rs1080985 — CYP2D6 *2A promoter

CYP2D6 Promoter *2A — The Upstream Switch That Controls Drug Metabolism

CYP2D6 is responsible for metabolizing roughly 25% of all prescribed medications, yet the amount of CYP2D6 enzyme your liver produces is not fixed — it varies with a cluster of regulatory variants that act like volume controls on the gene. One of the most consistently studied of these is rs1080985¹¹ rs1080985
the -1584C>G polymorphism located in the CYP2D6 promoter region, approximately 1,584 base pairs upstream of the coding sequence, a nucleotide change that alters how efficiently the gene is transcribed into protein. The C allele at this position is associated with reduced promoter activity — meaning cells produce less CYP2D6 enzyme, and drug metabolism is correspondingly slower.

The Mechanism

The GRCh38 reference allele at rs1080985 is G (the most common allele in European and global populations). The minor C allele is found in roughly 17% of Europeans but is nearly absent in East Asian and African populations. This strong ancestry-stratification is itself a clue about selective pressure on CYP2D6 expression levels.

A landmark 2001 study using 76 human liver biopsies²² A landmark 2001 study using 76 human liver biopsies
Zanger et al. Comprehensive analysis of the genetic factors determining expression and function of hepatic CYP2D6. Pharmacogenetics, 2001 established that individuals carrying the G allele at position -1584 had consistently higher CYP2D6 protein levels and enzyme activity compared to C allele carriers. The G allele functions as a stronger promoter, driving higher transcription rates. The C allele, by contrast, creates a less efficient promoter, resulting in reduced CYP2D6 expression — even when the coding sequence itself is intact.

This promoter variant is closely linked — though not identical — to the CYP2D6*2A haplotype, which combines this promoter change with coding variants including rs16947 (Arg296Cys). The rs1080985 variant is also in high linkage disequilibrium with the CYP2D6 enhancer variant rs5758550, located over 100 kb downstream. The net functional outcome therefore depends on the full haplotype context, but the -1584C allele consistently tracks with reduced expression in liver expression studies.

The Evidence

The strongest clinical evidence for rs1080985 comes from studies of donepezil — a cholinesterase inhibitor used to treat Alzheimer's disease that is substantially metabolized by CYP2D6. Because donepezil requires an active enzyme for clearance, lower CYP2D6 activity (C allele) was hypothesized to result in higher drug accumulation and, counterintuitively, better therapeutic response — or alternatively, altered exposure patterns affecting efficacy.

A prospective cohort of 127 Alzheimer's patients³³ A prospective cohort of 127 Alzheimer's patients
Pilotto et al. Effect of a CYP2D6 polymorphism on the efficacy of donepezil in patients with Alzheimer disease. Neurology, 2009 found that G allele carriers (normal/higher CYP2D6 expression) had a significantly lower likelihood of poor donepezil response — OR 3.43 (95% CI 1.49–7.90, p=0.013) — meaning C allele carriers were over three times more likely to respond poorly to the drug. This landmark finding was replicated in a larger study of 415 patients (OR 1.74, p=0.04).

A 2016 meta-analysis of 1,266 donepezil-treated patients⁴⁴ A 2016 meta-analysis of 1,266 donepezil-treated patients
Xiao et al. Effect of the CYP2D6 and APOE Polymorphisms on the Efficacy of Donepezil. CNS Drugs, 2016 confirmed that rs1080985 C allele carriers have a significantly elevated risk of poor donepezil response, and found that carrying both the C allele and APOE-ε4 further increased non-response risk (OR 1.73, p=0.03).

A smaller pharmacokinetic study of 40 patients⁵⁵ A smaller pharmacokinetic study of 40 patients
Chou et al. Impact of the CYP2D6 SNP on the concentration of and therapeutic response to donepezil. J Formos Med Assoc, 2022 found that G/G homozygotes had higher donepezil plasma concentrations and a 90% response rate compared to 50% in C allele carriers (OR 9.0, p=0.015). The underlying mechanism may involve CYP2D6 channeling donepezil into specific metabolic pathways that modulate the drug's binding properties in the brain, though this remains under investigation.

For broader drug metabolism, Llerena et al.⁶⁶ Llerena et al.
Llerena et al. CYP2D6 -1584C>G promoter polymorphism and debrisoquine ultrarapid hydroxylation. Pharmacogenomics, 2013 showed in 320 healthy volunteers that G allele carriers had lower metabolic ratios (faster debrisoquine hydroxylation), confirming the G allele's promoter-boosting effect on CYP2D6 enzyme activity.

Not all studies show consistent effects — three negative replication studies in specific ethnic populations (Polish, Chinese) found no significant association with donepezil response, highlighting that rs1080985's impact interacts with background haplotypes and is not identical across all ancestries.

Practical Actions

For individuals carrying one or two C alleles, the most clinically actionable implication is awareness before initiating donepezil therapy in Alzheimer's disease, or when prescribed other CYP2D6-substrate medications. The C allele's reduced promoter efficiency means the liver produces less CYP2D6 enzyme, shifting drug metabolism toward slower clearance — which affects both prodrug activation (e.g. codeine, which needs CYP2D6 to form morphine) and active drug elimination (e.g. tricyclic antidepressants that accumulate when metabolism is slow).

The practical implication depends on the specific drug: for prodrugs (codeine, tramadol), lower CYP2D6 activity means less activation and potentially reduced efficacy. For active drugs metabolized by CYP2D6 (many antidepressants, tamoxifen, beta-blockers), lower activity means slower elimination and higher plasma concentrations. Discuss your rs1080985 genotype with prescribing clinicians before starting any high-priority CYP2D6-substrate drug.

Interactions

rs1080985 interacts closely with rs16947 (CYP2D6*2 coding variant, Arg296Cys) and the enhancer SNP rs5758550. The net CYP2D6 activity score for any individual depends on the complete haplotype structure across these loci, not on any single SNP in isolation. In particular, a CYP2D6*2A haplotype that carries the -1584C allele plus the rs16947 A allele and lacks the rs5758550 enhancer variant may show meaningfully reduced activity compared to an individual with the G allele at all three positions. Full clinical CYP2D6 pharmacogenomic testing that resolves star alleles and copy number provides a more complete picture than individual SNPs. The CYP2D6*4 variant (rs3892097) is the major no-function allele in Europeans; if combined with a reduced-function *2A haplotype, the diplotype may approach intermediate metabolizer territory.