rs55687265 — ATP8B4 ATP8B4 Phospholipid Flippase F436L

ATP8B4 F436L — When a Membrane Pump Fails the Immune System

Your immune cells depend on a precisely organized cell membrane to function. The outer leaflet of every immune cell membrane is kept nearly free of phosphatidylserine — a lipid that, when exposed on the surface, signals "eat me" to phagocytes and dampens T-cell activation. A family of proteins called P4-ATPases (phospholipid flippases) does this housekeeping work, pumping phosphatidylserine¹¹ phosphatidylserine
A negatively charged phospholipid that is normally sequestered on the inner face of the plasma membrane; when it flips to the outer leaflet, it triggers apoptosis recognition and dampens immune signaling from the outer leaflet back inside. ATP8B4 is one of these flippases, expressed at especially high levels in bone marrow and immune-relevant tissues. The rs55687265 variant — a C allele on the plus strand creating the amino acid change Phe436Leu in the protein's transmembrane transport domain — appears to compromise this pumping activity, with measurable consequences for autoimmune disease risk.

The Mechanism

ATP8B4 belongs to the P4-type ATPase subfamily (also called type IV ATPases or phospholipid flippases). These enzymes use ATP hydrolysis to flip specific phospholipids from the outer to the inner leaflet of cellular membranes — maintaining the lipid asymmetry that is fundamental to healthy cell signaling. Disruption of this asymmetry in immune cells alters how those cells interpret and transmit inflammatory signals. In dendritic cells and macrophages, improper phosphatidylserine distribution on the plasma membrane affects toll-like receptor clustering²² toll-like receptor clustering
TLRs are pattern-recognition receptors that initiate innate immune responses; their signaling efficiency depends partly on local membrane lipid composition and downstream NF-kB activation dynamics.

The F436L substitution replaces a phenylalanine with a leucine in the transmembrane transport domain of the enzyme. Phenylalanine at this position is conserved across P4-ATPase family members and is predicted to participate in the phospholipid-binding pocket's geometry. Changing it to leucine — a smaller, less aromatic residue — is predicted to reduce substrate affinity or transport rate. Differential expression³³ Differential expression
SSc patients showed significantly altered ATP8B4 transcript levels compared to controls (P = 0.0005), consistent with a functional role in disease of ATP8B4 was observed between SSc patients and controls at the mRNA level, supporting biological involvement beyond just the genetic association.

The Evidence

The primary evidence comes from a whole-exome sequencing study⁴⁴ whole-exome sequencing study
WES sequences the protein-coding regions of the genome; rare variant burden analyses aggregate low-frequency variants by gene to detect associations too rare to find with single-SNP tests by Gao et al. (2016, Arthritis & Rheumatology, PMID 26473621) that performed gene-level burden analysis in European-American systemic sclerosis patients with and without pulmonary arterial hypertension. The rs55687265 variant emerged as the single strongest association signal within ATP8B4 — discovery cohort P = 9.35 × 10⁻¹⁰, OR 6.11. Replication in 415 SSc cases and 2,848 controls yielded OR 1.86 (P = 0.012). Meta-analysis across both cohorts: OR 2.5 (P = 1.92 × 10⁻⁷). Gene-level burden for all ATP8B4 rare variants reached P = 2.77 × 10⁻⁷.

A subsequent analysis by López-Isac et al.⁵⁵ López-Isac et al.
A multi-center European consortium follow-up study examining the F436L variant in an independent Spanish, Italian, German, and Dutch SSc cohort (2017, PMID 28141915) specifically assessed rs55687265 in a larger European-American SSc population. ATP8B4 gene-level burden was also detected in African American SSc patients⁶⁶ African American SSc patients
Whole-exome sequencing in 148 African American SSc cases and 2,397 controls; ATP8B4 among enriched genes (Gourh et al., 2018, PMID 29732714), extending the biological signal beyond European ancestry.

In parallel, a landmark Nature Genetics exome study⁷⁷ Nature Genetics exome study
16,036 Alzheimer's disease cases and 16,522 controls; gene-level burden test using SKAT-O across all rare damaging variants per gene (Holstege et al., 2022, PMID 36411364) of 32,558 individuals identified rare damaging variants across ATP8B4 as a significant Alzheimer's disease risk gene — placing it alongside TREM2, SORL1, and ABCA1 as genes involved in lipid membrane homeostasis in brain tissue, particularly in microglia.

The evidence across systemic sclerosis and Alzheimer's disease points to a shared mechanism: defective phospholipid flipping in immune cells (peripheral in SSc, central microglia in Alzheimer's) disrupts membrane lipid asymmetry, alters phagocytic and inflammatory signaling, and ultimately increases susceptibility to chronic dysregulated inflammation.

Practical Actions

The C allele at rs55687265 is rare globally (~1.4% overall, ~1.5% in Europeans, essentially absent in East Asians). Most carriers will be heterozygous GC. The elevated OR from the discovery cohort (OR 6.11) may reflect ascertainment bias from a highly selected SSc-with-PAH enriched sample; the replication OR of 1.86 and meta-analytic OR of 2.5 are better estimates of effect size in the general population. This is a meaningful but not deterministic risk factor — the C allele raises susceptibility, it does not guarantee disease.

Given the phospholipid transport mechanism, interventions that support membrane lipid composition and reduce chronic immune activation are most relevant. Omega-3 fatty acids (EPA/DHA) directly incorporate into immune cell membranes, modifying the pool of lipid substrates that flippases act on. Antioxidants that protect phospholipids from oxidation — particularly astaxanthin and vitamin E — reduce the load of oxidatively damaged phospholipids that accumulate when flipping is impaired.

Interactions

ATP8B4 belongs to the same P4-ATPase family as ATP8B1 (expressed in hepatocytes and bile ducts) and ATP8B2. The biological overlap with ABCA1 is particularly notable — ABCA1 is another lipid transporter independently identified as an Alzheimer's disease risk gene in the same Holstege et al. study, and both genes participate in cholesterol and phospholipid homeostasis in macrophages and microglia. Carriers of risk variants in both ATP8B4 and ABCA1 may face compounding lipid transport defects in immune cells, though no published compound analysis exists yet for this pair.

Systemic sclerosis as a disease involves dysregulated fibrotic and vascular responses alongside immune activation. Established SSc risk loci include rs2476601 (PTPN22), rs3087243 (CTLA4), and rs6920220 (TNFAIP3), which operate through T-cell and B-cell regulation rather than membrane lipid transport. ATP8B4 represents a distinct biological pathway into SSc susceptibility.