The technology platform:
non-replicative HSV-1 constructs
Core to gene therapy is the ability of the vector used to ensure safe transgene expression without compromising the integrity of the target cells or their surrounding tissue. Once within peripheral neurons, the HSV-1 virus naturally enters a non-toxic latent state. However, it needs to be engineered into a replication-defective vector in order to avoid reactivation from latency in neurons or to compromise the peripheral tissue in which it is administered. EG 427 has produced such non-replicative latent vectors that open gene therapy to a variety of indications that affect the peripheral nervous system.
Blocking immediate early genes to create non-replicative vectors
The multiplication of HSV-1 requires the expression of a gene cascade that starts with a group of immediate early (IE) genes, responsible for evading the host’s immune system as well as triggering the expression of early (E), and then late (L) genes [Honess R.W., Roizman B.]. Among IE genes, ICP4 and ICP27 are central to the entry into replication. Their deletion, either alone or in combination, produces a replication-defective virus, unable to multiply in non-neuronal cells and to reactivate in peripheral neurons. The deleted genome persists in a latent state in neurons and is able to support ectopic gene expression [ Marconi, P. et al. | Goins, W. F. et al. ]. While such vectors overcome the safety issues associated with the naturally replicative virus, their propagation in vitro still requires replication of the viral particles, which is achieved in cell lines that express complementing levels of the deleted genes. The resulting vectors are valuable gene therapy vehicles for use in peripheral neurons.
In addition to ICP4 and ICP27 the vector genome contains other deletions not indicated in the text.
Transgene variations for selective expression in specific cells of the PNS
While HSV-1 vectors naturally target PNS neurons, an additional layer of specificity is required to limit the expression of a transgene to a subset of neurons, depending on the indication to be treated. This is achieved by choosing a promoter that is specific to the targeted cell type. For instance, different promoters can be used for expression within sensory, sympathetic or parasympathetic neurons. Importantly, HSV-1 vectors can carry large transgenic constructs and therefore accommodate regulatory sequences that contribute to a controlled and selective transgene expression.
The defective viruses used by EG 427 provide a platform for gene therapy using large and complex transgene constructs, not only to compensate for gene mutations associated with rare, inherited and potentially life-threatening disorders, but also to deliver therapeutic gene sequences to treat acquired prevalent diseases.
While a gene therapy vector for the treatment of neurogenic bladder undergoes preclinical development, the company has identified other target indications and is working towards building new HSV-1-based vectors with the appropriate transgenes and promoters to address them.
What Our scientific founder says
HSV-1-based vectors provide the opportunity to specifically interfere with aberrant neurotransmission patterns responsible for diseases of the peripheral nervous system. Neurogenic bladder is one such disease for which we have developed an HSV-1-based gene therapy vector. But there are other indications for which we aim to develop similar therapeutic strategies, including neuropathic pain and other neuropathies.
François Giuliano, MD, PhD
Expert in translational urology