HNMT Thr105Ile

HNMT Thr105Ile - When the Tissue Histamine Enzyme Is Unstable

The Thr105Ile | Threonine to isoleucine at position 105 variant (rs11558538) is a well-characterized missense mutation in
the HNMT gene that replaces threonine with isoleucine at position 105. Unlike the
3'UTR variant that affects how much enzyme is made, this variant changes the
enzyme's structural stability and catalytic efficiency.

The Mechanism

Position 105 lies near the active site of HNMT where SAM and histamine bind.
The isoleucine substitution (T allele) destabilizes the protein, leading to faster
degradation and lower steady-state enzyme levels in cells. Studies using
recombinant HNMT | Recombinant protein is produced in laboratory cells to study enzyme properties in isolation from other cellular factors have shown that the Ile105 variant has reduced thermal stability
and lower catalytic activity compared to the wild-type Thr105 enzyme. The threonine
residue creates a more accessible conformation of substrate binding residues than
the isoleucine variant, resulting in higher enzymatic activity.

The Evidence

Preuss et al. (1998) | Preuss CV et al. Human Histamine N-Methyltransferase Pharmacogenetics: Common Genetic Polymorphisms That Alter Activity. Mol Pharmacol, 1998 demonstrated that individuals with the TT genotype had
significantly lower HNMT enzyme activity in red blood cells. Subsequent studies
confirmed that this variant is associated with increased susceptibility to allergic
diseases, asthma, and histamine-related symptoms, particularly in European
populations. The variant is relatively uncommon in homozygous form (about 2% of
Europeans), but heterozygous carriers (about 18%) may experience subtle effects,
particularly when combined with other histamine pathway variants. Interestingly,
meta-analyses | Thr105Ile and Parkinson disease meta-analysis have suggested that the Ile105 variant may be associated
with reduced risk of Parkinson disease, possibly through altered brain histamine levels.

Brain Histamine

HNMT is the only enzyme that degrades histamine in the brain, where histamine acts
as a neurotransmitter | Brain histamine is released by tuberomammillary neurons in the hypothalamus and helps regulate the sleep-wake cycle involved in wakefulness, appetite, and cognition. Reduced HNMT
activity can alter brain histamine signaling, which may partly explain why some
individuals with HNMT variants report sleep disturbances, anxiety, or cognitive
effects in response to histamine triggers.

Practical Considerations

If you carry the T allele, supporting your methylation pathway (which supplies SAM
for HNMT) becomes even more important, since your enzyme is already working at
reduced capacity. Combined with DAO variants, this can create a significant
histamine clearance deficit that benefits from both dietary management and
methylation support.