IL1B -3737 G/A — The Peritoneal Fire Switch in Endometriosis
The IL1B gene encodes interleukin-1 beta (IL-1β) | IL-1β is a master
pro-inflammatory cytokine produced primarily by activated macrophages; it
triggers NF-κB signaling and coordinates broad inflammatory gene
expression, one of the most
potent drivers of peritoneal inflammation in the body. In endometriosis,
IL-1β plays a central and well-documented pathogenic role: it is
elevated in the peritoneal fluid of affected women, secreted at higher
levels by peritoneal macrophages, and creates the inflammatory milieu
that allows ectopic endometrial tissue to implant, survive, and expand.
The rs4848306 variant sits 3,737 base pairs upstream of the IL1B
transcription start site (in the promoter region), where it can
influence how much IL-1β the body produces in response to inflammatory
stimuli — including the monthly retrograde menstruation that seeds the
peritoneum with endometrial fragments.
The Mechanism
rs4848306 maps to chromosome 2, position 112,840,530 (GRCh38), within
the promoter region of IL1B. The IL1B gene is encoded on the minus
strand of chromosome 2; the promoter variant is therefore read in the
complement direction, with papers frequently describing this position
using the coding-strand notation as IL-1B -3737. The reference allele
on the plus strand is G (~61% globally); the alternate allele is A
(~39%). Haplotype studies in schizophrenia genetics | A field that has
intensively studied IL1B promoter variants because neuroinflammation
is central to psychosis pathophysiology; the same promoter logic applies
to peritoneal inflammation in endometriosis
establish that the G allele at rs4848306 is part of the haplotype
associated with elevated IL1B mRNA expression, while the A allele
(coding-strand T) predicts reduced IL1B transcriptional output.
This makes rs4848306 a regulatory variant that fine-tunes the
amplitude of the IL-1β inflammatory response.
IL-1β in the peritoneal cavity acts at multiple points in endometriosis
pathogenesis: it promotes adhesion of ectopic endometrial stromal
cells to mesothelial surfaces, stimulates vascular endothelial growth
factor (VEGF) | VEGF drives the neovascularization that sustains
implant growth production
to vascularize nascent implants, activates matrix metalloproteinases
for tissue invasion, and suppresses natural killer cell cytotoxicity
that would otherwise clear ectopic cells. A promoter variant that
drives higher IL-1β output — the G allele — would amplify each of
these steps during the inflammatory cascade that follows retrograde
menstruation.
The Evidence
Elevated IL-1β in the peritoneal environment of women with endometriosis
is one of the most replicated findings in the field. Sikora et al. found
higher peritoneal fluid IL-1β and elevated macrophage IL-1β secretion in
women with endometriosis compared to controls |
Sikora J, et al. Pro-IL-1β and macrophage IL-1β secretion in
endometriosis. Ginekologia Polska, 2016,
with pro-IL-1β disproportionately elevated, suggesting
increased inflammasome-driven IL-1 processing. A complementary study
found that IL-1β and ICE (IL-1β converting enzyme) levels were
significantly higher in endometriosis peritoneal fluid | Sikora J, et al.
IL-1β, IL-18 and ICE in endometriosis. Ginekologia Polska, 2012,
implicating dysregulated cytokine maturation in disease progression.
Akoum et al. documented a specific imbalance between IL-1β and its
decoy inhibitory receptor (IL-1R2) in endometriosis peritoneal fluid |
Akoum A, et al. Imbalance between IL-1β and IL-1R2 in endometriosis.
Journal of Reproductive Immunology, 2008,
most pronounced in infertile patients, linking the defect in local IL-1
control directly to infertility risk. This biochemical finding is
complemented by genetic evidence: Mier-Cabrera et al. showed that
IL1B variants (the +3954 allele, rs1143634) were enriched in stage IV
endometriosis cases compared to controls (OR 2.69) | Mier-Cabrera J,
et al. TNF-α, IL-1β and IL1-Ra polymorphisms and endometriosis severity.
BMC Women's Health, 2022,
supporting IL1B genetic variation as a modifier of disease severity
rather than merely a bystander.
