Expanding the genetic landscape of inherited metabolic diseases using long-read sequencing and transcriptomic profiling.

Although next-generation sequencing has emerged as a powerful tool for diagnosing rare diseases (RD), many cases of inherited metabolic diseases (IMD) remain unsolved, hindering the diagnosis, clinical and therapeutic management of the patients. The primary aim of this study is to address the most elusive cases by applying long-read sequencing (LRS) targeted to the gene of interest on seven patients (FARS2, GYS2, PEX1, SLC2A1, AGL, ACAT1, and ACADM), identifying six novel pathogenic variants including two intronic variants, a structural variant and three transposable elements (TE) insertions. In addition, we have demonstrated the effect on splicing of an exonic variant previously reported as missense. Functional genetic tests specific for the expected effect of each variant of uncertain significance were designed, such as minigenes analysis or chromatin conformation capture assay. From the TE insertions, two were located in the genomic region of GYS2 or PEX1, causing a reduction in their mRNA expression. The third was located 7.6 kb downstream of SLC2A1; it alters the interaction between the SLC2A1 promoter and its distal regulatory element via the establishment of a loop with the 3' border of the native topologically associating domain. This study shows that the combination of LRS and functional genetic assays confers a powerful approach for expanding the mutational spectrum of IMD, adding data to improve the diagnosis of this large group of RD.