Gene therapy prevents photoreceptor death and preserves retinal function in a Bardet-Biedl syndrome mouse model

Abstract

Patients with Bardet-Biedl syndrome (BBS) experience severe retinal degeneration as a result of impaired photoreceptor transport processes that are not yet fully understood. To date, there is no effective treatment for BBS-associated retinal degeneration, and blindness is imminent by the second decade of life. Here we report the development of an adeno-associated viral (AAV) vector that rescues rhodopsin mislocalization, maintains nearly normalappearing rod outer segments, and prevents photoreceptor death in the Bbs4-null mouse model. Analysis of the electroretinogram awave indicates that rescued rod cells are functionally indistinguishable from wild-type rods. These results demonstrate that gene therapy can prevent retinal degeneration in a mammalian BBS model. ciliopathies | intra-flagellar transport | electroretinography B ardet-Biedl syndrome (BBS) is clinically diagnosed by the presence of at least four of the following signs: retinal dystrophy, polydactyly, obesity, learning disabilities, male hypogonadism, and renal anomalies (1). Of these, the visual phenotype is particularly devastating: the average BBS patient will progress to legal blindness before his or her 16th birthday. There are currently 15 genetic loci (Online Mendelian Inheritance in Man #209900) that are known to be associated with BBS, and although inheritance of this disease was once thought to follow a classic autosomal recessive pattern, recent evidence suggests that a more complex pattern, termed "triallelic inheritance," is involved at certain BBS loci (2). Only within the last decade has evidence emerged that BBS proteins play a role in ciliary function (3). More specifically, Bbs4 and the other BBSome components seem to be involved in both recruitment of cargo toward the ciliary basal body (4, 5) and in intraflagellar transport along the cilium (5-7). Several reports have elucidated the mechanisms by which disruption of BBS proteins give rise to the individual phenotypes seen in this highly pleiotropic syndrome (8-11).Retinal degeneration is a central feature of all BBS mouse models generated to date (8,(12)(13)(14)(15)(16). In rod and cone photoreceptors, the connecting cilium is a highly specialized ciliary structure that serves as the sole conduit from the inner segment to the outer segment. Because protein synthesis occurs proximal to the outer segment, rhodopsin and other visual proteins must be trafficked through the connecting cilium to reach their site of action in the outer segment. Abd-El-Barr et al. (11) showed that when Bbs4 was deleted in mice, rhodopsin and cone opsins became grossly mislocalized in rod and cone photoreceptors, respectively. This was followed by apoptotic photoreceptor death and deterioration of the electroretinogram (ERG) a-and b-waves (14). Ultrastructural analysis of rods from young animals revealed normal-appearing connecting cilia and basal body structures. This latter finding has important implications for gene therapy: because the structural transport apparatus seems to develop normally in Bbs4-nu...

Trehalose reduces retinal degeneration, neuroinflammation and storage burden caused by a lysosomal hydrolase deficiency

“…The a-wave responses from the 2 brightest stimuli were fitted using a formula devised by Cideciyan and Jacobson [45] to determine the maximum response amplitude (R max ) for each group (for detailed methodology see [46]). R max was 309.0 ± 9.1 µV for WT mice and 125.2 ± 6.0 µV for naglu –/ – mice at 28 weeks (60% reduction, P  < 0.01).…”

Successful arrest of photoreceptor and vision loss expands the therapeutic window of retinal gene therapy to later stages of disease

“…Successful rescue of photoreceptors with gene augmentation has been difficult to achieve when applied late in the disease natural history. There have been reports of success (31,49,50) but with the caveat of a lack of quantitative definition of spatiotemporal natural history of retinal degeneration at the time of the intervention.…”

“…Over the last decades, several therapeutic strategies that include the use of neuroprotective agents, antiapoptotic factors, immunotherapy, and corrective gene therapy have been developed with the ultimate goal of preventing or halting photoreceptor degeneration. With few exceptions (31,49,50), preclinical evidence gathered to support the initiation of clinical trials has been based on proof-of-concept studies conducted in animal models at a predegenerative stage (11,(58)(59)(60)(61). A limitation is that these experiments at predegenerate stages fail to recapitulate the cellular and molecular environment of surviving photoreceptors in patients with advanced disease.…”

Gene supplementation therapy for recessive forms of inherited retinal dystrophies

“…91 Mice with a disruption of the Bbs4 gene show early mislocalisation of Rhodopsin protein in the photoreceptor cell bodies and a progressive loss of photoreceptor cells over the course of 4 months. 92 Subretinal administration of self-complementary AAV5 carrying the Bbs4 gene in 2-week-old Bbs4 À/À mice results in a substantial reduction in the number of photoreceptors showing mislocalisation of Rhodopsin, with four layers of photoreceptor cells remaining after 16 weeks in the treated area of the retina compared with very few remaining cells in untreated areas and in untreated eyes. These findings suggest that the rescue of BBS4 function may slow the progressive retinal degeneration in the humans with BBS4 mutations.…”

“…These findings suggest that the rescue of BBS4 function may slow the progressive retinal degeneration in the humans with BBS4 mutations. 92 Of the three syndromic retinal dystrophies that have been treated in pre-clinical gene therapy studies, BBS4 appears to be the most attractive candidate for clinical application. The retinal degeneration in BBS is more severe than other candidate syndromic conditions and the efficacy of treatment for this disorder is most convincingly demonstrated.…”