Biology of the Adrenal Gland Cortex Obviates Effective Use of Adeno-Associated Virus Vectors to Treat Hereditary Adrenal Disorders

Abstract

Congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder occurring in 1:10,000 to 1:20,000 live births. In >95% of the cases, CAH results from mutations in the CYP21A2 gene, encoding the adrenal steroid enzyme 21-hydroxylase (21OH). Cardinal phenotypic features of CAH include genital ambiguity and sexual precocity, and in severe cases, neonatal salt loss and death. Current standard of care consists of lifelong oral steroid replacement to reverse the cortisol deficiency. Although significant advances in the treatment of CAH have been made, the burden of a lifelong therapeutic intervention is not ideal for quality of life. Gene therapy for CAH by adeno-associated virus (AAV) vectors has been shown to efficiently transduce the adrenal cortex, restoring normal steroidogenesis in the short term. However, adrenocortical cells are continuously renewed by stem cells located at the adrenal capsule, which differentiate as they centripetally migrate towards the adrenal medulla where they undergo apoptosis. In this context, we hypothesized that AAV-mediated genetic correction of the adrenal cortex will work short term but will eventually lead to a loss of correction. To test this hypothesis, we administered intravenously an AAV serotype rh.10 gene transfer vector (AAVrh.10-21OH-HA) to 21-hydroxylase deficient mice (21OH). The data demonstrates that a single intravenous administration efficiently transduces adrenocortical cells leading to 21OH-HA expression and restoration of normal steroidogenesis. However, the duration of therapeutic efficacy lasted for only 8 weeks, accompanied by loss of 21OH-HA expression in the adrenal gland. Analysis in immunodeficient mice confirmed that the disappearance of transgene expression was not due to an antiviral/transgene immune response. Taken together, these results demonstrate that a single treatment with an adeno-associated viral vector expressing a functional copy of the mutated gene can only transiently treat adrenocortical hereditary disorders and that strategies to genetically modify the adrenocortical stem cells population will likely be required.

How to discuss gene therapy for haemophilia? A patient and physician perspective

“…Trials generally exclude those with inhibitors, which would include approximately one‐third of people with haemophilia A. All of the published phase 1/2 studies for haemophilia A and B excluded patients with active hepatitis B or C (generally defined as active hepatitis antigen, DNA positivity or RNA viral load positivity) and active HIV infection (generally defined as positive serological test for HIV plus a CD4 T‐cell count of ≤200 per μL and detectable HIV viral load), although one study also excluded patients who were HIV positive . To date, all trials have also excluded patients with pre‐existing antibodies to the AAV serotype specific to each of the investigational products; however, at least one phase 3 trial for haemophilia B [NCT03569891] has lifted that exclusion criterion due to lack of evidence for associated reduced efficacy or immune responses due to pre‐existing low‐titre neutralising antibodies to AAV5 .…”

“…Given that recombinant AAV does not generally integrate into host genomes, levels of transduction are expected to fall as cells turnover and die. 8,37 Under normal conditions, most hepatocytes are in a quiescent state with <1%-2% undergoing turnover at any time. 38,39 While there is some uncertainty due to limited data, each non-resting hepatocyte has an estimated lifespan of 200-300 days, so it is likely that the documented stable FIX levels reflect low hepatocyte turnover.…”

“…In terms of the physician perspective, setting patient expectations, both for clinical trials and ultimately for gene therapy as an approved treatment option for haemophilia, will be important, as some PWH will not qualify for treatment. The standard inclusion/exclusion criteria employed in clinical trials to date have limited participation to adult patients with severe or moderately severe haemophilia, 33,36,37 with exposure to factor treatment for defined periods of time study also excluded patients who were HIV positive. 37 To date, all trials have also excluded patients with pre-existing antibodies to the AAV serotype specific to each of the investigational products; however, at least one phase 3 trial for haemophilia B [NCT03569891] has lifted that exclusion criterion due to lack of evidence for associated reduced efficacy or immune responses due to pre-existing low-titre neutralising antibodies to AAV5.…”

“…The standard inclusion/exclusion criteria employed in clinical trials to date have limited participation to adult patients with severe or moderately severe haemophilia, 33,36,37 with exposure to factor treatment for defined periods of time study also excluded patients who were HIV positive. 37 To date, all trials have also excluded patients with pre-existing antibodies to the AAV serotype specific to each of the investigational products; however, at least one phase 3 trial for haemophilia B [NCT03569891] has lifted that exclusion criterion due to lack of evidence for associated reduced efficacy or immune responses due to pre-existing low-titre neutralising antibodies to AAV5. 45 Exclusion of key populations such children and adolescents, women with haemophilia, 46 and those with a history of inhibitors to factor replacement is consistent across Phase 1-3 haemophilia GT trials, but a GT trial in people with haemophilia A and inhibitors has been announced.…”

“…Within the limited follow‐up of current trials, GT for haemophilia B has resulted in stable FIX expression for up to eight years, however, the longer‐term durability of expression remains to be determined. Given that recombinant AAV does not generally integrate into host genomes, levels of transduction are expected to fall as cells turnover and die . Under normal conditions, most hepatocytes are in a quiescent state with <1%‐2% undergoing turnover at any time .…”