rs72547511 — CYP1A2 CYP1A2*15 (Pro42Arg)

Rare CYP1A2 missense variant at the critical Pro42 position; nearly abolishes enzyme activity, impairing metabolism of caffeine, clozapine, theophylline, and tizanidine

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CYP1A2 Pro42Arg — A Near-Silent Enzyme

Buried in exon 2 of CYP1A2 lies a single nucleotide change that can reduce the enzyme's activity to less than 1% of normal. The rs72547511 variant — replacing the amino acid proline with arginine at position 42 of the protein — is rare but clinically consequential for anyone who carries it and takes medications that depend on CYP1A2 for clearance.

CYP1A211 CYP1A2
CYP1A2 (Cytochrome P450 Family 1 Subfamily A Member 2) is the dominant hepatic enzyme for metabolizing caffeine, theophylline, clozapine, olanzapine, tizanidine, and melatonin. Together, CYP1A2 accounts for roughly 95% of caffeine clearance and a large fraction of several psychiatric medication pathways.

The Mechanism

Proline at position 42 sits in the N-terminal transmembrane anchor of CYP1A2, a region critical for correct membrane insertion and subsequent hemoprotein[| hemoprotein: a protein containing a heme iron group, essential for the oxidative chemistry CYP enzymes perform] folding. When arginine replaces proline, the structural rigidity provided by proline's cyclic sidechain is lost, and the resulting protein fails to properly incorporate its heme iron center.

Saito et al. (2005)22 Saito et al. (2005)
Saito Y et al. Functional analysis of three CYP1A2 variants found in a Japanese population. Drug Metab Dispos, 2005 expressed the Pro42Arg variant in Chinese hamster V79 cells and measured reduced CO difference spectra — a direct measure of functional hemoprotein formation. The peak at 450 nm characteristic of active CYP enzyme was barely detectable in the Pro42Arg variant, despite protein expression at ~66% of wild-type levels. Catalytic efficiency (Vmax/Km) for 7-ethoxyresorufin O-deethylation was less than 1% of wild type; phenacetin O-deethylation was similarly abolished. The researchers concluded that Pro42 is a critical residue for producing catalytically active CYP1A2.

The Evidence

The Pro42Arg change is catalogued as the CYP1A2*15 allele. Ito et al. (2015)33 Ito et al. (2015)
Ito M et al. Functional characterization of 20 allelic variants of CYP1A2. Drug Metab Pharmacokinet, 2015 confirmed CYP1A2*15 belongs to the group of variants with inactive or near-absent enzymatic function, alongside *4, *6, *8, *16, and *21.

For clozapine — an antipsychotic with a narrow therapeutic window[| narrow therapeutic window: the dose range between efficacy and toxicity is small, so small PK changes matter greatly] — Kappel et al. (2024)44 Kappel et al. (2024)
Kappel DB et al. Rare variants in pharmacogenes influence clozapine metabolism in individuals with schizophrenia. Eur Neuropsychopharmacol, 2024 found that rare loss-of-function CYP1A2 variants collectively had the strongest pharmacogenomic effect of any gene on clozapine metabolism (β = 0.324, p = 0.009). Carriers of rare no-function alleles showed substantially elevated dose-adjusted clozapine concentrations.

A systematic review and meta-analysis by Santes-Palacios et al. (2018)55 systematic review and meta-analysis by Santes-Palacios et al. (2018)
Santes-Palacios R et al. The impact of genetic polymorphisms on CYP1A2 activity in humans: a systematic review and meta-analysis. Pharmacogenomics J, 2018 confirmed the importance of CYP1A2 genetic variation for inter-individual differences in drug clearance, though most studies focus on the common rs762551 variant. Rare coding variants like Pro42Arg have larger per-allele effects but lower population impact due to their rarity.

Practical Actions

This variant's rarity means most people will never encounter it. For those who carry one copy: caffeine half-life can extend significantly — a morning coffee may still be clearing at bedtime. For CYP1A2-metabolized medications, standard doses can produce plasma concentrations well above therapeutic targets. Prescribers should be aware when treating carriers with theophylline (asthma, COPD), clozapine or olanzapine (schizophrenia), or tizanidine (spasticity/muscle spasm).

A note on the second alternate allele: rs72547511 is multiallelic — the C>T change at the same position produces Pro42Leu. This variant has not been functionally characterized in the published literature as of the research date, but given that Pro42 is structurally critical, Pro42Leu likely also impairs CYP1A2 function.

Interactions

Carriers of rs72547511 (CYP1A2*15) who are also homozygous for the rs762551 C allele (slow inducibility) face a compounded reduction: one allele near-absent in activity, the other with reduced inducibility. For drugs like clozapine, this combination could produce clozapine levels equivalent to those seen during CYP1A2 inhibitor co-administration (e.g., ciprofloxacin, fluvoxamine), both of which are associated with toxic clozapine concentrations in case reports.

Drug Interactions

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

Nutrient Interactions

caffeine altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

CC Normal Metabolizer Normal

Normal CYP1A2 activity at this position

You carry the common reference allele at this position. Your CYP1A2 Pro42 residue is intact, meaning this variant does not impair your enzyme's heme-binding capacity. The vast majority of people worldwide share this genotype at rs72547511.

CG Reduced Metabolizer (Carrier) Reduced

One copy of the near-inactive CYP1A2*15 allele — reduced overall CYP1A2 capacity

CYP1A2 operates in an additive manner: with one near-zero activity allele, total hepatic enzyme output drops significantly. For drugs with a narrow therapeutic window (theophylline, clozapine, tizanidine), even a 40–50% reduction in clearance can shift plasma concentrations from therapeutic to toxic at standard doses.

Functional studies in Chinese hamster V79 cells expressing the Pro42Arg variant found reduced CO-difference spectral peaks, indicating failure of heme group incorporation despite near-normal protein expression levels. This suggests the mechanism is primarily a folding/assembly defect, not a transcription or translation defect.

Environmental CYP1A2 inducers (cigarette smoking, cruciferous vegetables) may partially compensate by upregulating the remaining wild-type allele, but this cannot rescue activity from the variant allele.

GG Poor Metabolizer (Homozygous) Absent

Two copies of the near-inactive CYP1A2*15 allele — severely impaired CYP1A2 activity

With both CYP1A2 alleles carrying the Pro42Arg substitution, normal hepatic CYP1A2 catalytic capacity is essentially absent. Plasma concentrations of clozapine, theophylline, tizanidine, and other CYP1A2-primary substrates can rise 3- to 10-fold above expected levels on standard doses.

This is analogous to CYP2D6 or CYP2C19 poor metabolizer status, but with even fewer documented cases. Non-genetic CYP1A2 inducers (smoking, cruciferous vegetables, AhR-activating foods) cannot rescue function from structurally defective enzyme.

Rare homozygous cases like this require pharmacogenomically-guided dosing from the start of any CYP1A2-substrate therapy; standard titration protocols are not adequate.