13.3 Clinical Pharmacogenomics (PGx) Core Alleles
Key Takeaways
- Pharmacogenomics utilizes star-allele (*allele) nomenclature to classify Phase I/II drug-metabolizing enzymes, drug transporters, and HLA loci into Poor, Intermediate, Normal, Rapid, and Ultrarapid Metabolizer phenotypes.
- CYP2D6 is a highly polymorphic Phase I enzyme with loss-of-function (*4, *5), decreased function (*10, *17, *41), and gene duplication (*1xN, *2xN) alleles; Ultrarapid Metabolizers convert codeine to morphine excessively, risking fatal toxicity.
- CYP2C19 loss-of-function variants (*2, *3) prevent bioactivation of the prodrug clopidogrel (Plavix), leading to antiplatelet treatment failure and stent thrombosis, whereas CYP2C19*17 confers increased transcription.
- Warfarin dosing requires combinatorial genotyping of CYP2C9 (*2, *3 impair S-warfarin clearance) and VKORC1 (-1639G>A reduces target enzyme expression), where variant carriers require substantially lower starting doses.
- Dihydropyrimidine dehydrogenase (DPYD *2A) and thiopurine methyltransferase (TPMT *2, *3A, *3C) / NUDT15 (*3) deficiencies cause lethal toxicity upon exposure to standard doses of 5-fluorouracil and thiopurines, respectively.
Last updated: August 2026
13.3 Clinical Pharmacogenomics (PGx) Core Alleles
Quick Summary: Clinical Pharmacogenomics (PGx) investigates how inherited germline genomic variations modulate inter-individual drug absorption, distribution, metabolism, and elimination (ADME), as well as therapeutic efficacy and adverse drug reactions (ADRs). Variations are categorized using the standardized star-allele ($\text{*}$) nomenclature curated by the Pharmacogene Variation Consortium (PharmVar) and translated into clinical guidelines by the Clinical Pharmacogenetics Implementation Consortium (CPIC). Key clinical targets include Phase I Cytochrome P450 enzymes (CYP2D6, CYP2C19, CYP2C9), Phase II conjugating enzymes (TPMT, NUDT15, DPYD, UGT1A1), drug transporters (SLCO1B1), and Human Leukocyte Antigen (HLA) alleles (HLA-B*57:01, HLA-B*15:02, HLA-B*58:01).
1. Pharmacogenomic Foundations, Star-Allele Nomenclature & Phenotypes
+----------------------------------------------------------------------------------------------------+
| STAR-ALLELE (\*) FUNCTIONAL SCORING |
+-------------------+-------------------+-------------------+----------------------------------------+
| Functional Group | Assigned Activity | Hallmark Star (\*)| Biochemical & In Vivo Consequence |
| | Score per Allele | Allele Examples | |
+-------------------+-------------------+-------------------+----------------------------------------+
| **Reference / | **1.0** | **\*1** | Fully functional wild-type enzyme |
| Normal Function** | | | activity; standard baseline |
+-------------------+-------------------+-------------------+----------------------------------------+
| **Decreased | **0.5** (or 0.25) | **CYP2D6\*10, | Partial activity; missense changes |
| Function** | | \*17, \*41; | impairing substrate affinity or |
| | | CYP2C9\*2** | splicing efficiency |
+-------------------+-------------------+-------------------+----------------------------------------+
| **No Function / | **0.0** | **CYP2D6\*3, \*4, | Total absence of active enzyme; |
| Null Allele** | | \*5; CYP2C19\*2, | frameshift, premature stop, canonical |
| | | \*3; DPYD\*2A** | splice defect, or whole-gene deletion |
+-------------------+-------------------+-------------------+----------------------------------------+
| **Increased | **> 1.0** (or gene| **CYP2C19\*17; | Promoter gain-of-function mutations or |
| Function** | copy multiplier) | CYP2D6\*1xN, \*2xN| genomic tandem gene duplications |
+-------------------+-------------------+-------------------+----------------------------------------+
+----------------------------------------------------------------------------------------------------+
| METABOLIZER PHENOTYPE STRATIFICATION |
+-------------------+-------------------+------------------------------------------------------------+
| Phenotype Category| Activity Score | In Vivo Drug Dynamics & Clinical Impact |
+-------------------+-------------------+------------------------------------------------------------+
| **Poor | **AS = 0.0** | **Active Drug**: Toxic drug accumulation -> Reduce dose. |
