Genetic Therapy Approaches for Ornithine Transcarbamylase Deficiency

Ornithine transcarbamylase deficiency (OTCD) is the most common urea cycle disorder with high unmet needs, as current dietary and medical treatments may not be sufficient to prevent hyperammonemic episodes, which can cause death or neurological sequelae. To date, liver transplantation is the only curative choice but is not widely available due to donor shortage, the need for life-long immunosuppression and technical challenges. A field of research that has shown a great deal of promise recently is gene therapy, and OTCD has been an essential candidate for different gene therapy modalities, including AAV gene addition, mRNA therapy and genome editing. This review will first summarise the main steps towards clinical translation, highlighting the benefits and challenges of each gene therapy approach, then focus on current clinical trials and finally outline future directions for the development of gene therapy for OTCD

Targeting the liver to treat the eye

Over the last two decades, gene therapy has given hope of potential cure for many rare diseases. In the simplest form, gene therapy is the transfer or editing of a genetic material to cure a disease via nonviral or viral vehicles. Gene therapy can be performed either in vivo by injecting a vector carrying the gene or tools for gene editing directly into a tissue or into the systemic circulation, or ex vivo when patient cells are genetically modified outside of the body and then introduced back into the patient (Yilmaz et al, 2022). Adeno-associated viral vectors (AAV) have been the vectors of choice for in vivo gene therapy. There has been a lot of promising research on the development of novel tissue and cell-specific serotypes in order to improve efficacy and safety for clinical applications (Kuzmin et al, 2021). In this issue of EMBO Molecular Medicine, Boffa and colleagues present a novel AAV-based liver-directed gene therapy for ornithine aminotransferase deficiency

AAV-mediated gene therapy for rare metabolic disorders: turning a promise into a reality

Gene therapy is emerging as the realistic treatment option for inborn errors of metabolism (IEMs) and, with the promising safety and efficacy evidence from the proof-of-concept studies, adeno-associated virus (AAV) has become the frontrunner among viral vector candidates for these monogenic disorders. Different AAV capsids exhibit specific tissue tropisms, which can considerably increase the efficiency of gene transfer to particular organs. Here, we will discuss two distinct diseases: ornithine transcarbamylase (OTC) deficiency and Niemann–Pick disease type C, in which significant advances have been achieved in AAV-based gene therapy trials

Features of Congenital Arthrogryposis Due to Abnormalities in Collagen Homeostasis, a Scoping Review

Congenital arthrogryposis (CA) refers to the presence of multiple contractures at birth. It is a feature of several inherited syndromes, notable amongst them are disorders of collagen formation. This review aims to characterize disorders that directly or indirectly impact collagen structure and function leading to CA in search for common phenotypic or pathophysiological features, possible genotype–phenotype correlation, and potential novel treatment approaches based on a better understanding of the underlying pathomechanism. Nine genes, corresponding to five clinical phenotypes, were identified after a literature search. The most notable trend was the extreme phenotype variability. Clinical features across all syndromes ranged from subtle with minimal congenital contractures, to severe with multiple congenital contractures and extra-articular features including skin, respiratory, or other manifestations. Five of the identified genes were involved in the function of the Lysyl Hydroxylase 2 or 3 enzymes, which enable the hydroxylation and/or glycosylation of lysyl residues to allow the formation of the collagen superstructure. Whilst current treatment approaches are post-natal surgical correction, there are also potential in-utero therapies being developed. Cyclosporin A showed promise in treating collagen VI disorders although there is an associated risk of immunosuppression. The treatments that could be in the clinical trials soon are the splice correction therapies in collagen VI-related disorders

Markers of cognitive function in individuals with metabolic disease: Morquio Syndrome and Tyrosinemia Type III

We characterized cognitive function in two metabolic diseases. MPS–IVa (mucopolysaccharidosis IVa, Morquio) and tyrosinemia type III individuals were assessed using tasks of attention, language and oculomotor function. MPS–IVa individuals were slower in visual search, but the display size effects were normal, and slowing was not due to long reaction times (ruling out slow item processing or distraction). Maintaining gaze in an oculomotor task was difficult. Results implicated sustained attention and task initiation or response processing. Shifting attention, accumulating evidence and selecting targets were unaffected. Visual search was also slowed in tyrosinemia type III, and patterns in visual search and fixation tasks pointed to sustained attention impairments, although there were differences from MPS–IVa. Language was impaired in tyrosinemia type III but not MPS–IVa. Metabolic diseases produced selective cognitive effects. Our results, incorporating new methods for developmental data and model selection, illustrate how cognitive data can contribute to understanding function in biochemical brain systems

Three-dimensional Characterization of Interorganelle Contact Sites in Hepatocytes using Serial Section Electron Microscopy

