough examination of the most appropriate genetic models for amyotrophic lateral sclerosis (ALS). The pathology and epidemiology of ALS have been intensively studied since Adar, Charcot, and Duchenne first described the disease in the 1860s. Results of genetic studies that have emerged over the past two decades have led to the identification of SOD1 as a well-established causative gene for ALS. However, the identification of SOD1 has not been followed up by the identification of other genes responsible for classic ALS. This leads to the speculation that more complex genetic mechanisms are involved than initially assumed. While mutations in single genes are still likely to constitute a small proportion of ALS cases, the genes responsible for ALS in families with clusters of two or three affected individuals, and more particularly in sporadic cases, are far from being determined. Multigenic, somatic mutation, and gene–environment models may all contribute to the genetic etiology of ALS. The challenge now lies in determining which models are the most appropriate to dissect out the genetic components involved. This research will ultimately aid in identifying the cumulative risk of developing ALS. GLOSSARY: ALS = amyotrophic lateral sclerosis; FALS = familial ALS; FTD = frontotemporal dementia; HSP = hereditary spastic paraplegia; PDC = Parkinson-dementia complex; PLS = primary lateral sclerosis; PON = paraoxonase; SALS = sporadic ALS; SBMA = spinal and bulbar muscular atrophy; SMA = spinal muscular atrophy; SNP = single nucleotide polymorphism.