Pharmacogenetics, Gene Therapy, and Genome Editing: Educational Guide

Short version. Pharmacogenetics studies how heritable variation in drug-metabolising enzymes, transporters, targets, and HLA alleles shapes individual response to a small-molecule or biologic therapy. Friedrich Vogel coined the term in 1959 (Acta Genetica 9:312); Werner Kalow's 1962 textbook Pharmacogenetics: Heredity and the Response to Drugs consolidated the field. Sixty years later, the canonical pharmacogenetic axes have been mapped in detail across the cytochrome P450 system and the HLA locus, codified in CPIC guidelines and curated in PharmGKB. In parallel, the therapeutic stack has expanded to include AAV-delivered gene therapy, antisense oligonucleotides, ex vivo gene-modified cell therapies, and CRISPR-derived genome editing. This page is an educational summary of the published research literature on both threads.

Origins of the field

The first observations of heritable drug response collected in the late 1940s and 1950s. Pseudocholinesterase deficiency, identified after prolonged apnoea following suxamethonium anaesthesia, was traced to inherited variants of butyrylcholinesterase. Primaquine-induced haemolysis in some African-American soldiers during the 1950s was linked to glucose-6-phosphate dehydrogenase deficiency, an X-linked enzymopathy that turned out to be the most common enzyme defect in humans. Slow versus rapid acetylation of isoniazid, the cornerstone tuberculosis drug, was shown by Evans and others to be a bimodally distributed trait reflecting NAT2 polymorphism. The pattern across these examples — an enzyme-mediated step in drug disposition, varying between individuals on a heritable basis — pointed at a unified field.

Friedrich Vogel introduced the term Pharmakogenetik in a 1959 review in Ergebnisse der Inneren Medizin und Kinderheilkunde (republished in Acta Genetica et Statistica Medica 9:312, 1959), and Werner Kalow's textbook Pharmacogenetics: Heredity and the Response to Drugs (W. B. Saunders, 1962) consolidated what was then known. The contemporary literature builds directly on that synthesis, with the cytochrome P450 system as the dominant locus.

The cytochrome P450 system

The cytochromes P450 (CYP) are a superfamily of haem-containing mono-oxygenases. Five isoforms — CYP3A4, CYP2D6, CYP2C9, CYP2C19, and CYP3A5 — between them metabolise a substantial fraction of marketed small-molecule drugs. Each carries common variant alleles whose effects on substrate clearance have been mapped in detail and are catalogued by phenotype: poor metaboliser (PM), intermediate (IM), normal (NM), rapid (RM), and ultrarapid (UM).

CYP2D6 is unusual in showing very large between-individual variation, with copy-number variants on top of the more usual SNV polymorphism. The CYP2D6*4 null allele is common in European populations; full gene duplication produces the ultrarapid-metaboliser phenotype, more common in some North African and Middle Eastern populations. Codeine is a prodrug bioactivated by CYP2D6 O-demethylation to morphine; ultrarapid metabolisers convert more aggressively, and the published literature includes case reports of opioid toxicity in nursing infants of ultrarapid-metaboliser mothers. Tamoxifen activation to endoxifen is also CYP2D6-mediated, although the magnitude of the clinical effect on outcomes has been debated. Tricyclic antidepressants and many antipsychotics are also CYP2D6 substrates.

CYP2C19 activates the antiplatelet prodrug clopidogrel; loss-of-function alleles (CYP2C19*2, *3) reduce active-metabolite formation and have been associated with reduced antiplatelet effect after percutaneous coronary intervention. CYP2C19 is also the principal metaboliser of proton-pump inhibitors and of voriconazole; the literature on dose adjustment by genotype is more developed for PPIs and voriconazole than for clopidogrel.

CYP2C9 and VKORC1 + warfarin. Warfarin dose requirement is jointly influenced by CYP2C9 (clearance of the more pharmacologically active S-warfarin) and VKORC1 (the warfarin target, vitamin K epoxide reductase). Several pharmacogenetic dosing algorithms have been published; the comparative-effectiveness trials have produced mixed results, and the clinical question of whether genotype-guided initiation outperforms clinical-only algorithms remains contested in the literature.

