Clinical features and genetics in non-5q spinal muscular atrophy caused by acid ceramidase deficiency

Spinal muscular atrophy (SMA) is a spectrum of genetically and clinically heterogeneous diseases leading to the progressive degeneration of peripheric motor neurons with subsequent muscle weakness and atrophy. More than 95% of the cases of SMA are represented by homozygous mutations of the SMN1 gene (5q-SMA). Because this disease represents the leading cause of death due to a genetic cause and due to the availability of genetic therapies which can now save the life of the patient and stop the progress of the disease, early diagnosis is crucial. This report presents the case of a 13-year-old patient admitted to our hospital in 2018 who presented a phenotype typical to 5q-SMA. Next-generation sequencing (NGS) and Sanger sequencing of the SMN1 gene were performed, and a negative result was obtained. Consequently, we continued testing using whole-exome sequencing and discovered three mutations in the ASAH1 gene (one pathogenic and two variants of uncertain significance). Pathogenic mutations in the ASAH1 gene are responsible for spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME) and Farber disease, which overlapped with our patient’s phenotype. Currently, there are 45 SMA cases caused by mutations in the ASAH1 gene reported worldwide; however, the present case is the first reported in Romania.

Acid Sphingomyelinase Deficiency Ameliorates Farber Disease

Farber disease is a rare lysosomal storage disorder resulting from acid ceramidase deficiency and subsequent ceramide accumulation. No treatments for Farber disease are clinically available, and affected patients have a severely shortened lifespan. We have recently reported a novel acid ceramidase deficiency model that mirrors the human disease closely. Acid sphingomyelinase is the enzyme that generates ceramide upstream of acid ceramidase in the lysosomes. Using our acid ceramidase deficiency model, we tested if acid sphingomyelinase could be a potential novel therapeutic target for the treatment of Farber disease. A number of functional acid sphingomyelinase inhibitors are clinically available and have been used for decades to treat major depression. Using these as a therapeutic for Farber disease, thus, has the potential to improve central nervous symptoms of the disease as well, something all other treatment options for Farber disease can’t achieve so far. As a proof-of-concept study, we first cross-bred acid ceramidase deficient mice with acid sphingomyelinase deficient mice in order to prevent ceramide accumulation. Double-deficient mice had reduced ceramide accumulation, fewer disease manifestations, and prolonged survival. We next targeted acid sphingomyelinase pharmacologically, to test if these findings would translate to a setting with clinical applicability. Surprisingly, the treatment of acid ceramidase deficient mice with the acid sphingomyelinase inhibitor amitriptyline was toxic to acid ceramidase deficient mice and killed them within a few days of treatment. In conclusion, our study provides the first proof-of-concept that acid sphingomyelinase could be a potential new therapeutic target for Farber disease to reduce disease manifestations and prolong survival. However, we also identified previously unknown toxicity of the functional acid sphingomyelinase inhibitor amitriptyline in the context of Farber disease, strongly cautioning against the use of this substance class for Farber disease patients.