SLC19A1 G80A (His27Arg)

SLC19A1 — The Folate Gateway

SLC19A1 (Solute Carrier Family 19 Member 1), also known as the reduced folate
carrier (RFC1), is the primary transporter responsible for moving folate from
your blood into your cells. Even if you produce adequate methylfolate (via MTHFR)
or take methylfolate supplements, this transporter determines how efficiently
that folate actually reaches the inside of your cells where it is needed.

The Mechanism

The G80A variant (rs1051266) causes a histidine-to-arginine substitution | Histidine-to-arginine substitution at position 27 of the transporter protein (p.His27Arg) at
position 27 of the transporter protein, located in transmembrane domain 1 (TMD1),
a region implicated in substrate binding and translocation. The T allele (arginine
variant) has altered transport kinetics, resulting in reduced folate uptake into
cells. This creates a situation where blood folate levels may appear normal on a
standard test, but intracellular folate levels are suboptimal.

Clinical Relevance

This variant is particularly important in the context of other methylation
variants. If you have reduced MTHFR activity (making less methylfolate) AND
reduced RFC1 transport (getting less folate into cells), the combined effect
can be more significant than either variant alone. Studies have also linked
this variant to altered methotrexate response | Methotrexate is an antifolate drug used for cancer and autoimmune diseases — it competes with folate for the same RFC1 transporter, since methotrexate uses the
same transporter. A PharmGKB summary | Gong L et al. SLC19A1 Pharmacogenomics Summary, 2010
documents the pharmacogenomic relevance of this transporter.

The Bigger Picture

The folate pathway is like a production line: MTHFR converts folate to its
active form, SLC19A1 transports it into cells, and MTHFD1 helps process it
further. Bottlenecks at any step can reduce overall methylation capacity | This is why looking at individual SNPs in isolation can be misleading — the whole pathway matters. By
understanding which steps are compromised, you can target your supplementation
more effectively.

Practical Implications

If you carry the T allele, ensuring adequate (or slightly above average) folate
intake becomes important. Methylfolate may have an advantage over folic acid since
it is already in the active form and may be transported more efficiently. Higher
doses may help compensate for reduced transport efficiency.

Interactions

SLC19A1 interacts with MTHFR (rs1801133, rs1801131) — if both folate production
and transport are impaired, the combined effect is greater. It also interacts with
MTHFD1 (rs2236225) for downstream folate processing.