CYP3A4 and CYP3A5. CYP3A4 is the highest-abundance hepatic P450 and metabolises a very broad substrate range; the principal sources of variation are environmental (induction and inhibition by drugs and food). CYP3A5 is more polymorphic, with the CYP3A5*3 non-expresser allele common in European populations and minority status in some sub-Saharan African populations. The published-literature application is most developed for tacrolimus dosing in transplantation.

HLA-drug associations

Severe cutaneous adverse drug reactions are over-represented for particular drug-HLA-allele pairings. Two associations are textbook. HLA-B*57:01 and abacavir hypersensitivity (Mallal et al. 2008, NEJM 358:568; the PREDICT-1 trial) showed that excluding HLA-B*57:01 carriers from abacavir initiation eliminated immunologically confirmed hypersensitivity in the prospectively screened arm. HLA-B*15:02 and carbamazepine-associated Stevens-Johnson syndrome / toxic epidermal necrolysis in some Asian populations is a parallel association, with strong genetic predisposition concentrated in populations of Han Chinese, Thai, and Malay ancestry.

Other canonical gene-drug pairs

TPMT and NUDT15 + thiopurines. Thiopurine methyltransferase (TPMT) and NUDT15 variants attenuate clearance of azathioprine, 6-mercaptopurine, and thioguanine; homozygous loss-of-function carriers face severe myelosuppression on standard doses. NUDT15 risk variants are more frequent in East Asian populations than in European populations.
UGT1A1 + irinotecan. The UGT1A1*28 promoter TA repeat polymorphism (Gilbert syndrome) reduces glucuronidation of the SN-38 active metabolite of irinotecan, raising the risk of severe diarrhoea and neutropenia.
DPYD + 5-fluorouracil and capecitabine. Variants in dihydropyrimidine dehydrogenase (the rate-limiting catabolic enzyme for fluoropyrimidines) reduce clearance and predispose to severe mucositis, diarrhoea, and myelotoxicity.
SLCO1B1 + simvastatin. The SLCO1B1 c.521T>C variant reduces hepatic uptake of simvastatin (and other statins to a lesser extent) and is associated with increased simvastatin myopathy risk.
G6PD + oxidative-stress drugs. G6PD deficiency, a common X-linked enzymopathy, predisposes to haemolysis on exposure to a defined list of drugs including primaquine, dapsone, rasburicase, and methylene blue.

CPIC, PharmGKB, and structured guidelines

The Clinical Pharmacogenetics Implementation Consortium (CPIC) publishes structured, peer-reviewed guideline documents for individual gene-drug pairs — describing the variant nomenclature (mostly the star-allele system maintained by PharmVar), the phenotype mapping, and the published evidence on dosing or therapeutic alternatives. The development process is described in Caudle et al. 2014, Clin Pharmacol Ther 96:542. CPIC documents are educational and reference resources; their adoption into clinical use, where it occurs, sits with healthcare systems and prescribers, not with the published guideline itself.

The Pharmacogenomics Knowledge Base (PharmGKB) at Stanford curates the underlying evidence base, with allele-function tables, evidence summaries, and gene-drug dosing guidelines. Variant nomenclature is maintained at PharmVar. These three resources together — CPIC, PharmGKB, PharmVar — are the standard educational references for pharmacogenetic content.

Therapeutic strategies based on genetic mechanism

Pharmacogenetics describes how a heritable variant changes the response to an existing drug. The complementary direction — using genetic mechanism to design new therapies — has produced an expanding stack over the past two decades. The categories below summarise published research-literature reports, not recommendations.

