(1) Background: Most rare disease patients endure long delays in obtaining a correct diagnosis, the so-called "diagnostic odyssey", due to a combination of the rarity of their disorder and the lack of awareness of rare diseases among both primary care professionals and specialists. Next-generation sequencing (NGS) techniques that target genes underlying diverse phenotypic traits or groups of diseases are helping reduce these delays; (2) Methods: We used a combination of biochemical (thin-layer chromatography and high-performance liquid chromatography-tandem mass spectrometry), NGS (resequencing gene panels) and splicing assays to achieve a complete diagnosis of three patients with suspected metachromatic leukodystrophy, a neurologic lysosomal disorder; (3) Results: Affected individuals in each family were homozygotes for harmful variants in the ARSA gene, one of them novel (c.854+1dup, in family 1) and the other already described (c.640G>A, p.(Ala214Thr), in family 2). In addition, both affected individuals in family 2 were carriers of a known pathogenic variant in an additionallysosomal disease gene, GNPTAB (for mucolipidosis III). This additional variant may modify the clinical presentation by increasing lysosomal dysfunction. (4) Conclusions: We demonstrated the deleterious effect of the novel variant c.854+1dup on the splicing of ARSA transcripts. We also confirmed the involvement of variant c.640G>A in metachromatic leukodystrophy. Our results show the power of diagnostic approaches that combine deep phenotyping, NGS, and biochemical and functional techniques.
Keywords: NGS; functional assays; lysosomal diseases; metachromatic leukodystrophy; phenotypic variation; rare diseases.