Years ago, the biomedical community convinced itself that, once we knew the sequence of the human genome, we would hold the keys to the curable universe. Clearly, this is far from the case. Unlocking the human genome made us realize human genetics is more complex than we ever imagined.

To be fair, gene-targeted therapies have already yielded enormous triumphs and promise for the conquest of developmental disorders of the nervous system. But they have also yielded many challenges and demanded more and different solutions.¹

Every cell in the body has the same complement of DNA at least during some phase of its lifetime. But a pars compacta neuron is a pars compacta neuron and not a reticular formation astrocyte. This is partly because the DNA represents the whole repertoire of proteins that could be produced by a cell, but not which ones are produced by a particular cell. Histone and DNA methylation, histone acetylation, transcription factor activation and binding, and the presence of enhancers and suppressors of transcription all play roles in gating production of proteins from DNA and mRNA. In addition, the modification status of individual and networks of genes changes over time.²

For this reason, understanding the impact of errors in or modifications of the DNA sequence requires an anatomical and temporo-physiological map of each of these biochemical processes. The ability to do single-cell transcriptomics, methylomic and acetylomic analyses, and cryo-electron microscopy should aid this basic science enterprise and its translation to an understanding of genetic and metabolic disease cell by cell, organ by organ, and system by system.³

Antisense therapies have been implemented with varying degrees of success for several developmental neurological diseases. Challenges include the need for and cost of lifelong therapy, off-target delivery, and the adverse effects of both the construct itself and its delivery vehicle or chemical modification. Furthermore, variability of disease severity and course makes it difficult to decide for whom the risks of therapy outweigh the risks of the disease.⁴ It is critical that we develop standardized metrics for disease course and progression and, with them, a robust understanding of the natural history and deep phenotypic subtypes of each disease in diverse cohorts of patients. We must also use the prodigious tools of basic science to understand the relationship between antisense sequences and chemical modifications and the organs they target, both therapeutically and adversely.

Gene therapies are most often delivered by viral vehicles. From study to study and investigator to investigator, delivery viruses vary widely. To date, there is no road map for which virus targets which tissue or cell type or causes what off-target effects. Virally-mediated gene therapy has been known to cause acute hepatic failure and microangiopathic thrombocytopenia with renal failure. In addition, immune reactions to the viral vehicle mean that, if the first construct is not successful in correcting the disorder, the patient likely cannot safely and effectively receive another.¹ We need to codify what viruses are best for delivery to a specific tissue and cell type. We need effective, non-toxic methods for building tolerance to a given delivery virus. In addition, we need to explore non-viral delivery systems, such as nanoparticles, to avoid the challenges presented by currently available delivery strategies.

Because each answer begets another question and because a fuller understanding reveals the unimaginable complexity of biological systems, there is much work yet to be done to safely effect cures. Only through innovative, collaborative efforts will we achieve this goal for children and families with neurogenetic disorders.⁵