Antisense oligonucleotides

Synthetic short oligonucleotides bind a target RNA and modulate splicing, translation, or transcript stability. Nusinersen (Spinraza) for spinal muscular atrophy is the canonical example: an intrathecally delivered antisense oligonucleotide that promotes inclusion of SMN2 exon 7, raising production of full-length SMN protein in motor neurones. Trial outcomes were reported by Finkel et al. for the infant-onset population (Finkel et al. 2017, NEJM 377:1723) and by Mercuri et al. for the later-onset population. Other published antisense agents include eteplirsen and golodirsen for exon-skipping in DMD-mutated Duchenne muscular dystrophy and inotersen for hereditary transthyretin amyloidosis.

AAV gene therapy

Recombinant adeno-associated virus (AAV) vectors deliver a functional copy of a gene to non-dividing cells, with serotype selection driving tissue tropism. The therapeutic gene typically integrates only at low frequency and persists episomally; the published-literature concern is durability over the lifetime of the recipient.

Voretigene neparvovec (Luxturna) for biallelic RPE65-mediated Leber congenital amaurosis was the first in vivo AAV gene therapy with a published phase 3 trial (Russell et al. 2017, Lancet 390:849): a subretinal AAV2 vector restoring functional RPE65 to retinal pigment epithelium. Onasemnogene abeparvovec (Zolgensma) for SMN1-deficient spinal muscular atrophy delivers an SMN1 cDNA via an AAV9 capsid that crosses the blood-brain barrier (Mendell et al. 2017, NEJM 377:1713; the START trial). Subsequent published AAV programmes target haemophilia A and B (factor VIII and IX), Duchenne muscular dystrophy (micro-dystrophin constructs), and a number of metabolic disorders.

Ex vivo gene-modified cell therapy

Cells — usually CD34+ haematopoietic stem and progenitor cells, or T cells — are removed from the patient, modified ex vivo using lentivirus, gamma-retrovirus, or genome editing, and re-infused after conditioning. The established clinical examples are CAR-T cells for B-cell lymphoid malignancies (tisagenlecleucel, axicabtagene ciloleucel, brexucabtagene autoleucel, lisocabtagene maraleucel, idecabtagene vicleucel for myeloma) and lentivirally modified haematopoietic stem cells for β-thalassaemia and sickle cell disease. The lentiviral programmes from bluebird bio, including betibeglogene autotemcel (Zynteglo) and lovotibeglogene autotemcel (Lyfgenia), and the related approaches for cerebral adrenoleukodystrophy and metachromatic leukodystrophy, are reported in the published trial literature.

CRISPR-Cas9 genome editing

The CRISPR-Cas9 system, characterised biochemically by Jinek et al. 2012, Science 337:816 and reviewed by Doudna and Charpentier 2014, Science 346:1258, uses a single guide RNA to direct the Cas9 endonuclease to a defined genomic site, where it produces a double-strand break. Repair via non-homologous end joining typically introduces small insertions or deletions, disabling the targeted sequence; homology-directed repair, with a supplied template, can install precise edits but is much less efficient in non-dividing cells.

The first published CRISPR-derived therapy in the human-trial literature is exagamglogene autotemcel (exa-cel, marketed as Casgevy) for sickle cell disease and transfusion-dependent β-thalassaemia. The approach uses ex vivo Cas9 editing of the BCL11A erythroid enhancer in autologous CD34+ cells, releasing fetal-haemoglobin transcriptional repression and elevating γ-globin expression after re-infusion. Phase 1/2 trial results were reported by Frangoul et al. 2021, NEJM 384:252. Casgevy received its first regulatory authorisation in 2023.

Base editing and prime editing

Two refinements of CRISPR architecture extend its scope without requiring a double-strand break. Base editing, introduced by David Liu's group (Komor et al. 2016, Nature 533:420), fuses a catalytically impaired Cas9 nickase to a cytidine deaminase (or, in subsequent adenine base editors, an evolved adenine deaminase), enabling targeted C-to-T or A-to-G transitions at the protospacer-proximal window. Prime editing (Anzalone et al. 2019, Nature 576:149) fuses a Cas9 nickase to a reverse transcriptase, with a "prime editing guide RNA" (pegRNA) that both directs the editor and templates the desired edit. Both technologies have moved into early-phase trial reports; the literature on long-term durability and off-target effects is still maturing. Casgevy is editing-by-disruption rather than editing-to-restore; published base- and prime-editing programmes targeting causative variants in inherited retinal disease, hypercholesterolaemia (PCSK9), and other disorders are at earlier trial stages at the time of writing.

