PI-041 - A COMPREHENSIVE DEEP MUTATIIONAL SCAN OF OATP1B1: IMPLICATIONS FOR DRUG RESPONSE

J. Yang¹, C. Macdonald¹, S. Yee¹, C. Chien¹, T. Haldar¹, A. Oni-Orisan¹, W. Coyote-Maestas¹, K. Giacomini²; ¹UCSF, University of California, San Francisco, CA, US, ²UCSF, San Francisco, CA, USA.

Background: OATP1B1 (SLCO1B1) is a critical hepatic transporter mediating the uptake of various endogenous compounds and drugs. Genetic variations in SLCO1B1 can significantly influence transporter-mediated drug uptake, affecting the pharmacokinetics and toxicity of statins, anti-cancer drugs, and anti-diabetes medications. Many SLCO1B1 variants remain functionally uncharacterized, hindering our understanding of interindividual differences in drug response.

Methods: We employed our saturation mutagenesis to generate a comprehensive variant library encompassing all single codon missense and deletion mutations in OATP1B1. Stable HEK293T cell lines, each harboring a unique genomic variant, were established using a landing pad attachment site. To assess the impact of mutations on protein folding and stability, we utilized VAMP-seq (variant abundance by massively parallel sequencing) to quantify overall protein abundance. Functional changes were evaluated in a screen where cell viability was linked to transporter-mediated uptake of fluorescein methotrexate. We explored the frequency of OATP1B1 coding variants in the gnomAD browser and analyzed their collective association with bilirubin levels in the UK Biobank.

Results: Our comprehensive variant library and cell lines enabled a systematic assessment of over 16,000 OATP1B1 variants. Folding was a primary determinant of variant function, with 33.8% of missense and deletion variants showing significant loss of function. We generated functional scores for over 900 OATP1B1 missense variants present in gnomAD, previously lacking functional annotation, with 235 displaying poor function. Notably, South Asians had the lowest total allele frequency of poor-functioning variants. Combining 54 poor-function OATP1B1 variants in a Burden test showed a strong association with total bilirubin (p = 1.6 × 10⁻²³), which attenuated with 141 normal-function variants (p = 0.01).

Conclusion: Our deep mutational scanning study provides a comprehensive understanding of OATP1B1's mutational landscape. By integrating with UK Biobank data, we demonstrate the impact of these mutations on human physiology. Our findings highlight the importance of OATP1B1 pharmacogenomics in personalized medicine and provide a foundation for future research into drug transporter mutations.