| Metabolizer (PM)**| (Two null alleles)| **Prodrug**: Inability to bioactivate -> Therapeutic failure|
+-------------------+-------------------+------------------------------------------------------------+
| **Intermediate | **AS = 0.5 to 1.0**| Reduced clearance; modest risk of toxicity or reduced |
| Metabolizer (IM)**| (Decreased/null) | prodrug activation; often requires moderate dose titration |
+-------------------+-------------------+------------------------------------------------------------+
| **Normal / Exten- | **AS = 1.25 to | Standard therapeutic drug clearance and expected bio- |
| sive (NM / EM)** | 2.0** | activation; standard dosing per manufacturer label |
+-------------------+-------------------+------------------------------------------------------------+
| **Rapid / Ultra- | **AS > 2.0** | **Active Drug**: Hyper-clearance -> Subtherapeutic efficacy|
| rapid (RM / UM)** | (Gain/Duplication)| **Prodrug**: Rapid conversion -> Severe acute overdose risk|
+-------------------+-------------------+------------------------------------------------------------+
ACTIVE DRUG VS. PRODRUG METABOLIC DYNAMICS
[ ACTIVE PARENT DRUG ] ----(CYP Metabolism)----> [ INACTIVE METABOLITE ]
* Poor Metabolizer: Drug accumulation -> Severe Toxicity! (Lower Dose Needed)
* Ultrarapid Metabolizer: Rapid clearance -> Therapeutic Failure! (Higher Dose Needed)
[ INACTIVE PRODRUG ] ----(CYP Bioactivation)--> [ ACTIVE PHARMACOLOGICAL AGENT ]
* Poor Metabolizer: Zero active drug formed -> Therapeutic Failure! (Switch Drug!)
* Ultrarapid Metabolizer: Massive burst of active drug -> Fatal Toxicity! (Contraindicated!)
2. The Core Cytochrome P450 Enzymes: CYP2D6, CYP2C19, and CYP2C9
+----------------------------------------------------------------------------------------------------+
| MAJOR CYTOCHROME P450 PHARMACOGENES |
+-------------------+-------------------+-------------------+----------------------------------------+
| Gene & Chromosome | Common Star (\*) | Substrate Drugs | Key Clinical Consequences & Guidance |
| | Alleles & Nature | | |
+-------------------+-------------------+-------------------+----------------------------------------+
| **CYP2D6** | **\*1** (Normal) | **Codeine, | **Prodrugs (Codeine/Tramadol/Tamoxifen)**:|
| (22q13.2) | **\*4** (Splice) | Tramadol, | PMs have no analgesia / poor endocrine |
| Non-inducible; | **\*5** (Deletion)| Tamoxifen, | response. UMs have **fatal morphine |
| >100 alleles; | **\*10, \*17, \*41| Fluoxetine, | overdose**; codeine contraindicated in |
| Pseudogenes | **\*1xN, \*2xN** | Metoprolol, | nursing mothers and pediatric tonsil- |
| *CYP2D7/8* | (Duplications) | Venlafaxine** | lectomy patients |
+-------------------+-------------------+-------------------+----------------------------------------+
| **CYP2C19** | **\*1** (Normal) | **Clopidogrel | **Clopidogrel (Prodrug)**: PMs and IMs |
| (10q23.33) | **\*2** (Splice) | (Plavix), | (\*2, \*3) fail to form active thiol |
| Highly polymorphic| **\*3** (Stop) | Omeprazole (PPIs),| metabolite -> High risk of stent throm-|
| in East Asians | **\*17** (Promoter| Citalopram, | bosis; CPIC mandates alternative |
| | Gain) | Voriconazole** | antiplatelet (Prasugrel, Ticagrelor) |
+-------------------+-------------------+-------------------+----------------------------------------+
| **CYP2C9** | **\*1** (Normal) | **Warfarin | S-warfarin is 3–5x more potent than |
| (10q23.33) | **\*2** (Arg144Cys)| (S-enantiomer), | R-warfarin. \*2 and \*3 carriers have |
| Major hepatic | **\*3** (Ile359Leu)| Phenytoin, | severely impaired clearance -> Extreme |
| Phase I enzyme | | Celecoxib** | bleeding risk on standard doses |
+-------------------+-------------------+-------------------+----------------------------------------+
Molecular Challenges in CYP2D6 Testing
Testing CYP2D6 is technically demanding due to the adjacent pseudogenes CYP2D7 and CYP2D8, which share $>90%$ sequence homology. Gene conversions, hybrid genes (e.g., CYP2D6/CYP2D7 fusions), whole-gene deletions (*5), and high-order copy number amplifications (*1xN, *2xN, *4xN) require combining Long-Range PCR (XL-PCR), Multiplex Ligation-dependent Probe Amplification (MLPA), or quantitative TaqMan Copy Number Assays (CNV) to avoid miscalling duplications of inactive alleles.
