Abstract
Tech wire
Defeating Dystrophy with Multiexon Skipping
Unlike replacement gene therapy, in which a functional copy of the disease gene is added, exon skipping aims to correct the translational reading frame by removing (or skipping) the exon containing the mutation from the affected gene during the endogenous splicing process. One way to achieve exon skipping is to use synthetic antisense oligonucleotides (AONs) known as morpholinos; these molecules bind endogenous mRNA and can modify splicing or prevent translation through steric interference.
Duchenne muscular dystrophy (DMD) is a severe X-linked form of muscular dystrophy characterized by rapid progression of muscle degeneration. Mutations in the DMD gene encoding the muscle protein dystrophin are the underlying cause of DMD and its milder form, Becker muscular dystrophy (BMD). Genetic defects in DMD lead to significant dystrophin dysfunction whereas mutations in BMD result in truncated protein with residual activity. Interestingly, some BMD patients with deletions as large as 33 exons show little or no clinical symptoms. This raises the possibility of using morpholino-mediated removal of exons carrying nonsense mutations, or whose presence disrupts the open reading frame at the site of the mutation, so as to restore the translational reading frame and convert DMD to a milder BMD phenotype. Many DMD patients will likely require skipping of two or more exons to restore the reading frame. As such, it is thought that cocktails of morpholino molecules targeting multiple exons would permit design of quasi-dystrophin proteins that retain increased functionality while enabling the treatment of a large group of DMD patients with distinct but overlapping deletions. Clinical trials have provided proof-of-principle results for the exon-skipping strategy (1), but have only targeted a single exon in a localized application.
A new study published in the Annals of Neurology (2) now demonstrates that intravenous delivery of a cocktail of different morpholinos is safe in beagle dogs with a naturally occurring form of canine muscular dystrophy (CMD). Beagles with CMD have at least two exons that must be skipped (multiexon skipping) to restore the open reading frame; therefore, it is more challenging to rescue CMD dogs with an exon-skipping strategy. The authors show that the morpholino cocktail targeting exons 6 through 9 resulted in systemic production of dystrophin and improvement in the dogs' functional abilities. These findings suggest multiexon skipping is a feasible strategy for the treatment of muscular dystrophies involving mutations in more than one exon.
Clinical wire
Ribozyme Cuts HIV
Results have emerged from the first randomized, double-blinded, placebo-controlled, phase II gene transfer clinical trial for HIV (1). The multicenter study enrolled 74 HIV-positive volunteers. Participants received their own CD34+ blood stem cells, either untreated or genetically modified using OZ1, a Moloney murine leukemia virus-based, replication-incompetent retroviral vector. OZ1 vectors contain a gene that encodes a ribozyme that targets the overlapping vpr and tat reading frames of HIV-1. Previous phase I studies have demonstrated the safety and feasibility of this approach. In this study, between 0.3 and 20.2 million CD34+ cells were infused once per subject to examine whether these cells would engraft, divide, and differentiate into T cells that are protected from HIV-1 infection. Subjects were requested to interrupt their antiretroviral treatment for a period of time after CD34+ administration to provide a selective pressure for the OZ1-transduced cells. The study was unable to demonstrate a significant reduction of viral loads and therefore did not reach its primary efficacy end point. CD4+ T cell counts, however, were higher in the OZ1-treated participants.
The study reports that approximately 54% of the infused OZ1-treated cells were successfully transduced in the treatment group. Four weeks after infusion, 94% of the participants demonstrated evidence of expression of the transgenic product, which waned to 29% of the enrolled patients by week 48. Mean viral loads were reduced in the OZ1 group compared with the placebo group but not in a statistically significant manner. A weak negative correlation between the cell dose and the viral load was observed in the treated but not the control group. Importantly, there was no evidence of malignant transformation due to insertional mutagenesis of the vector.
Industry wire
Cell Genesys (San Francisco, CA) reports that it has ended development of the GVAX immunotherapy product for prostate cancer. GVAX products are irradiated whole tumor cells that are modified to secrete granulocyte-macrophage colony-stimulating factor (GM-CSF) to activate the immune system. The product has demonstrated a favorable safety profile in several phase I and II clinical trials in multiple types of cancer. Cell Genesys will continue efforts in lung cancer, leukemia, and pancreatic cancer with GVAX.
Amsterdam Molecular Therapeutics (AMT, Amsterdam, The Netherlands) reports the resignation of its chief executive officer (CEO), Ronald Lorijn. AMT also announced that Glybera, the company's lead AAV-based drug for LPL deficiency, will be filed later this year. In addition, the company announced the start of a clinical multicenter trial for hemophilia B this year.
Targeted Genetics (Seattle, WA) has granted Celladon Corporation certain proprietary AAV vector technologies enabling it to manufacture MYDICAR, an enzyme replacement therapy for heart failure in exchange for increased revenue for Targeted Genetics. The company's current efforts are focused on a treatment for severe retinal dystrophies, Huntington's disease, and amyotrophic lateral sclerosis (ALS).
Généthon (Évry Cédex, France), a nonprofit organization, announced the production of the first batch of HIV-1-based vector according to GMP-standards for a trial for Wiskott-Aldrich syndrome, a rare immunodeficiency.