The MAPT gene exists in two major haplotypes—H1 and H2—defined by an inversion of a 900 kb genomic region at 17q21.31[@baker1999]. The H1 haplotype is strongly associated with increased risk of sporadic 4R-tauopathies including Progressive Supranuclear Palsy (PSP), Corticobasal Degeneration (CBD), and Argyrophilic Grain Disease (AGD)[@sala2021].
H1 vs H2 Haplotypes
Haplotype Distribution by Disease
Progressive Supranuclear Palsy (PSP)
H1/H1 Frequency: >95% in sporadic PSP cases
Odds Ratio: 5.5-7.8 for H1/H1 vs other genotypes
Key Subhaplotype: H1c (also called H1P)
Reference: [Baker et al., 1999](https://doi.org/10.1093/hmg/8.4.711)
PSP shows the strongest association with the MAPT H1 haplotype, with nearly universal H1 homozygosity in sporadic cases.
Corticobasal Degeneration (CBD)
H1/H1 Frequency: 90-95%
Odds Ratio: 3.0-5.0
Key Subhaplotype: H1b
CBD shows similar but slightly weaker H1 association compared to PSP[@taga2021].
Argyrophilic Grain Disease (AGD)
H1/H1 Frequency: 80-85%
Odds Ratio: 1.5-2.5
AGD shows a more modest H1 association than PSP or CBD[@zeithmann2020].
Globular Glial Tauopathy (GGT)
H1/H1 Frequency: ~85%
Odds Ratio: 2.0-3.0
GGT shows intermediate H1 association with limited available data.
FTDP-17 (Familial)
Pathogenic MAPT mutations are fully penetrant regardless of haplotype. However, H1 may accelerate disease onset in mutation carriers. Different subhaplotypes interact with various mutations to modify phenotype.
Subhaplotypes and Fine-Structure
Beyond H1/H2, finer genetic substructure exists within the MAPT locus:
Functional Mechanisms
How H1 Increases Tauopathy Risk
Increased 4R Tau Expression: H1 haplotype is associated with increased exon 10-containing (4R) tau isoforms[@vandrovcova2019]
Altered Splicing Regulation: Specific variants influence splice site selection, favoring exon 10 inclusion
[Baker et al., Association of an extended haplotype in the tau gene with progressive supranuclear palsy (1999) (1999)](https://doi.org/10.1093/hmg/8.4.711)
[Sala Frigerio et al., The MAPT H1 haplotype is a risk factor for tauopathies: meta-analysis (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/33592689/)
[Taga et al., Association of MAPT haplotypes with progressive supranuclear palsy in a US cohort (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/34227150/)
[Zeithmann et al., H1 haplotype of MAPT and the risk of argyrophilic grain disease (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/32891426/)
[Vandrovcova et al., Differential association of the MAPT H1 subhaplotype with neurodegenerative diseases (2019) (2019)](https://doi.org/10.1016/j.neurobiolaging.2019.06.001)