rs72547515 — CYP1A2 *16 (Arg377Gln)

Nearly-inactive CYP1A2 missense variant — carriers have severely impaired metabolism of caffeine, theophylline, tizanidine, and other CYP1A2 substrates

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CYP1A2*16 — The Near-Silent Metabolizer Allele

CYP1A2 is the liver enzyme responsible for clearing approximately 95% of the caffeine you consume, along with clinically critical medications including the muscle relaxant tizanidine, the antipsychotic clozapine, the bronchodilator theophylline, and the antidepressant melatonin. The rs72547515 variant, catalogued as the [CYP1A2*16 star allele | Star alleles are named variants in drug-metabolizing genes that define activity level; *16 denotes a severely reduced-function allele], carries a missense substitution at codon 377 that converts arginine to glutamine (Arg377Gln, c.1130G>A). This change nearly abolishes enzymatic activity.

The Mechanism

Arginine-377 sits within a structurally critical region of the CYP1A2 protein. Saito et al. (2005)11 Saito et al. (2005)
Saito Y et al. Functional analysis of three CYP1A2 variants found in a Japanese population. Drug Metab Dispos, 2005 showed that the Arg377Gln substitution reduces CYP1A2 protein expression to approximately 30% of wild-type, and — more significantly — increases the Km (substrate binding constant) approximately 9-fold while reducing Vmax to less than 3% and intrinsic clearance (Vmax/Km) to less than 1% of the wild-type enzyme. The authors concluded that Arg377 is a critical residue for producing catalytically active CYP1A2 holoenzyme, likely required for correct protein folding and incorporation of the heme cofactor that drives oxidation chemistry. A subsequent study by Ito et al. (2015)22 Ito et al. (2015)
Ito M et al. Functional characterization of 20 allelic variants of CYP1A2. Drug Metab Pharmacokinet, 2015 characterizing 20 CYP1A2 alleles confirmed that CYP1A2*16 is inactive toward both phenacetin and 7-ethoxyresorufin — the two canonical CYP1A2 substrates used in functional assays.

The Evidence

The functional data for CYP1A2*16 are derived from recombinant expression systems rather than clinical pharmacokinetic studies, which is unsurprising given the extreme rarity of the allele (approximately 1–4 carriers per 10,000 people in Japanese cohorts, and even rarer globally). For CYP1A2 substrates, the pharmacokinetic consequences of near-complete enzyme inactivation are well-understood from drug interaction studies using CYP1A2 chemical inhibitors. For tizanidine, Backman et al. (2008)33 Backman et al. (2008)
Backman JT et al. Effects of gender and moderate smoking on the pharmacokinetics and effects of the CYP1A2 substrate tizanidine. Eur J Clin Pharmacol, 2008 demonstrated that even partial CYP1A2 suppression (via smoking cessation) raises tizanidine plasma levels substantially; a moderate CYP1A2 inhibitor (mexiletine) increased tizanidine peak concentration and area-under-the-curve significantly in a separate study (PMID 19789372). For theophylline, a narrow-therapeutic- index bronchodilator, CYP1A2 handles the majority of clearance — reduced enzyme function elevates plasma concentrations into the toxic range (arrhythmia, seizures). For caffeine, the half-life extends from approximately 4–6 hours in normal metabolizers to potentially 12–24+ hours when CYP1A2 is severely impaired.

The Dutch Pharmacogenetics Working Group (DPWG) reviewed CYP1A2 variants for clozapine and olanzapine in its 2023 guideline update and concluded that no dose adjustment is warranted solely on the basis of CYP1A2 genotype for these antipsychotics, citing lack of demonstrated pharmacokinetic effect in available studies. This conclusion was based on the more common polymorphisms (rs762551 and similar); it does not specifically evaluate the severely reduced-function *16 allele.

