Acid Ceramidase Deficiency: New Insights on SMA-PME Natural History, Biomarkers, and In Cell Enzyme Activity Assay

Abstract

Background and objectives: Spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME) due to acid ceramidase deficiency is a rare disorder, allelic with Farber disease, resulting from recessive ASAH1 variants. Patients present in early childhood with muscle weakness due to anterior horn degeneration and/or progressive drug-resistant myoclonic epilepsy. Death usually results from respiratory complications or status epilepticus during adolescence.

Methods: We identified 9 patients with SMA-PME from 5 different families followed in neurology, rehabilitation, and genetics departments of university hospitals in France and the United States. During disease progression, motor functional scores were assessed for seven of them and C26-ceramide quantification on dried blood spots (DBSs) was performed for 4 of them. An in cell assay, measuring the degradation rate of ceramides in living skin fibroblasts, was also performed in 2 patients. Finally, a literature review was conducted.

Results: Twelve years after the molecular characterization of SMA-PME, here we present the detailed history of 9 patients from 5 different families with 4 new ASAH1 variants. The prospective follow-up for 4 of them allows us to evaluate the relevance of functional scales and of C26-ceramide assay on DBS, as a biomarker. In addition, an in cell assay could provide a more reliable level of the residual ceramidase activity. Based on a comprehensive literature review, we provide a detailed description of the natural history of the 44 patients with SMA-PME diagnosed to date and show a genotype-phenotype correlation for the 2 main variants and the disease onset.

Discussion: This study presents the detailed natural history of SMA-PME. Given the rarity of this disease and the current lack of a reliable biomarker for patient follow-up, this work may serve as a retrospective control group for future therapeutic trials.

Figure 1. Motor Functional Scores According to Patients' Age

Left, the gross motor function classification score from GMFC-MLD ranges from 0 to 6. Level 0: walking without support with normal performance. Level 1: walking without support with reduced performance. Level 2: walking with support. Level 3: walking with or without support not possible, but sitting without support and locomotion such as crawling or rolling possible. Level 4: sitting without support but no locomotion, or sitting without support not possible, but locomotion such as crawling or rolling possible. Level 5: no locomotion nor sitting without support, but head control possible. Level 6: loss of any locomotion, head and trunk control. The first circled point corresponds to the age at symptom onset. Right, the total Motor Function Measurement–32 items (MFM-32) score decreases from 100% to 0% with the progression of the motor impairment.

Figure 2. Electron Microscopy (EM) Findings in Skin Biopsy of Patient P5

(A) “Farber bodies” (red arrows) appear as curvilinear tubular bodies of approximately 15-20 nm in diameter; ×24000. (B) “Banana bodies” (red arrows), also suggestive of FD, are large, clear, spindle-shaped vacuoles of approximately 2 µm; ×12000. (C) “Zebra bodies” (red arrows) are lysosomal inclusions also found in other sphingolipidoses, which appear as vacuoles with transverse membranes of approximately 2 µm; ×12000. FD = Farber disease.

Figure 3. Evolution of C26-Ceramide Blood Concentration in Patients

C26-ceramide level (nmol/L) extracted from DBS discs and measured by LC-MS/MS for patients P1–4 during follow-up (left ordinate, spots, and solid lines), according to their age (abscissa) and the evolution of their GMFC-MLD scores (right ordinate, red dashed lines). The orange strip represents the mean control level of C26-ceramide ± 1 SD (mean = 305.38 nmol/L, SD = 57.30 nmol/L). DBS = dried blood spot.

Figure 4. Natural History of 44 Patients With SMA-PME (9 Reported Here and 35 Studied From the Literature)

(A) First signs revealing SMA-PME. (B and C) Epileptic phenotype in SMA-PME: (B) types of seizures revealing epilepsy and (C) occurring during the course of the disease. (D) Symptoms in SMA-PME, ordered by age at onset. For each symptom, the cross represents the mean, the central bar represents the median, the peripheral bars represent the first and last quartiles, and the vertical lines represent the extreme ages. (E) Overall survival and (F) loss of ambulation in patients with classic SMA-PME, represented on the Kaplan-Meier curve. 95% CI: dotted line. SMA-PME = spinal muscular atrophy with progressive myoclonic epilepsy.

Figure 5. Schematic Representation of ACDase, the 27 Different Variants Found in Patients With SMA-PME, and Age at First Symptom of SMA-PME Depending on the Genotype of the Patients

(A) Heterodimeric structure of ACDase consists of α-subunit (13 kDa, residues 22–142) and β-subunit (40 kDa, residues 143–395), and the 27 different variants found in the 44 studied patients with SMA-PME. *Novel variants in SMA-PME. Created with ProteinPaint. (B) Genotype-phenotype correlation for the symptom onset, with T42M patients carrying at least one p.(Thr42Met) variant, K152N carrying at least one p.(Lys152Asn) variant, and others carrying none of these. Comparisons are made for (a) any symptom, (b) seizure, and (c) muscular weakness.