P634: Clinical impact of RNA sequencing on VUS resolution in a diverse rare disease cohort of over 100,000 patients*

Introduction: Application of clinical exome sequencing (ES) is especially transformative for pediatric rare diseases; however, equitable access is a key barrier especially in under-privileged communities.The Genomics for KIDS in ASEAN project aims to establish a clinical workflow for providing clinical ES to 510 under-resourced families in Southeast Asia and to build a knowledgebase in collaboration with regional experts to better serve the specific needs of Southeast Asian families.Methods: We developed an International Organization for Standardization (ISO)-accredited clinical workflow for germline ES with our regional partners using Twist Exome 2.0 panel on Illumina short-read sequencing platform with in-house developed framework for clinical bioinformatics.Variants were interpreted according to ACMG and ClinGen guidelines.Results: To date, we have 112 participating Southeast Asian families across 12 ethnicities from 3 study sites in Singapore, Malaysia and the Philippines.All but 1 are proband-parent trio analyses; 65% (n=73) involved a proband with neurodevelopmental disorder with/without congenital abnormalities, 17% (n=19) with multiple congenital anomalies, 7% (n=8) with developmental/intellectual disability and 11% (n=12) reported isolated congenital anomalies.Probands ranged from age 7 days to 42 years, with median age 2 years, all of whom reported phenotypes before adulthood.Reportable findings were detected in 75/112 (67%) probands, of whom 67 carried at least a pathogenic/likely pathogenic variant (PV) and 8 carried variants of uncertain significance (VUS) in genes relevant to their clinical presentation.Diagnostic yield was highest in probands manifesting neurodevelopmental disorder with/without congenital abnormalities (75%, 55/73), whereas only 25% (2/8) of probands with developmental/intellectual disability had a reportable finding.Among 67 probands with PVs detected, 10 (15%) harbored large copy-number variants and 1 with multi-exon intragenic deletion.Of the remaining 56 probands, 4 were detected with multiple PVs and the rest harbored small nucleotide variants associated with autosomal dominant (n=40), recessive (n=8) and X-linked (n=4) conditions.Collectively, 80 PVs were reported, with the majority being missense variants (41%, n=33) and protein-truncating variants (35%, n=28).Two probands had PVs in genes affecting cellular pathways which are targetable by FDA-approved drugs.Conclusion: Our study provided clinical ES test to 112 under-resourced families with pediatric rare disease in Southeast Asia and demonstrated a diagnostic yield of 67%, with the highest rate among those presenting neurodevelopmental phenotypes with/without congenital anomalies.Our genotype-first approach to variant analysis also revealed approximately 4% of pediatric rare disease cases may harbor multiple PVs associated with multiple conditions and demonstrated the opportunities for immediate therapeutic intervention following a molecular diagnosis through clinical ES.Finally, our study demonstrated that regional collaborations can positively impact families with rare disease, especially those with limited access to genomic testing in underserved communities.