The possibility that AAV vector-mediated gene therapy is linked to the development of hepatic tumors in ornithine transcarbamylase (OTC)-deficient mice is an evolving story that has held the attention of researchers at the junction of the fields of biochemical genetics and gene therapy. Moscioni and colleagues previously presented an abstract reporting effective AAV-mediated gene therapy in two strains (spf and spf-ash) of OTC-deficient mice [1]. The discovery of hepatic tumors in some treated mice at the end of this long-term study was an important caveat however. In this issue of Molecular Therapy, two new articles from this group describe the results of efficient AAV-mediated gene therapy in these OTC-deficient mice and provide further details of the incidence of hepatic tumors in these strains of mice [2, 3].

Improved therapeutic options are clearly needed for the X-linked human metabolic disorders known as OTC deficiency. OTC deficiency can lead to early lethality due to neonatal-onset hyperammonemia in severely affected males despite the availability of dietary and drug therapy. A subset of females carrying an inactivating mutation of the OTC gene also exhibit symptoms because of skewed inactivation of the X chromosome. However, females can also present with life-threatening hyperammonemia. Mutation of the OTC gene in humans has not been linked to an increased risk of hepatic tumors in surviving patients, such as mildly affected males, or in female carriers. Although liver transplantation can be effective, many affected males die prior to transplantation, leading some metabolic specialists to forego treatment of severely affected patients [4]. The previously reported evaluation of adenovirus vector-mediated gene therapy for OTC was complicated by transient transgene expression and marked toxicity [5, 6]. The isolation of additional AAV serotypes from primates has vastly improved the prospects for liver-targeted therapy in disorders of metabolism such as OTC deficiency [7]. In the first of the companion studies reported in this issue, Moscioni and colleagues evaluated delivery of the mouse OTC gene by AAV vectors pseudotyped with AAV2, AAV7, AAV8, or AAV9 capsids in adult OTC-deficient mice of the spf and spf-ash strains [3]. Vectors pseudotyped with AAV7, 8 or 9 capsids were effective, as evidenced by normalization of a disease-related biomarker, urinary orotate, fifteen days after administration of the therapy. Furthermore, functional testing consisting of a hyperammonemic challenge confirmed the benefit of vector treatment from 21 to over 250 days post-administration. Interestingly, prior to 21 days, the pseudotyped AAV vectors did not protect against symptoms of hyperammonemia, underlining the potential importance of early stabilization of disease progression in patients prior to the onset of therapeutic OTC expression during a future clinical trial. Detectable transgene was expressed in the mice by the vector for up to one year. However, the development of hepatic tumors in mice monitored for longer periods of time raised the spectre that AAV vectors could be tumorigenic, as had been suggested by a previous long-term study involving gene therapy in a mouse model of Sly disease, a lysosomal storage disorder [8]. In addition, while the new paper reports compelling data regarding the efficacy of pseudotyped AAV vectors in OTC deficiency, important questions remain regarding the feasibility of AAV vector-mediated gene therapy in this disorder, given the evidence for the need for early treatment of this disorder and the potential for reduced efficacy of AAV vectors in very young animals [9, 10]. The importance of long-term follow-up and …