Genetic Disease and Therapy

Abstract

Genetic diseases cause numerous complex and intractable pathologies. DNA sequences encoding each human's complexity and many disease risks are contained in the mitochondrial genome, nuclear genome, and microbial metagenome. Diagnosis of these diseases has unified around applications of next-generation DNA sequencing. However, translating specific genetic diagnoses into targeted genetic therapies remains a central goal. To date, genetic therapies have fallen into three broad categories: bulk replacement of affected genetic compartments with a new exogenous genome, nontargeted addition of exogenous genetic material to compensate for genetic errors, and most recently, direct correction of causative genetic alterations using gene editing. Generalized methods of diagnosis, therapy, and reagent delivery into each genetic compartment will accelerate the next generations of curative genetic therapies. We discuss the structure and variability of the mitochondrial, nuclear, and microbial metagenomic compartments, as well as the historical development and current practice of genetic diagnostics and gene therapies targeting each compartment.

Keywords: clinical genetics; gene editing; gene therapies; genetic diagnostics; genetic disease.

Figure 1

Gene therapies based on bulk replacement or selection of genetic compartments. (a) The mitochondrial genome of an affected mother’s oocyte can be replaced through transfer of its nucleus into a donor mother’s oocyte, which contains mitochondria unaffected by the mutation. (b) The nuclear genome can be selected through preimplantation diagnosis of in vitro–fertilized zygotes.

Figure 2

Gene therapies based on nontargeted genetic addition or targeted gene editing. (a) Mutated genes in the mitochondrial genome can be integrated into the nuclear genome, with their protein products targeted for import into the mitochondria. Direct delivery of genetic material to the mitochondrial genome poses a greater challenge. (b) Adding or editing genetic material in the nuclear genome of the human germline poses significant ethical concerns. (c) Nontargeted addition or targeted editing in somatic cells, such as cells cultured ex vivo (e.g., hematopoietic stem cells and T cells). (d) Nontargeted addition or targeted editing in somatic cells in vivo, as in retinal cells, hepatocytes, or myocytes, critically depends on delivery platforms to carry DNA, RNA, and/or protein cargos to the cell type of interest.

Figure 3

Modular systems for genetic diagnosis and therapy. Gene therapies in all four human genetic compartments depend on the modular process of diagnosis, therapeutic design, and compartment-specific delivery of therapeutic reagents. Diagnosis of genetic disease is now centered around next-generation DNA sequencing to detect errors in the mitochondrial and nuclear genomes. Therapy can be based on bulk replacement or selection of the genetic compartment or the therapeutic nontargeted addition of new genetic material or targeted correction of causative mutations through gene editing. Delivery platforms targeting each genomic compartment in somatic cells, whether in vivo or ex vivo, can carry gene addition and editing reagents with distinct therapeutic sequences or specificities depending on the genetic diagnosis. Abbreviations: AAV, adeno-associated virus; CRISPR/Cas9, clustered regularly interspaced short palindromic repeats/caspase-9; TALEN, transcription activator–like effector nuclease; ZFN, zinc-finger nuclease.