Transmission electron microscopy has been long considered to be the gold standard for the visualization of cellular ultrastructure. However, analysis is often limited to two dimensions, hampering the ability to fully describe the three-dimensional (3D) ultrastructure and functional relationship between organelles. Volume electron microscopy (vEM) describes a collection of techniques that enable the interrogation of cellular ultrastructure in 3D at mesoscale, microscale, and nanoscale resolutions. This protocol provides an accessible and robust method to acquire vEM data using serial section transmission EM (TEM) and covers the technical aspects of sample processing through to digital 3D reconstruction in a single, straightforward workflow. To demonstrate the usefulness of this technique, the 3D ultrastructural relationship between the endoplasmic reticulum and mitochondria and their contact sites in liver hepatocytes is presented. Interorganelle contacts serve vital roles in the transfer of ions, lipids, nutrients, and other small molecules between organelles. However, despite their initial discovery in hepatocytes, there is still much to learn about their physical features, dynamics, and functions. Interorganelle contacts can display a range of morphologies, varying in the proximity of the two organelles to one another (typically ~10-30 nm) and the extent of the contact site (from punctate contacts to larger 3D cisternal-like contacts). The examination of close contacts requires high-resolution imaging, and serial section TEM is well suited to visualize the 3D ultrastructural of interorganelle contacts during hepatocyte differentiation, as well as alterations in hepatocyte architecture associated with metabolic diseases

Buffy Coat Score as a Biomarker of Treatment Response in Neuronal Ceroid Lipofuscinosis Type 2

The introduction of intracerebroventricular (ICV) enzyme replacement therapy (ERT) for treatment of neuronal ceroid lipofuscinosis type 2 (CLN2) disease has produced dramatic improvements in disease management. However, assessments of therapeutic effect for ICV ERT are limited to clinical observational measures, namely the CLN2 Clinical Rating Scale, a subjective measure of motor and language performance. There is a need for an objective biomarker to enable assessments of disease progression and response to treatment. To address this, we investigated whether the proportion of cells with abnormal storage inclusions on electron microscopic examination of peripheral blood buffy coats could act as a biomarker of disease activity in CLN2 disease. We conducted a prospective longitudinal analysis of six patients receiving ICV ERT. We demonstrated a substantial and continuing reduction in the proportion of abnormal cells over the course of treatment, whereas symptomatic scores revealed little or no change over time. Here, we proposed the use of the proportion of cells with abnormal storage as a biomarker of response to therapy in CLN2. In the future, as more tissue-specific biomarkers are developed, the buffy coats may form part of a panel of biomarkers in order to give a more holistic view of a complex disease

A survival analysis of ventricular access devices for delivery of cerliponase alfa

OBJECTIVE: Late infantile neuronal ceroid lipofuscinosis type 2 (CLN2) is a rare autosomal recessive disease caused by tripeptidyl peptidase 1 enzyme deficiency. At the authors' center, the medication cerliponase alfa is administered every 2 weeks via the intracerebroventricular (ICV) route. This requires the placement of a ventricular access device (VAD) or reservoir and frequent percutaneous punctures of this device over the child's lifetime. In this study, the authors audited the longevity and survival of these VADs and examined the causes of device failure. METHODS: A single-center survival analysis of VAD insertions and revisions (January 2014 through June 2020) was conducted. All children received cerliponase alfa infusions through a VAD. Patient characteristics and complications were determined from a prospectively maintained surgical database and patient records. For the VAD survival analysis, the defined endpoint was when the device was removed or changed. Reservoir survival was assessed using Kaplan-Meier curves and the log-rank (Cox-Mantel) test. RESULTS: A total of 17 patients had VADs inserted for drug delivery; median (range) age at first surgery was 4 years 4 months (1 year 8 months to 15 years). Twenty-six VAD operations (17 primary insertions and 9 revisions) were required among these 17 patients. Twelve VAD operations had an associated complication, including CSF infection (n = 6) with Propionibacterium and Staphylococcus species being the most prevalent organisms, significant surgical site swelling preventing infusion (n = 3), leakage/wound breakdown (n = 2), and catheter obstruction (n = 1). There were no complications or deaths associated with VAD insertion. The median (interquartile range) number of punctures was 59.5 (7.5-82.0) for unrevised VADs (n = 17) versus 2 (6-87.5) for revised VADs (n = 9) (p = 0.70). The median survival was 301 days for revisional reservoirs (n = 9) versus 2317 days for primary inserted reservoirs (n = 17) (p = 0.019). CONCLUSIONS: In the context of the current interest in intrathecal drug delivery for rare metabolic disorders, the need for VADs is likely to increase. Auditing the medium- to long-term outcomes associated with these devices will hopefully result in their wider application and may have potential implications on the development of new VAD technologies. These results could also be used to counsel parents prior to commencement of therapy and VAD implantation

Diagnostic and Management Issues in Patients with Late-Onset Ornithine Transcarbamylase Deficiency

Ornithine transcarbamylase deficiency (OTCD) is the most common inherited disorder of the urea cycle and, in general, is transmitted as an X-linked recessive trait. Defects in the OTC gene cause an impairment in ureagenesis, resulting in hyperammonemia, which is a direct cause of brain damage and death. Patients with late-onset OTCD can develop symptoms from infancy to later childhood, adolescence or adulthood. Clinical manifestations of adults with OTCD vary in acuity. Clinical symptoms can be aggravated by metabolic stressors or the presence of a catabolic state, or due to increased demands upon the urea. A prompt diagnosis and relevant biochemical and genetic investigations allow the rapid introduction of the right treatment and prevent long-term complications and mortality. This narrative review outlines challenges in diagnosing and managing patients with late-onset OTCD