Immune Response Poses Major Challenge in AAV Gene Therapy

In recent years, adeno-associated virus (AAV) vectors have emerged as the most widely used method of gene therapy delivery. A small, non-enveloped virus with a single-stranded DNA genome, AAV is able to enter specific target cells and transfer its genetic material to the nucleus, making it a highly effective treatment for diseases of the brain, lungs, liver, muscle, and retina.

Since AAV gene therapy relies on a virus to transfer genetic information to a host, the body’s natural immune response often presents a major challenge for gene delivery. The immune response occurs almost entirely via cell-mediated pathways, typically involving cytotoxic T cells and neutralizing antibodies. In a study conducted by Chirmule et al., 96 percent of subjects had AAV antibodies circulating in their bodies. The immune response also depends on factors such as AAV serotype and route of administration, further complicating the relationship between AAV administration and humoral response.

Researchers have developed a number of strategies to help circumvent the body’s immune response to AAV. One strategy is to suppress the specific parts of the body’s immune system that inhibit AAV administration. This could be by immune suppressive drugs or by physical removal of the antibodies before AAV administration. Another strategy involves modifying the AAV capsid (through point mutagenesis or directed evolution) to evade pre-existing neutralizing antibodies.

The CRISPR Genome Editing Tool

One of the most exciting developments in molecular biology in recent years, the CRISPR genome editing tool may have profound implications for genome editing and gene therapy. Although gene editing is nothing new for the scientific community, CRISPR enables researchers to edit genomes with accuracy and efficiency that far surpasses anything else that is currently available.

Short for “clustered regularly interspaced short palindromic repeats,” CRISPR refers to repeating chains of DNA found in some bacterial genomes. Between these clusters are unique segments of DNA nucleotides, which match specific sequences on the DNA of harmful viruses. Utilizing these sequences as a signaling agent, bacteria can use defensive enzymes known as Cas, short for “CRISPR-associated proteins,” which recognize viral DNA and destroy it before it can proliferate.

In terms of genomic editing, researchers can isolate the gene for the Cas9 enzyme and connect it with a sequence of interest. Using the natural properties of the Cas9 enzyme, researchers can cut highly specific segments of DNA and paste in new genetic material. Unlike other enzymes, which often cut DNA indiscriminately at many points, Cas9 can accommodate templates of up to 20 bases, making it much easier to target specific gene sequences.

GlaxoSmithKline Gets Approval Recommendation for Gene Therapy

Recently, Strimvelis received a positive regulatory opinion from EMA’s Committee for Medicinal Products for Human Use (CHMP). Strimvelis is a gene therapy treatment developed by GlaxoSmithKline to treat a rare type of severe combined immunodeficiency (ADA-SCID). If Strimvelis obtains approval, it will be the first gene therapy approved for children in Europe. Before reaching the market, the treatment must receive final clearance from the European Commission.

Severe combined immunodeficiency is a crippling disorder, and Strimvelis offers hope to patients with one form of the rare ailment. Typical treatment for this disorder is a bone marrow transplant, but finding a suitable donor can take time, and the disease often proves fatal within a year.

Strimvelis involves the extraction of a patient’s own bone marrow cells, which are repaired by the addition of a missing enzyme, which is inserted into the genome using a lentivirus, before being placed back into the body. Clinical trial results have been promising, with 100 percent of treated patients still alive after a typical seven-year follow-up.

Market approval of ex vivo gene therapy for SCID has been a long time coming, beginning with the first clinical trials in the 1990s. This recent article highlights progress in the field and ongoing challenges to clinical development of gene therapy vectors: http://goo.gl/hBO3um