Practical Implications

Because rs72547515 is one of the rarest pharmacogenomic variants in the CYP1A2 gene, heterozygous carriers (AG) are the only clinically relevant genotype; homozygous AA individuals are essentially non-existent in population data. A single copy of the *16 allele reduces overall CYP1A2 capacity partially — the remaining wild-type allele continues to produce functional enzyme, but total metabolic throughput is reduced. The practical impact is most significant when CYP1A2-metabolized drugs are prescribed at standard doses, or when multiple CYP1A2 substrates are used simultaneously.

Carriers should inform prescribers before starting tizanidine (muscle relaxant), theophylline or aminophylline (asthma/COPD), or any other narrow-therapeutic- index CYP1A2 substrate. Therapeutic drug monitoring (plasma level measurement) is a straightforward risk-reduction strategy for drugs with established target ranges.

Interactions

CYP1A2 activity is strongly modulated by inducers (cigarette smoke, charcoal- grilled meat, omeprazole, cruciferous vegetables) and inhibitors (fluvoxamine, ciprofloxacin, oral contraceptives, vemurafenib). For a carrier of the *16 allele, starting a strong CYP1A2 inhibitor like fluvoxamine on top of a reduced-function allele creates additive suppression; starting an inducer might partially compensate. The common CYP1A2 variant rs762551 (*1F) acts via inducibility — its effect is modified by smoking; the *16 variant is a structural (protein-level) defect that inducers cannot overcome. Carriers should be aware that this variant interacts differently with environmental exposures than the more common CYP1A2 regulatory polymorphisms.

Drug Interactions

caffeine dose_adjustment literature
theophylline increased_toxicity DPWG
tizanidine increased_toxicity FDA
clozapine dose_adjustment literature
olanzapine dose_adjustment literature
melatonin dose_adjustment literature

Nutrient Interactions

caffeine altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal Metabolizer Normal

Standard CYP1A2 activity — typical caffeine and drug metabolism

You carry two copies of the reference (G) allele at rs72547515. Your CYP1A2*16 allele is absent, meaning your CYP1A2 enzyme is not impaired by this specific variant. This is by far the most common genotype — approximately 99.98% of people worldwide share this result. Your caffeine clearance and metabolism of CYP1A2-dependent medications proceeds through the normal range for this variant (other CYP1A2 variants such as rs762551 may still affect your metabolism separately).

AG Intermediate Metabolizer Intermediate

One CYP1A2*16 allele — reduced CYP1A2 capacity, drug interactions warrant attention

In functional studies, CYP1A2*16 produced less than 3% of wild-type Vmax and less than 1% of wild-type intrinsic clearance (Saito et al. 2005, PMID 16174806). A second independent study (Ito et al. 2015, PMID 26022657) confirmed complete inactivity of the *16 allele toward canonical CYP1A2 substrates. As a heterozygous carrier, your functional CYP1A2 capacity is driven almost entirely by your normal (GG) allele. The degree of total capacity reduction depends on allele dosage and protein expression levels — empirical pharmacokinetic data specific to heterozygous *16 carriers are not available given the allele's rarity, but the structural nature of the defect (a misfolded enzyme, not simply a regulatory change) means the affected allele contributes negligibly to substrate clearance regardless of induction status.

AA Poor Metabolizer Poor

Homozygous CYP1A2*16 — near-complete CYP1A2 loss of function

Both copies of your CYP1A2 gene carry the Arg377Gln substitution, which in functional studies produced less than 3% of wild-type Vmax and less than 1% of wild-type intrinsic clearance per allele. With both alleles affected, residual CYP1A2 activity would be negligible; minor alternative pathways (CYP3A4, CYP2E1) can partially compensate for some substrates (e.g., acetaminophen metabolites), but for caffeine and theophylline, no adequate backup pathway exists. The clinical situation would resemble pharmacological CYP1A2 inhibition with a strong inhibitor such as fluvoxamine — where tizanidine is contraindicated and theophylline doses need to be reduced by 50% or more. Given the extreme rarity of this genotype, a phenotyping confirmation (caffeine metabolic ratio test) is advisable before making irreversible clinical decisions.