rs4900442 — CYP46A1

Intronic variant in the brain cholesterol 24-hydroxylase gene; C allele associated with a higher 24S-hydroxycholesterol/cholesterol ratio in CSF and modestly elevated Alzheimer's disease risk, primarily in East Asian populations

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CYP46A1 — Brain Cholesterol Turnover and Alzheimer's Risk

Your brain makes its own cholesterol — and unlike the rest of your body, it has almost no way to get rid of it. The blood-brain barrier blocks cholesterol from simply diffusing out, so the brain relies almost entirely on a single enzyme, cholesterol 24-hydroxylase11 cholesterol 24-hydroxylase
encoded by CYP46A1, a cytochrome P450 enzyme expressed almost exclusively in neurons, to convert excess cholesterol into 24S-hydroxycholesterol (24S-OHC) — a metabolite that can cross the blood-brain barrier and exit the brain. This makes CYP46A1 the primary gatekeeper of brain cholesterol homeostasis and a direct regulator of how much cholesterol accumulates in neurons over time.

The Mechanism

rs4900442 sits in intron 3 of the CYP46A1 gene (the IVS3+43 C>T variant), placing it in a non-coding region that likely influences mRNA splicing efficiency or regulatory element binding rather than the amino acid sequence of the enzyme itself. The C allele, which is the reference allele and the globally more common variant (~55% frequency), appears to be associated with altered enzyme activity or expression — manifesting as a higher ratio of 24S-hydroxycholesterol to cholesterol in cerebrospinal fluid22 24S-hydroxycholesterol to cholesterol in cerebrospinal fluid
CSF levels of 24S-OHC reflect the rate of CYP46A1-catalysed cholesterol turnover in the brain; an elevated ratio suggests dysregulated flux among CC homozygotes with Alzheimer's disease. Disrupted brain cholesterol turnover is mechanistically plausible as an Alzheimer's risk pathway: cholesterol accumulation in neuronal membranes can promote amyloid-beta production and tau phosphorylation, both core features of AD pathology.

The Evidence

The original association was reported by Kölsch et al. in 200233 Kölsch et al. in 2002
Kölsch H et al. Polymorphism in the cholesterol 24S-hydroxylase gene is associated with Alzheimer's disease. Mol Psychiatry 2002;7:899–902 in a German cohort of 114 AD patients and 144 healthy controls. They found the C allele of the IVS3+43 C>T variant significantly more prevalent in AD patients, and additionally demonstrated that CC genotype carriers had an elevated 24S-OHC/cholesterol ratio in CSF — providing a direct biochemical link between genotype and brain cholesterol metabolism.

A 2016 meta-analysis by Jia et al.44 2016 meta-analysis by Jia et al.
Jia F et al. The association between CYP46A1 rs4900442 polymorphism and the risk of Alzheimer's disease: a meta-analysis. Neurosci Lett 2016;620:83–87 pooled six case-control studies covering 1,555 AD cases and 1,347 controls. The overall analysis found no statistically significant association (allele model OR=0.947), but a significant protective effect of the T allele emerged specifically in Chinese populations (OR=0.780, 95% CI 0.628–0.968), with no equivalent signal in Caucasian cohorts. This population-specific finding is consistent with the broader literature on CYP46A1 variants, where effect sizes and directions vary substantially across ancestries.

A related but distinct variant (rs754203, the intron 2 T/C polymorphism) has been more extensively studied and shows similarly mixed results in the largest meta-analysis to date — 21 studies, 4,875 AD cases, 4,874 controls — where the CC genotype of that neighboring variant confers modestly increased risk (recessive model OR=1.20) and the effect is amplified among APOE ε4 carriers55 amplified among APOE ε4 carriers
Li L et al. CYP46A1 T/C polymorphism associated with the APOE ε4 allele increases the risk of AD. J Neurol 2013;260:1446–1455. The rs4900442 and rs754203 variants are in partial linkage disequilibrium and likely tag overlapping haplotypes within the CYP46A1 locus.

The overall evidence picture is moderate: the biological mechanism is well-established (CYP46A1 controls the dominant route of brain cholesterol elimination), the CSF biomarker data provide functional support, but the genetic association is population-specific and the effect size is modest.