For rs4848306 specifically, the direct endometriosis evidence is
pathway-based rather than variant-specific: the A allele reduces IL1B
promoter activity, and lower IL-1β production capacity would logically
dampen the peritoneal inflammatory cascade. This is classified as
emerging evidence — the pathway plausibility is strong, but
genome-wide significant association data for rs4848306 in endometriosis
cohorts is not yet published.
Practical Actions
The G allele at rs4848306 is associated with higher IL-1β promoter
activity. In the context of endometriosis, where peritoneal IL-1β excess
is a documented and replicated finding, GG homozygotes may carry a
heightened inflammatory tone that could favor lesion establishment and
progression. Because this is an emerging-evidence variant, actions
should focus on monitoring and evidence-based lifestyle factors known
to modulate the IL-1β/IL-1Ra balance — specifically, dietary omega-3
fatty acids, which suppress IL-1β production, and awareness of symptoms
that suggest active peritoneal inflammation.
Interactions
IL1A rs6542095: The IL-1α cytokine (encoded by IL1A) shares the
same receptor as IL-1β and operates in the same peritoneal inflammatory
cascade. Women carrying both the IL1A rs6542095-C risk allele (elevated
IL-1α susceptibility) and the IL1B rs4848306 GG genotype (elevated IL-1β
promoter activity) may experience compounded peritoneal IL-1 signaling —
two branches of the same inflammatory pathway simultaneously amplified.
IL1RN rs2234663 (IL1B VNTR): The interleukin-1 receptor antagonist
(IL-1Ra) competitively inhibits both IL-1α and IL-1β signaling. The IL1RN
intron 2 VNTR (rs2234663, IL-1RN*2 allele) has been associated with lower
IL-1Ra production in some tissues. A compounded state of high IL-1β (GG
at rs4848306) with low IL-1Ra (IL1RN*2 carrier) would represent maximal
unchecked IL-1 signaling in the peritoneal cavity.
IL1B rs1143634 (+3954): This IL1B exon 5 synonymous variant is the
most studied IL1B variant in endometriosis; the *2 allele was associated
with OR 2.69 for stage IV endometriosis in the Mier-Cabrera 2022 cohort.
The two variants tag different haplotypes within the IL1B locus and may
independently contribute to IL-1β expression amplitude.
Alla genotyper
One copy each of A and G — intermediate IL-1β promoter activity, common genotype
You carry one A allele and one G allele at rs4848306. This is the most common genotype globally (~48% of people). At this IL1B promoter locus, you carry one copy of the A allele that is associated with reduced IL-1β expression and one copy of the G allele that is associated with higher promoter activity. The net effect is intermediate IL-1β production capacity, representing typical function for this locus. In the context of endometriosis, where elevated peritoneal IL-1β is a replicated finding, your genotype does not place you at the higher end of the IL-1β expression spectrum at this particular variant.
Two copies of the A allele — reduced IL-1β promoter activity, potentially lower peritoneal inflammatory tone
You carry two copies of the A allele at rs4848306. This genotype is associated with reduced IL-1β promoter activity at this IL1B locus. Haplotype studies establish that the A allele (coding-strand T) predicts lower IL1B mRNA expression than the G allele. In the context of endometriosis, where peritoneal IL-1β excess drives ectopic tissue implantation and inflammatory adhesion formation, having a reduced-output IL-1β promoter may confer modest protective benefit against the most IL-1β-driven forms of disease progression. About 15% of people carry this genotype.
Two copies of the G allele — higher IL-1β promoter activity, modestly elevated peritoneal inflammatory tone
You carry two copies of the G allele at rs4848306. This is the homozygous state for the allele associated with higher IL-1β promoter activity. Haplotype studies identify the G allele as part of the higher-expression IL1B haplotype. Approximately 37% of people carry this GG genotype globally. In the context of endometriosis, where peritoneal IL-1β is consistently elevated in affected women — driving ectopic tissue adhesion, inflammatory angiogenesis, and the chronic inflammatory state that sustains lesions — a genotype associated with higher IL-1β production capacity may modestly amplify this pathway. The evidence for rs4848306 specifically in endometriosis is classified as emerging; the pathway plausibility is strong, but direct variant-level association studies in endometriosis cohorts are pending.