Lentivirus: a vector for nervous system applications

The ABCs of Stargardt disease: the latest advances in precision medicine

Stargardt disease (STGD) is the most common form of inherited juvenile macular dystrophy and is caused by sequence variants in the ABCA4 gene. Due to its genetic complexity and phenotypic variability, STGD poses significant therapeutic challenges. In the past decade, a lot of progress has been made regarding our understanding of the molecular and clinical aspects of STGD, along with its mechanisms. This has led to the development of new therapies, and there are human clinical trials currently ongoing. This paper evaluates the emergence of pharmacological approaches targeting the visual cycle to mitigate retinal damage, the role of gene therapy in correcting specific genetic defects, and the use of stem cell therapies aimed at retinal regeneration by showcasing the latest clinical trials and precision medicine approaches.

Gene Therapy of ABCA4-Associated Diseases

The ATP-binding cassette (ABC) transporter gene, ABCA4 (ABCR), was characterized in 1997 as the causal gene for autosomal recessive Stargardt disease (STGD1). Shortly thereafter several other phenotypes were associated with mutations in ABCA4, which now have collectively emerged as the most frequent cause of retinal degeneration phenotypes of Mendelian inheritance. ABCA4 functions as an important transporter (or "flippase") of vitamin A derivatives in the visual cycle. Several ways to alleviate the effects of the defective ABCA4 protein, which cause accumulation of 11-cis and all-trans-retinal in photoreceptors and lipofuscin in the retinal pigment epithelium, have been proposed. Although ABCA4 has proven to be a difficult research target, substantial progress through genetic, functional, and translational studies has allowed major advances in therapeutic applications for ABCA4-associated pathology, which should be available to patients in the (near) future. Here, we summarize the status of the gene therapy-based treatment options of ABCA4-associated diseases.

Correction of the disease phenotype in the mouse model of Stargardt disease by lentiviral gene therapy

Autosomal recessive Stargardt disease (STGD1) is a macular dystrophy caused by mutations in the ABCA4 (ABCR) gene. The disease phenotype that is most recognized in STGD1 patients, and also in the Abca4-/- mouse (a disease model), is lipofuscin accumulation in retinal pigment epithelium. Here, we tested whether delivery of the normal (wt) human ABCA4 gene to the subretinal space of the Abca4 -/- mice via lentiviral vectors would correct the disease phenotype; that is, reduce accumulation of the lipofuscin pigment A2E. Equine infectious anemia virus (EIAV)-derived lentiviral vectors were constructed expressing either the human ABCA4 gene or the LacZ reporter gene under the control of the constitutive (CMV) or photoreceptor-specific (Rho) promoters. Abca4-/- mice were injected subretinally with 1 microl ( approximately 5.0 x 10(5) TU) of each EIAV vector in one eye at postnatal days 4 and 5. An injection of saline, an EIAV-null vector, or an uninjected contralateral eye served as a control. Mice were killed at various times after injection to determine photoreceptor (PR) transduction efficiency and A2E concentrations. EIAV-LacZ vectors transduced from 5 to 20% of the PRs in the injected area in mice. Most importantly, a single subretinal injection of EIAV-CMV-ABCA4 to Abca4-/- mouse eyes substantially reduced disease-associated A2E accumulation compared to untreated and mock-treated control eyes. Treated eyes of Abca4-/- mice accumulated 8-12 pmol per eye (s.d.=2.7) of A2E 1 year after treatment, amounts comparable to wt controls, whereas mock-treated or untreated eyes had 3-5 times more A2E (27-39 pmol per eye, s.d.=1.5; P=0.001-0.005). Although extrapolation to humans requires caution, the high transduction efficiency of both rod and cone photoreceptors and the statistically significant reduction of A2E accumulation in the mouse model of STGD1 suggest that lentiviral gene therapy is a potentially efficient tool for treating ABCA4-associated diseases.

Novel gene therapeutic strategies for neurodegenerative diseases

The convergent pathobiologic model of Parkinson's disease stipulates that disparate insults initiate a disease process that obligately share a common pathway leading to cell death. A combinatorial treatment which targets various steps in this pathway is likely to be the most successful therapeutic strategy. As advances are made in the field of neuroimaging and pharmacogenomics, early detection of sporadic PD will become a reality. Early intervention will likely spare more dopaminergic neurons and extend the quality of life for the patient. Continued advancements in the fields of pharmacology, neurosurgery, and gene therapy will strengthen the armamentarium available for the treatment of PD patients.