3. Warfarin Sensitivity: The CYP2C9, VKORC1, and CYP4F2 Dosing Triad
Warfarin is a narrow therapeutic index anticoagulant administered as a racemic mixture of $R$- and $S$-enantiomers. Inter-individual therapeutic maintenance dose requirements vary up to 10-fold across patients.
WARFARIN PHARMACOGENOMIC MECHANISM
S-Warfarin (Potent) Dietary Vitamin K
| |
v (CYP2C9 Inactivation) v
Inactive Hydroxy-Warfarin Vitamin K Epoxide (Inactive)
| ^
| | (Inhibited by Warfarin!)
VKORC1 Reductase |
| |
v |
Vitamin K Hydroquinone (Active)
|
v (GGCX Carboxylation)
Clotting Factors II, VII, IX, X
(Functional Anticoagulation!)
- CYP2C9 (**2, *3): Inactivates potent $S$-warfarin via 7-hydroxylation. The $*2$ (c.430C>T) and $*3$ (c.1075A>C) alleles reduce enzymatic activity by $\sim 30%$ and $\sim 80–90%$, respectively, prolonging $S$-warfarin half-life.
- VKORC1 (c.-1639G>A, rs9923231): Located in the promoter region of Vitamin K Epoxide Reductase Complex Subunit 1 on chromosome 16p11.2. The variant $A$ allele suppresses transcription factor binding, reducing baseline VKORC1 protein expression. Individuals harboring $A/A$ or $G/A$ genotypes require significantly lower warfarin doses to achieve therapeutic target INR ($2.0–3.0$).
- CYP4F2 (c.1297G>A, p.Val433Met, **3): Removes vitamin K from the cycle by converting vitamin $\text{K}_1$ to hydroxyvitamin $\text{K}_1$. The $*3$ variant reduces enzyme activity, shunting more vitamin K toward clotting factor activation and requiring slightly higher warfarin maintenance doses.
4. Phase II Enzymes & Chemotherapy Pharmacogenomics
+----------------------------------------------------------------------------------------------------+
| CHEMOTHERAPY PHARMACOGENOMICS & SEVERE TOXICITY GENES |
+-------------------+-------------------+-------------------+----------------------------------------+
| Gene & Enzyme | Critical Alleles | Drug Substrates | Clinical Manifestation & Guidelines |
+-------------------+-------------------+-------------------+----------------------------------------+
| **TPMT** | **\*2** (c.238G>C)| **6-Mercaptopurine| Inactivates thiopurines by S-methyl- |
| (6p22.3) | **\*3A** (c.460A+ | (6-MP), | ation. Homozygous null or compound |
| S-methyltransferase| c.719G in cis) | Azathioprine (AZA)| heterozygotes shunt drugs into cyto- |
| | **\*3C** (c.719A>G)| 6-Thioguanine (6TG)| toxic TGNs -> **Lethal bone marrow |
| | | | aplasia / pancytopenia**; reduce dose |
| | | | by 90% or switch agents |
+-------------------+-------------------+-------------------+----------------------------------------+
| **NUDT15** | **\*3** | **6-Mercaptopurine| Hydrolyzes toxic thiopurine triphos- |
| (13q14.2) | (c.415C>T, | Azathioprine** | phates (dTGTP); **Primary genetic |
| Nudix Hydrolase | p.Arg139Cys) | | driver of thiopurine-induced leukopenia|
| | | | in East Asians & Hispanics** |
+-------------------+-------------------+-------------------+----------------------------------------+
| **DPYD** | **\*2A** (c.1905+1| **5-Fluorouracil | Rate-limiting catabolic enzyme (>80% of|
| (1p21.3) | G>A splice) | (5-FU), | 5-FU cleared by DPYD). Non-functional |
| Dihydropyrimidine | **\*13** (c.1679T>G)| Capecitabine** | alleles cause drug accumulation -> |
| Dehydrogenase | **c.2846A>T** | | **Fatal neurotoxicity, mucositis, |
| | **HapB3** | | myelosuppression, and diarrhea** |
+-------------------+-------------------+-------------------+----------------------------------------+
| **UGT1A1** | **\*28** | **Irinotecan | Glucuronidates active SN-38 metabolite.|
| (2q37.1) | (c.-53_-52insTA, | (Camptosar)** | Homozygous \*28/\*28 (TA)7/(TA)7 alleles|
| Glucuronosyl- | (TA)$_7$ promoter | | (Gilbert syndrome) have impaired SN-38 |
| transferase | vs (TA)$_6$ \*1) | | clearance -> **Severe neutropenia & |
| | | | life-threatening diarrhea** |
+-------------------+-------------------+-------------------+----------------------------------------+
5. HLA-Mediated Severe Cutaneous Adverse Reactions (SCAR) & Transporters
Severe, life-threatening idiosyncratic drug reactions are frequently mediated by specific Human Leukocyte Antigen (HLA) class I alleles that bind small-molecule drugs directly in the antigen-binding groove, triggering massive cytotoxic T-cell attacks against skin and viscera (Stevens-Johnson Syndrome [SJS], Toxic Epidermal Necrolysis [TEN], and Drug Reaction with Eosinophilia and Systemic Symptoms [DRESS]).
