Accurate Molecular Diagnosis of Gaucher Disease Using Clinical Exome Sequencing as a First-Tier Test

Gaucher disease (GD) is an autosomal recessive lysosomal disorder due to beta-glucosidase gene (GBA) mutations. The molecular diagnosis of GD is complicated by the presence of recombinant alleles originating from a highly homologous pseudogene. Clinical exome sequencing (CES) is a rapid genetic approach for identifying disease-causing mutations. However, copy number variation and recombination events are poorly detected, and further investigations are required to avoid mis-genotyping. The aim of this work was to set-up an integrated strategy for GD patients genotyping using CES as a first-line test. Eight patients diagnosed with GD were analyzed by CES. Five patients were fully genotyped, while three were revealed to be homozygous for mutations that were not confirmed in the parents. Therefore, MLPA (multiplex ligation-dependent probe amplification) and specific long-range PCR were performed, and two recombinant alleles, one of them novel, and one large deletion were identified. Furthermore, an MLPA assay performed in one family resulted in the identification of an additional novel mutation (p.M124V) in a relative, in trans with the known p.N409S mutation. In conclusion, even though CES has become extensively used in clinical practice, our study emphasizes the importance of a comprehensive molecular strategy to provide proper GBA genotyping and genetic counseling.

GBA Analysis in Next-Generation Era: Pitfalls, Challenges, and Possible Solutions

Mutations in the gene encoding the lysosomal enzyme acid β-glucosidase (GBA) are responsible for Gaucher disease and represent the main genetic risk factor for developing Parkinson disease. In past years, next-generation sequencing (NGS) technology has been applied for the molecular analysis of the GBA gene, both as a single gene or as part of gene panels. However, the presence of complex gene-pseudogene rearrangements, resulting from the presence of a highly homologous pseudogene (GBAP1) located downstream of the GBA gene, makes NGS analysis of GBA challenging. Therefore, adequate strategies should be adopted to avoid misdetection of GBA recombinant mutations. Here, we validated a strategy for the identification of GBA mutations using parallel massive sequencing and provide an overview of the major drawbacks encountered during GBA analysis by NGS. We implemented a NGS workflow, using a set of 38 patients with Gaucher disease carrying different GBA alleles identified previously by Sanger sequencing. As expected, the presence of the pseudogene significantly affected data output. However, the combination of specific procedures for the library preparation and data analysis resulted in maximal repeatability and reproducibility, and a robust performance with 97% sensitivity and 100% specificity. In conclusion, the pipeline described here represents a useful approach to deal with GBA sequencing using NGS technology.

Investigation on acute effects of enzyme replacement therapy and influence of clinical severity on physiological variables related to exercise tolerance in patients with late onset Pompe disease

Exercise intolerance is one of the clinical hallmarks of late-onset Pompe disease (LOPD). We studied the acute effects of ERT on the physiological variables associated with exercise tolerance in patients chronically ERT treated. Moreover, we assessed the influence of clinical severity on the investigated variables. The day before (B) and the day after (A) ERT injection, 11 LOPD patients performed on a cycle-ergometer an exercise tolerance test to voluntary exhaustion; VO₂, HR, RPE, and GAA activity were determined in B and A. The disease severity was characterized by Walton scale, 6MWT, and pulmonary function tests. No significant differences in the variables related to exercise tolerance were found in A vs B, despite a significant increase in GAA activity in peripheral lymphocytes. No differences in VO₂ peak were observed between patients with only skeletal muscle impairment and patients with both skeletal and respiratory muscle impairment. Distance walked at 6MWT was significantly higher than VO₂ peak expressed as percentage of normal values. In conclusion, in LOPD patients the exercise tolerance test is not acutely affected by ERT administration; the peripheral muscle component seems more prominent in determining the VO₂ peak decrease than the respiratory component; VO₂ peak might be more sensitive than 6MWT in estimating exercise tolerance in LOPD.

Sequence and copy number analyses of HEXB gene in patients affected by Sandhoff disease: functional characterization of 9 novel sequence variants

Sandhoff disease (SD) is a lysosomal disorder caused by mutations in the HEXB gene. To date, 43 mutations of HEXB have been described, including 3 large deletions. Here, we have characterized 14 unrelated SD patients and developed a Multiplex Ligation-dependent Probe Amplification (MLPA) assay to investigate the presence of large HEXB deletions. Overall, we identified 16 alleles, 9 of which were novel, including 4 sequence variation leading to aminoacid changes [c.626C>T (p.T209I), c.634C>A (p.H212N), c.926G>T (p.C309F), c.1451G>A (p.G484E)] 3 intronic mutations (c.1082+5G>A, c.1242+1G>A, c.1169+5G>A), 1 nonsense mutation c.146C>A (p.S49X) and 1 small in-frame deletion c.1260_1265delAGTTGA (p.V421_E422del). Using the new MLPA assay, 2 previously described deletions were identified. In vitro expression studies showed that proteins bearing aminoacid changes p.T209I and p.G484E presented a very low or absent activity, while proteins bearing the p.H212N and p.C309F changes retained a significant residual activity. The detrimental effect of the 3 novel intronic mutations on the HEXB mRNA processing was demonstrated using a minigene assay. Unprecedentedly, minigene studies revealed the presence of a novel alternative spliced HEXB mRNA variant also present in normal cells. In conclusion, we provided new insights into the molecular basis of SD and validated an MLPA assay for detecting large HEXB deletions.