Off-target effects, durability, and the open questions

The published literature emphasises three open questions across the editing platforms. Off-target editing, especially at near-cognate genomic sites, has been characterised by genome-wide assays (GUIDE-seq, CIRCLE-seq, Discover-seq) and by deep sequencing of predicted off-targets; refinements of Cas9 (high-fidelity variants, Cas12 orthologues) and base / prime editors continue to be reported. Bystander editing within the protospacer window is a base-editing-specific concern. Durability, particularly for AAV-delivered in vivo gene therapy in growing tissues and for CD34+ ex vivo programmes after transplant, remains a longitudinal-data question; published follow-up extends to several years for the earliest programmes but not yet to a lifetime. The emerging-therapeutics literature is moving fast, and the educational reading list needs to be refreshed against the current literature on each programme rather than a static citation list.

Educational and research framing

This page is an educational summary of the published research literature, written for students, researchers, and educators. It is not a recommendation about pharmacogenetic testing for any individual patient, and it is not a recommendation about any therapy. Genetic-testing decisions and therapeutic decisions belong with a qualified clinical-genetics, pharmacology, or specialist clinical service. Approval status of named therapeutic agents varies between regulatory jurisdictions, and the description above is a snapshot of the published trial and approval literature at the time this page was written; readers should consult the relevant primary source and the current regulatory status before drawing conclusions about availability.

Selected references

Vogel, F. (1959). Moderne Probleme der Humangenetik. Ergebnisse der Inneren Medizin und Kinderheilkunde 12:52–125; Acta Genetica et Statistica Medica 9:312.
Kalow, W. (1962). Pharmacogenetics: Heredity and the Response to Drugs. W. B. Saunders, Philadelphia.
Mallal, S. et al. (2008). HLA-B*5701 screening for hypersensitivity to abacavir. NEJM 358:568–579 (PREDICT-1). PubMed
Caudle, K. E. et al. (2014). Incorporation of pharmacogenomics into routine clinical practice: the CPIC guideline development process. Clin Pharmacol Ther 96:542–548. PubMed
Russell, S. et al. (2017). Efficacy and safety of voretigene neparvovec (AAV2-hRPE65v2) in patients with RPE65-mediated inherited retinal dystrophy. Lancet 390:849–860. PubMed
Mendell, J. R. et al. (2017). Single-dose gene-replacement therapy for spinal muscular atrophy. NEJM 377:1713–1722. PubMed
Finkel, R. S. et al. (2017). Nusinersen versus sham control in infantile-onset spinal muscular atrophy. NEJM 377:1723–1732. PubMed
Jinek, M. et al. (2012). A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science 337:816–821. PubMed
Doudna, J. A., and Charpentier, E. (2014). Genome editing: the new frontier of genome engineering with CRISPR-Cas9. Science 346:1258096. PubMed
Komor, A. C. et al. (2016). Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature 533:420–424. PubMed
Anzalone, A. V. et al. (2019). Search-and-replace genome editing without double-strand breaks or donor DNA. Nature 576:149–157. PubMed
Frangoul, H. et al. (2021). CRISPR-Cas9 gene editing for sickle cell disease and β-thalassemia. NEJM 384:252–260. PubMed
Clinical Pharmacogenetics Implementation Consortium (CPIC). cpicpgx.org
Pharmacogenomics Knowledge Base (PharmGKB). pharmgkb.org
PharmVar (variant nomenclature). pharmvar.org

Pharmacogenetics and emerging therapeutics