Background:　Although　genome-wide　association　studies　(GWAS)　have　identified　14　loci　associated　with　frailty　index　(FI)　susceptibility,　the　underlying　causative　genes　and　biological　mechanisms　remain　elusive.　Methods:　A　cross-tissue　transcriptome-wide　association　study　(TWAS)　was　conducted　utilizing　the　Unified　Test　for　Molecular　Markers　(UTMOST),　which　integrates　GWAS　summary　statistics　from　164,610　individuals　of　European　ancestry　and　10,616　Swedish　participants,　alongside　gene　expression　matrices　from　the　Genotype-Tissue　Expression　(GTEx)　Project.　Validation　of　the　significant　genes　was　performed　through　three　distinct　methods:　FUSION,　FOCUS,　and　Multiple　Marker　Analysis　of　Genome-wide　Annotation　(MAGMA).　Exploration　of　tissue　and　functional　enrichment　for　FI-associated　SNPs　was　conducted　using　MAGMA.　Conditional　and　joint　analyses,　along　with　fine　mapping,　were　employed　to　enhance　our　understanding　of　FI’s　genetic　architecture.　Mendelian　randomization　was　employed　to　ascertain　causal　relationships　between　significant　genes　and　FI,　and　co-localization　analysis　was　utilized　to　investigate　shared　SNPs　between　significant　genes　and　FI.　Results:　In　this　study,　two　novel　susceptibility　genes　associated　with　the　risk　of　FI　were　identified　through　the　application　of　four　TWAS　methods.　Mendelian　randomization　demonstrated　that　HTT　may　elevate　the　risk　of　developing　frailty,　whereas　LRPPRC　could　offer　protection　against　the　onset　of　frailty.　Additionally,　co-localization　analysis　identified　a　shared　SNP　between　LRPPRC　and　FI.　Tissue　enrichment　analyses　revealed　that　genomic　regions　linked　to　SNPs　associated　with　frailty　were　predominantly　enriched　in　various　brain　regions,　including　the　frontal　cortex,　cerebral　cortex,　and　cerebellar　hemispheres.　Conditional,　combined　analyses,　and　fine　mapping　collectively　identified　two　genetic　regions　associated　with　frailty:　2p21　and　4q16.3.　Functional　enrichment　analyses　revealed　that　the　pathways　associated　with　frailty　were　primarily　related　to　the　MHC　complex,　PD-1　signaling,　cognition,　inflammatory　response　to　antigenic　stimuli,　and　the　production　of　second　messenger　molecules.　Conclusion:　This　investigation　uncovers　two　newly　identified　genes　with　forecasted　expression　levels　associated　with　the　risk　of　FI,　offering　new　perspectives　on　the　genetic　architecture　underlying　FI.　Copyright　©　2024　Lin,　Wu,　Li,　Ye,　Pan,　Zheng　and　Gao.