rs242557 — MAPT H1c Sub-haplotype Tag

MAPT H1c — The Sub-haplotype That Amplifies Tauopathy Risk

Within the H1 haplotype of the MAPT gene — itself a well-established risk factor for neurodegenerative diseases involving tau protein — there exists a finer level of genetic variation. The rs242557 variant tags the H1c sub-haplotype, a distinct subset of H1 that carries substantially elevated risk for progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) compared to other H1 sub-haplotypes. This SNP adds resolution beyond the broad H1/H2 distinction captured by rs17649553, identifying individuals within the H1 majority who face the highest tau-related risk.

The Mechanism

Unlike the H1/H2 distinction — driven by a 900kb chromosomal inversion — H1c is a sub-haplotype within H1 defined by a specific combination of intronic variants including rs242557. The A allele at rs242557 tags the H1c configuration, which is associated with altered transcriptional regulation of the MAPT gene¹¹ tags the H1c configuration, which is associated with altered transcriptional regulation of the MAPT gene
The intronic variant likely affects splicing regulatory elements or transcription factor binding within the first large intron of MAPT.

A 2007 study found that the H1c haplotype specifically increases both the total level of MAPT transcripts and the proportion containing four microtubule-binding repeat domains (4R tau)²² A 2007 study found that the H1c haplotype specifically increases both the total level of MAPT transcripts and the proportion containing four microtubule-binding repeat domains (4R tau)
4R tau isoforms are the predominant component of pathological tau aggregates in PSP, CBD, and some forms of Alzheimer's disease. This mechanistic finding directly links the H1c genotype to the molecular pathology of 4R tauopathies: more total tau protein, with a higher fraction folded into the 4-repeat isoform that aggregates into neurofibrillary tangles and other tau deposits.

The Evidence

A 2017 meta-analysis of 82 case-control studies found that rs242557 (A allele) confers an odds ratio of 1.96 (95% CI 1.71–2.25) for PSP and 2.51 (95% CI 1.66–3.78) for CBD³³ A 2017 meta-analysis of 82 case-control studies found that rs242557 (A allele) confers an odds ratio of 1.96 (95% CI 1.71–2.25) for PSP and 2.51 (95% CI 1.66–3.78) for CBD
These effect sizes are among the largest identified for common variants in neurodegenerative disease risk. The CBD effect size (OR 2.51) is especially striking for a common variant, indicating that H1c roughly triples the risk of this rare condition compared to non-H1c individuals.

A 2015 GWAS of 219 CBD cases and 3,750 controls confirmed the H1c sub-haplotype marked by rs242557 as significantly associated with CBD risk (p = 7.91×10⁻⁶), and demonstrated that CBD and PSP share this genetic risk architecture⁴⁴ A 2015 GWAS of 219 CBD cases and 3,750 controls confirmed the H1c sub-haplotype marked by rs242557 as significantly associated with CBD risk (p = 7.91×10⁻⁶), and demonstrated that CBD and PSP share this genetic risk architecture
This shared genetics aligns with the clinical and neuropathological overlap between the two conditions.

For Alzheimer's disease, the H1c haplotype showed association with AD risk in two autopsy-confirmed case series (Myers et al., Human Molecular Genetics, 2005)⁵⁵ the H1c haplotype showed association with AD risk in two autopsy-confirmed case series (Myers et al., Human Molecular Genetics, 2005)
The AD association appears most relevant in APOE ε4 non-carriers, where MAPT haplotype becomes a more prominent risk factor. A 2007 quantitative trait study showed rs242557 drives the H1c association with cerebrospinal fluid tau levels in a dose-dependent manner (p = 0.002)⁶⁶ rs242557 drives the H1c association with cerebrospinal fluid tau levels in a dose-dependent manner (p = 0.002)
Higher CSF tau is a biomarker of neuronal injury and is elevated years before clinical Alzheimer's onset.

Practical Actions

The H1c genotype does not cause disease on its own — it modifies risk within a multifactorial landscape. Environmental and lifestyle factors that reduce tau hyperphosphorylation and aggregation are particularly relevant: avoiding head trauma, controlling metabolic risk factors for neurodegeneration (blood pressure, insulin resistance), and staying physically and cognitively active. No approved preventive pharmacotherapy targets tau pathology specifically in the presymptomatic period, but clinical trials of tau-targeting immunotherapies are ongoing.

Individuals carrying AA or AG genotypes at rs242557 who are also H1/H1 at rs17649553 face the combined burden of the broad H1 risk and the additional H1c refinement — neurological surveillance may be warranted if other risk factors (age, family history, additional genetic risk) are present.

Interactions

rs242557 adds independent risk stratification on top of the H1/H2 distinction captured by rs17649553. Nearly all individuals with the H1c sub-haplotype (A allele at rs242557) are also H1/H1 or H1/H2 at the broader haplotype level — these SNPs are in strong linkage disequilibrium but not perfectly correlated, so together they provide finer resolution. The rs2471738 variant (also listed among related SNPs) represents another H1 sub-haplotype marker with comparable effect sizes for PSP (OR 1.85) and CBD (OR 2.07), and may tag a partially overlapping but distinct risk subgroup.

In Alzheimer's disease, the MAPT H1c effect appears to interact with APOE status: the association is nominally significant in APOE ε4 carriers but may be more prominent in non-carriers, suggesting that H1c contributes to a distinct tauopathic path to AD independent of amyloid-driven mechanisms.