P335: Exome sequencing for diagnostic investigation of 267 children from Brazilian public health system (SUS) followed by reanalysis of negative/inconclusive cases

Introduction: Despite affecting few patients each, rare diseases' altogether impacts keep growing more and more evident.Even though genetic tests' constant advances over the last years, most notably next-generation sequencing (NGS), have provided better assistance for such cases, the path towards molecular diagnosis remains a long and slow one for many in Brazil, especially for patients and families relying exclusively on the public health system.The objective of this study was to confirm the genetic etiology through exome sequencing analysis from patients with rare diseases without previous known diagnosis, including reanalysis of the cases with initially negative or inconclusive molecular results.Methods: From 2017 and 2023, 267 patients with suspicion of rare disease due to unknown genetic etiology were selected from clinical follow-up at the Genetics Unit from the Instituto da Criana do Hospital das Clnicas da Universidade de So Paulo; blood samples from the probands were sent to Yokohama City University and submitted to exome sequencing, along with parents' samples.Retrospective data revision along with reanalysis of negative/inconclusive results was performed mostly in 2024.Results: From all cases analyzed, at least 124 returned with positive results (46,4%), most of them with some degree of intellectual disability or developmental delay.The large majority of positive cases were caused by pathogenic/likely pathogenic variants each located in over a hundred different genes; there were, however, few recurrent genes such as SCN4A and NSD1, each present in 4 unrelated families.Even though 65 cases, corresponding to over a half (52,4%) were inherited through autosomal dominant pattern, there was a significant amount of 35 cases with autosomal recessive (29%) and, in minor scale, 11 with Xlinked inheritance (8,8%), alongside with 12 cases (9,6%) due to copy-number variants (CNVs).On the other hand, the 127 cases with negative results (50,4%) and another 11 ones with inconclusive results (4,3%) were all submitted to reanalysis of exome sequencing data; this process enabled molecular diagnosis of at least 10 cases (7%) previously undetected.Conclusion: In this context, the data presented here, reinforces the importance of broad molecular investigation with exome sequencing in Brazilian patients; diagnostic yield obtained (nearly 45%) was in accordance with great cohorts and meta-analysis published around the world.As shown here, reanalysis of exome sequencing data, although limited, can improve this scenario.Finally, this seems to be the first large study with exome sequencing of cases with rare disease without previous known etiology with patients exclusively from the Brazilian public health system.