Albert Seymour, PhD
Albert Seymour, PhD, Chief Scientific Officer, Homology Medicines

Advances in gene therapy and gene editing represent the future for many rare diseases where a single administration may lead to durable and potentially curative treatments. These two approaches share a similar basis, but they are distinct technologies. In specific contexts, each may present different advantages.

Gene therapy aims to deliver functional copies of the causal gene to the exact location in the body where the disease manifests. Gene therapies reside inside the nucleus of target organ cells as standalone DNA constructs and do not integrate into the genome. These constructs can be lost as cells divide. Yet the constructs have the potential to cure those diseases where the target organ is comprised of slowly dividing or nondividing cells, such as the cells of the adult liver or central nervous system.

Conversely, the goal of gene editing is to insert a functional gene directly into the genome of an affected patient. In doing so, gene editing is designed to provide a permanent correction that may be maintained even as cells divide. This is achieved by designing the gene editing construct to specifically target a certain region of the genome. Then, by using the body’s natural DNA repair process, called homologous recombination, a functional gene can be inserted. Figure 1 shows the key differentiators between gene therapy and gene editing approaches.

Figure 1. A comparison of the gene therapy and gene editing approaches enabled by Homology Medicines’ platform, which deploys AAVHSCs, that is, adeno-associated virus (AAV) vectors derived from human hematopoietic stem cells (HSCs).

One disease, two approaches

To visualize the differences between each approach, it’s helpful to study a specific example. For people born with the rare genetic metabolic disorder phenylketonuria (PKU), each day is a challenge for both the affected individual and his/her caregiver(s). PKU patients must meticulously and continuously manage dietary phenylalanine (Phe), an essential amino acid derived from protein in the diet.

Newborn screening was implemented in the early 1960s and is still used to identify children with PKU, and parents must ensure their children maintain levels of Phe below a strict threshold to prevent irreversible neurological damage. Adult patients universally describe a “fog like” condition associated with high Phe levels and often struggle with anxiety or have difficulty with short-term memory or executive decision-making.

PKU affects approximately 50,000 people worldwide, with an estimated 1,000 to 1,500 newborns diagnosed each year. It has been known for some time that the gene that causes PKU produces an enzyme called phenylalanine hydroxylase (PAH). The PAH enzyme works predominantly in the liver to metabolize Phe into tyrosine, a precursor of neurotransmitters, while also tightly regulating the concentration of Phe required for protein synthesis. Patients with PKU have mutations in the PAH gene that result in a loss of PAH enzyme activity and cause Phe to build up in the body.

While restrictive diets are the standard of care, other available treatments for patients with PKU are limited. These therapies lower Phe by either increasing the residual activity of PAH or through a direct breakdown of the amino acid in the bloodstream. While these therapies represent important advances, the treatments do not reconstitute the normal biochemical pathway for ~95% of patients, and they require chronic dosing.

As a result, a significant unmet need remains for treatments that address the root cause of PKU and provide necessary relief for patients and their families. With this goal in mind and armed with the knowledge that loss of PAH activity in the liver causes PKU, researchers are now focusing on developing genetic medicines, specifically, gene therapy and gene editing approaches, to restore PAH activity in the liver and enable the normal biochemical pathway of processing Phe that results in the production of neurotransmitters.

An integrated approach

At Homology Medicines, we are developing a gene therapy and gene editing platform to deliver potential one-time treatments, and ultimately cures, using our naturally occurring adeno-associated virus (AAV) vectors, which are derived from human stem cells. The discovery of 15 novel AAV serotypes derived from human hematopoietic stem cells (HSCs)—that is, 15 novel AAVHSCs—has enabled researchers at Homology Medicines to develop a dual gene therapy and gene editing platform to target many rare diseases, including PKU. Unlike most gene therapy and gene editing approaches in development, our AAVHSCs are designed to deliver corrective genes to disease-relevant cells in vivo.

There are specific reasons why a dual platform may be an effective approach for treating a disease like PKU. Given that adult livers are fully developed and that cell division is slow, a gene therapy approach may provide a durable and potentially curative effect in adult patients. In the case of PKU, a specific AAVHSC with high tropism, or affinity, for the liver can be used to deliver a functional PAH gene expressed by a liver-specific promoter. This approach is being studied in a clinical trial, which showed initial encouraging clinical data.

In diseases where cells divide rapidly, such as diseases affecting the liver in pediatric patients or the bone marrow, a gene editing approach is required to provide a more permanent correction.

AAVHSCs can be designed to gene edit using the homologous recombination pathway. Unlike other editing technologies that use nucleases to cut DNA, AAVHSC-based editing is designed to insert a functional copy of the gene into a vector flanked by long stretches of DNA sequences called homology arms. The DNA sequences in the homology arms match with the specific location in the genome required to correct the disease and provide the proper instructions for insertion into the genome.

Using these instructions, AAVHSCs can be designed to be directly inserted into the mutated gene, or into other specified regions of the genome, properly enabling expression of the corrected gene. By leveraging the natural high precision of homologous recombination, the potential for mutations or off-target integration, which may complicate protein expression, can be avoided.

While genetic medicines are still nascent compared to other therapeutic classes, hundreds of clinical trials are underway. Gene therapy and gene editing approaches could offer a life-changing opportunity for people who screen positive for rare genetic diseases, such as PKU, to receive a one-time administration that may offer a durable and potentially curative treatment.


Albert Seymour, PhD, is the chief scientific officer of Homology Medicines.


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