Practical Actions

The actionable implication of impaired CYP46A1 function focuses on modifiable factors that can influence brain cholesterol homeostasis through parallel routes. The most important consideration is the well-documented interaction with APOE genotype: the risk associated with CYP46A1 C alleles appears substantially amplified in APOE ε4 carriers, making combined genotyping particularly informative.

Efavirenz66 Efavirenz
an antiretroviral drug that acts as a low-dose CYP46A1 activator — repurposing studies are underway has been investigated at sub-therapeutic doses as a pharmacological activator of CYP46A1, and early research suggests it may restore normal brain cholesterol turnover. This is a research-stage observation and not a current clinical recommendation.

Statins do not readily cross the blood-brain barrier and do not directly substitute for CYP46A1 function in the brain; they act on peripheral cholesterol, leaving brain cholesterol metabolism largely untouched.

Interactions

The most clinically relevant interaction is with rs429358 (APOE ε4 determinant). Li et al. 2013 demonstrated that APOE ε4 carriers with the CC genotype at the related rs754203 locus (and by extension, likely at rs4900442) face significantly greater Alzheimer's disease risk than either risk factor alone would predict. CYP46A1 and APOE are both central to brain lipid metabolism, and their interaction is mechanistically coherent: impaired 24S-OHC production compounds with APOE ε4-driven amyloid accumulation.

The intron 2 variant rs754203 is the most extensively studied CYP46A1 polymorphism in Alzheimer's disease research. rs4900442 and rs754203 are in partial LD and likely tag the same underlying haplotype structure within the CYP46A1 locus. Their combined effect has not been formally modeled in published studies.

Genotype Interpretations

What each possible genotype means for this variant.

TT Protective Genotype Normal

Both copies of the protective T allele — standard brain cholesterol turnover

You carry two copies of the T allele, the less common but protective variant. Studies suggest that TT individuals have a lower prevalence of the CYP46A1 haplotype associated with elevated 24S-hydroxycholesterol/cholesterol ratios in cerebrospinal fluid. In the 2016 meta-analysis of Chinese populations, the T allele was associated with reduced Alzheimer's disease risk (OR=0.78). About 20% of people globally share this genotype, with higher frequency in South Asian (~25%) and East Asian (~23%) populations and lower frequency in people of African ancestry (~11%).

CT Heterozygous Intermediate

One copy each of the C (risk) and T (protective) alleles — intermediate profile

You carry one copy of the C (risk) allele and one T (protective) allele. Heterozygous individuals show an intermediate profile in CYP46A1 studies. The 2016 meta-analysis (1,555 AD cases, 1,347 controls) found a trend toward additive risk contribution with each C allele, though the dominant model did not reach significance in Caucasian populations. CT is the most common genotype globally, shared by roughly 50% of people across most ancestry groups.

CC Homozygous Risk High Risk

Two copies of the C allele — associated with altered brain cholesterol turnover and modestly elevated Alzheimer's disease risk in East Asian populations

The CYP46A1 enzyme is responsible for converting approximately 6–7 mg of cholesterol per day in the adult human brain into 24S-hydroxycholesterol (24S-OHC), the only significant route by which neurons can shed excess cholesterol. An elevated 24S-OHC/cholesterol ratio in CSF observed in CC genotype Alzheimer's patients may reflect compensatory upregulation of a partly dysregulated enzyme, or it may indicate downstream disruption of the cholesterol flux control point.

Mechanistically, the rs4900442 variant lies in intron 3 (IVS3+43) and does not change the encoded protein. Instead, it likely affects intronic regulatory elements — splicing enhancers or silencers — that modulate CYP46A1 mRNA levels or alternative splicing. This class of intronic variants can alter enzyme expression without altering its structure.

The interaction with APOE ε4 genotype is particularly important to know. Li et al. (2013, PMID 23070465) demonstrated across 9,749 participants that APOE ε4 carriers with the CC genotype at the CYP46A1 locus faced substantially higher AD risk than either risk factor alone would predict — consistent with two independent pathways (amyloid accumulation from APOE ε4; cholesterol clearance impairment from CYP46A1) converging on the same disease process.