rs72547515 — CYP1A2 *16 (Arg377Gln)

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.