rs368234815 — IFNL4 ss469415590 (TT/ΔG)

IFNL4 — The Interferon That Backfires Against Hepatitis C

Interferon lambda 4¹¹ Interferon lambda 4
A newly discovered type III interferon, encoded by IFNL4 on chromosome 19q13.13, identified in 2013 as the causal gene behind the well-known "IL28B" hepatitis C association is one of biology's great counterintuitive discoveries: a protein that belongs to the antiviral interferon family yet actively impairs the body's ability to clear a specific virus — hepatitis C (HCV). The rs368234815 polymorphism (originally designated ss469415590) is the molecular switch that controls whether you can produce IFN-λ4 at all. If you carry the ΔG allele, your cells make functional IFN-λ4 and your chances of clearing HCV spontaneously or responding to treatment are significantly reduced. If you are homozygous TT, the gene is broken by a frameshift, IFN-λ4 is never produced, and your immune system is paradoxically better at eliminating HCV.

This is not a rare clinical curiosity. The ΔG allele is the ancestral form — humans evolved with IFN-λ4 as a functional gene. The TT null variant arose and was then driven to high frequency by positive selection²² driven to high frequency by positive selection
especially strong in East Asia, where TT frequencies approach 90%, suggesting a survival advantage during periods of HCV or other viral epidemics. Among people of African ancestry, up to 80% still carry the ancestral ΔG allele. This variant supersedes the previously used rs12979860 (the "IL28B" SNP) as the direct functional test for IFNL4 activity.

The Mechanism

The rs368234815 polymorphism is a dinucleotide indel³³ dinucleotide indel
A two-nucleotide insertion/deletion; the ΔG allele involves deletion of one thymine, shifting the reading frame in exon 1 of IFNL4. The ΔG allele creates an intact open reading frame, allowing translation of the full IFNL4 protein. The TT allele introduces a frameshift that generates a premature stop codon, producing only truncated, non-functional polypeptides. In this way, TT homozygotes are complete IFNL4 "knockouts" at the population level.

The paradox is that recombinant IFN-λ4 protein has real antiviral activity against HCV in cell culture — it signals through the IFN-λ receptor and activates hundreds of interferon-stimulated genes⁴⁴ interferon-stimulated genes
ISGs, whose protein products directly inhibit viral replication. Why, then, does IFN-λ4 production predict worse outcomes? Two mechanisms have emerged. First, IFN-λ4 is largely retained in the endoplasmic reticulum⁵⁵ endoplasmic reticulum
ER; only small amounts are secreted extracellularly, where it induces ER stress. ER-stressed cells are significantly weaker activators of HCV-specific CD8+ T cells, blunting the adaptive immune response that is ultimately required for viral clearance. Second, the chronically elevated ISG expression driven by IFN-λ4 may induce a state of "interferon exhaustion" in the liver — when hepatocytes are pre-saturated with interferon signaling, exogenous peginterferon therapy cannot drive ISG expression higher, and the additional antiviral effect is lost. Recent work also shows that IFN-λ4 impairs HCV antigen presentation to T cells⁶⁶ IFN-λ4 impairs HCV antigen presentation to T cells
potentially allowing virus to persist by avoiding immune recognition, suggesting a third mechanism by which functional IFN-λ4 undermines clearance despite appearing antiviral in isolation.

The Evidence

The original 2013 discovery by Prokunina-Olsson et al.⁷⁷ Prokunina-Olsson et al.
"A variant upstream of IFNL3 (IL28B) creating a new interferon gene IFNL4 is associated with impaired clearance of hepatitis C virus" used RNA-sequencing in primary human hepatocytes stimulated with double-stranded RNA to mimic HCV infection. The study found that rs368234815 ΔG carriers could be distinguished from TT individuals by a new induced transcript upstream of IFNL3⁸⁸ new induced transcript upstream of IFNL3
the newly discovered IFNL4 gene, and that rs368234815 predicted HCV clearance more accurately than the then-standard rs12979860 marker.

For treatment outcomes, a meta-analysis spanning multiple HCV genotypes, ethnicities, and treatment regimens⁹⁹ meta-analysis spanning multiple HCV genotypes, ethnicities, and treatment regimens
including both peginterferon/ribavirin and direct-acting antivirals confirmed that TT/TT individuals achieve higher sustained virologic response (SVR) rates regardless of therapy. In DAA-treated HCV genotype 1¹⁰¹⁰ DAA-treated HCV genotype 1
sofosbuvir-based regimens, SVR was 98.6% for TT/TT versus 86.8% for ΔG/ΔG (P=0.01) — a clinically meaningful difference even in the era of highly effective antivirals. For spontaneous clearance in hemodialysis patients¹¹¹¹ hemodialysis patients
a cohort where HCV exposure rates are high, TT/TT carriers had 6.38-fold higher odds of clearing HCV without treatment (OR 6.38, 95% CI 1.69–24.2, P=0.003).

The variant is also relevant for HCV genotype 4 patients treated with sofosbuvir plus ribavirin, for IFN-based regimens in HIV/HCV coinfection, and for predicting post-transplant fibrosis progression in HCV-positive liver transplant recipients. Beyond HCV, emerging data suggest IFNL4 genotype modestly influences HIV seroconversion rates (ΔG/ΔG carriers may have slightly lower HIV acquisition odds) and is associated with prostate cancer risk in men with high sexually-transmitted infection exposure — though these applications remain investigational.

Practical Implications

For the vast majority of people who have never been infected with HCV, this variant's clinical significance is limited. HCV is not transmitted through casual contact — it requires blood-to-blood exposure (sharing needles, unscreened blood transfusions before 1992, certain medical procedures in under-resourced settings, or rarely sexual transmission). If you have no HCV risk factors, carry no HCV infection, and have no family history of liver disease from HCV, this variant is informationally interesting but requires no immediate action.

If you have been diagnosed with chronic hepatitis C or are at elevated risk (healthcare workers, people who inject drugs, those who received blood products before 1992), your IFNL4 genotype directly informs treatment planning. ΔG carriers — especially ΔG/ΔG homozygotes — should discuss with their hepatologist whether a standard 8-week DAA course is sufficient or whether extended therapy is warranted. Several clinical studies support extended treatment duration for ΔG/ΔG individuals receiving sofosbuvir-based regimens.

Interactions

rs368234815 and rs12979860 are in strong linkage disequilibrium (LD) and measure the same biological effect — but rs368234815 is the direct functional variant. In African Americans, where LD between the two variants is lower, rs368234815 is a substantially better predictor. If you have been genotyped with the older rs12979860 (C/T) assay, the favorable C allele at rs12979860 corresponds to the TT allele at rs368234815; the unfavorable T allele at rs12979860 corresponds to the ΔG allele. The rs117648444 Pro70Ser variant¹²¹² rs117648444 Pro70Ser variant
a coding variant in IFNL4 that modulates IFN-λ4 protein activity is a secondary modifier among ΔG carriers — those carrying the S70 form of IFN-λ4 have impaired protein function and better treatment outcomes than P70 ΔG carriers, providing additional prognostic resolution within the heterozygous group.