Abstract 6321: Characterizing plasma protein LEG1 homolog, a novel potential risk factor for post-menopausal breast cancer

Abstract Background: We previously identified and confirmed a positive association between plasma protein LEG1 homolog and post-menopausal breast cancer risk in 2 cohort studies in models adjusted for breast cancer risk factors. This evolutionarily conserved protein is understudied. We now aim to characterize the protein’s within-person variation over time, its correlates and residual association after accounting for correlates. Methods: We analyzed data from the Atherosclerosis Risk in Communities study, in which we first identified the protein (Syleouni M et al. JNCI 2025). We included 4,403 post-menopausal women (74% not taking hormone replacement therapy [HRT]) without cancer. Plasma proteins were previously measured by SomaScan® 5K at Visits 2, 3 and 5 (3, 18 years apart). We used demographic, anthropometric, lifestyle, and reproductive data collected by interview or measured by trained staff. Previously, plasma estradiol and testosterone were measured. We assessed within-person variation over time (ICC) and identified correlates of LEG1 after log2 transformation (linear regression). We re-analyzed the LEG1-breast cancer association stratifying by or adjusting for the correlates using multivariable-adjusted Cox regression. For highly correlated factors, we first regressed LEG1 on each correlate and then modeled LEG1 residuals. Results: The ICC for LEG1 was 0.52 across the 3 time points. LEG1 was highly correlated with prolactin-inducible protein (r=0.95) and extracellular glycoprotein lacritin (r=0.86); these correlations were almost identical by age, race, HRT use, BMI, and diabetes. The remaining 4,709 proteins had r< |0.35|, including weak correlations for kidney function (albumin, ALT, AST) and inflammation (CRP) markers. LEG1 was only weakly correlated with estradiol (measured 3 years apart, N=316, r=0.13, p=0.02) and testosterone (measured 3 years apart, N=3,134, r=0.05, p=0.02). LEG1 declined modestly with age (p<0.0001). Adjusting for age, level was notably higher in Black than White women (p<0.0001). Adjusting for both age and race, LEG1 level was lower in women with higher BMI and diabetes. Direction of the association appeared to differ by race for HRT use (level was higher in users in White women only) and reproductive factors. The residuals of LEG1 after regression on prolactin-inducible protein (p=0.0001) or extracellular glycoprotein lacritin (p=0.015) were statistically significantly positively associated with breast cancer. Associations for LEG1 were stronger in women not using HRT, ≥median BMI, and with diabetes; we previously reported no difference in the association by race. Conclusions: This study provides continued support for plasma protein LEG1 homolog as a risk factor for post-menopausal breast cancer. This protein should be considered for its utility in current risk stratification tools. Mechanistic studies are needed. Support: NHLBI, NCI, NPCR Citation Format: Vernon A. Burk, Elizabeth A. Platz. Characterizing plasma protein LEG1 homolog, a novel potential risk factor for post-menopausal breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6321.