+----------------------------------------------------------------------------------------------------+
| HLA PHARMACOGENOMIC CONTRAINDICATIONS & TRANSPORTER GENES |
+-------------------+-------------------+-------------------+----------------------------------------+
| Pharmacogene | Associated Drug | Population Risk | Clinical Mandate & Pathology |
+-------------------+-------------------+-------------------+----------------------------------------+
| **HLA-B\*57:01** | **Abacavir** | Pan-ethnic | **Abacavir Hypersensitivity (AHR)**: |
| | (Ziagen / Triumeq | (~5–8% Caucasians)| Multi-organ immune reaction; **Testing |
| | for HIV-1)** | | is mandatory before prescribing; |
| | | | 100% negative predictive value** |
+-------------------+-------------------+-------------------+----------------------------------------+
| **HLA-B\*15:02** | **Carbamazepine** | Southeast Asians, | **SJS / TEN**: Severe epidermolysis; |
| | (Tegretol), | Han Chinese | FDA black-box warning; mandatory test |
| | Oxcarbazepine | (10–15% prevalence)| in high-prevalence Asian ancestry |
+-------------------+-------------------+-------------------+----------------------------------------+
| **HLA-A\*31:01** | **Carbamazepine** | Caucasians, | **DRESS / SJS / Maculopapular exanthem**|
| | | Japanese | Broad hypersensitivity spectrum |
+-------------------+-------------------+-------------------+----------------------------------------+
| **HLA-B\*58:01** | **Allopurinol** | Han Chinese, | **Allopurinol SCAR / SJS / DRESS**: |
| | (Zyloprim for | Koreans, Thai | Severe cutaneous reactions in gout |
| | gout/hyperuricemia)| (~10–15%) | patients; strongly recommended screen |
+-------------------+-------------------+-------------------+----------------------------------------+
| **SLCO1B1** | **Simvastatin** | Pan-ethnic | Encodes OATP1B1 hepatic uptake pump; |
| (12p12.1) | (Zocor), | (\*5 allele freq | \*5 (c.521T>C) impairs hepatic uptake, |
| *SLCO1B1\*5* | Atorvastatin | $\sim 15\%$) | elevating systemic blood levels -> |
| (p.Val174Ala) | | | **Severe statin-induced rhabdomyolysis**|
+-------------------+-------------------+-------------------+----------------------------------------+
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Test Your Knowledge
A 62-year-old patient undergoing percutaneous coronary intervention with drug-eluting stent placement is prescribed clopidogrel (Plavix). Pharmacogenomic testing reveals that the patient is homozygous for the CYP2C19*2 allele (*2/*2). What is the metabolic consequence of this genotype, and what clinical action is indicated?
A
B
C
D
Test Your Knowledge
Prior to initiating abacavir therapy for a patient newly diagnosed with HIV-1, why does clinical practice strictly require molecular testing for the HLA-B*57:01 allele?
A
B
C
D
Test Your Knowledge
A 7-year-old patient with newly diagnosed acute lymphoblastic leukemia (ALL) is scheduled to begin maintenance chemotherapy with 6-mercaptopurine (6-MP). Molecular testing identifies homozygous TPMT*3A alleles (*3A/*3A). What is the biochemical consequence of this genotype upon exposure to standard doses of 6-MP?
A
B
C
D