rs72547511 — CYP1A2 CYP1A2*15 (Pro42Arg)

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.