A novel C-terminal truncated mutation in hCDKL5 protein causing a severe West syndrome: Comparison with previous truncated mutations and genotype/phenotype correlation.

INTRODUCTION: West Syndrome is a severe epileptic encephalopathy characterized by epileptic spasms, hypsarrhythmia, and regression of psychomotor acquisitions beginning in the first year of life. ARX and CDKL5 genes were identified as linked to the most frequent genetic causes of West Syndrome. METHODS: The present study reports the clinical, molecular and bioinformatic investigation of the patient with severe West syndrome. RESULTS: Molecular analysis of the two candidate genes, i.e. ARX and CDKL5 showed the presence of a novel insertion c.2788insG in exon 19 of CDKL5 gene. This mutation causes changes in cis regulation elements of exon 19 splicing and in secondary pre-mRNA structure leading probably to inclusion of alternative exon 19 in hCDKL5_5 isoform for which foetal brain expression was recently confirmed. This insertion led also to a frameshift mutation and generated a premature stop codon (p.E930Gfs9X) in the C- terminal domain and causing the lack of a part of the signal recognized by proteasome as well as the lack of peptidase I serine active site. Moreover, we review previously described, truncated mutations occurring in different regions of the C- terminal domain, and we compared the subcellular mutated protein localization and their resulting patients' phenotypes. CONCLUSIONS: The impairment of alternative splicing of exon 19 and the lack of a part of the proteasome signal due to c.2788insG mutation could disrupt the dynamic regulation of isoform levels especially hCDKL5_5 and hCDKL5_1 during pre and postnatal neurodevelopment and then could cause pathogenic phenotype. Signal peptidase I serine active site seems to modulate hCDKL5_5 movements between nucleus and cytoplasm. We noticed that the resulting phenotypes from truncated mutations among the C-terminal domain of hCDKL5 are almost similar and are always severe.

Clinical, Molecular, and Computational Analysis Showed a Novel Homozygous Mutation Among the Substrate-Binding Site of ARSA Protein in Consanguineous Family with Late-Infantile MLD.

Metachromatic leukodystrophy (MLD) is a neurodegenerative disorder characterized by progressive demyelination resulting from impaired degradation and thus the accumulation of cerebroside-3-sulfate (sulfatide). It is caused by the deficiency of arylsulfatase A (ARSA) enzyme which is encoded by the ARSA gene. The present study reports the clinical, molecular, and bioinformatic investigation of three patients belonging to a consanguineous family with late-infantile MLD disorder. The results revealed a novel homozygous missense mutation c.699C>A (p.His231Gln) in exon 4 of ARSA gene in the three patients inherited from their heterozygous parents. Interestingly, this novel mutation is the second mutation identified in the substrate-binding site of ARSA protein and it was classified as damaging and deleterious by several bioinformatics tools. The c.699C>A (p.His231Gln) leads to changes in the pre-mRNA secondary structure and in the ARSA protein 3D structure with a significant root mean square deviation value which could probably affect its stability and function.

Novel mutations in the CDKL5 gene in complex genotypes associated with West syndrome with variable phenotype: First description of somatic mosaic state.

INTRODUCTION: West syndrome is a rare epileptic encephalopathy of early infancy, characterized by epileptic spasms, hypsarrhythmia, and psychomotor retardation beginning in the first year of life. METHODS: The present study reports the clinical, molecular and bioinformatic investigation in the three studied West patients. RESULTS: The results revealed a complex genotype with more than one mutation in each patient including the known mutations c.1910C>G (P2, P3); c.2372A>C in P3 and c.2395C>G in P1 and novel variants including c.616G>A, shared by the three patients P1, P2 and P3; c.1403G>C shared by P2 and P3 and c.2288A>G in patient P1. CONCLUSIONS: All the mutations were at somatic mosaic state and were de novo in the patients except ones (c.2372A>C). To our knowledge; the somatic mosaic state is described for the first time in patients with West syndrome. Five identified mutations were located in the C-terminal domain of the protein, while the novel mutation (c.616G>A) was in the catalytic domain. Bioinformatic tools predicted that this latter is the most pathogenic substitution affecting 3D protein structure and the secondary mRNA structure. Complex genotype composed of different combinations of mutations in each patient seems to be related to the phenotype variability.

Biochemical analyses and molecular modeling explain the functional loss of 17ß-hydroxysteroid dehydrogenase 3 mutant G133R in three Tunisian patients with 46, XY Disorders of Sex Development.

Mutations in the HSD17B3 gene resulting in 17ß-hydroxysteroid dehydrogenase type 3 (17ß-HSD3) deficiency cause 46, XY Disorders of Sex Development (46, XY DSD). Approximately 40 different mutations in HSD17B3 have been reported; only few mutant enzymes have been mechanistically investigated. Here, we report novel compound heterozygous mutations in HSD17B3, composed of the nonsense mutation C206X and the missense mutation G133R, in three Tunisian patients from two non-consanguineous families. Mutants C206X and G133R were constructed by site-directed mutagenesis and expressed in HEK-293 cells. The truncated C206X enzyme, lacking part of the substrate binding pocket, was moderately expressed and completely lost its enzymatic activity. Wild-type 17ß-HSD3 and mutant G133R showed comparable expression levels and intracellular localization. The conversion of Δ4-androstene-3,17-dione (androstenedione) to testosterone was almost completely abolished for mutant G133R compared with wild-type 17ß-HSD3. To obtain further mechanistic insight, G133 was mutated to alanine, phenylalanine and glutamine. G133Q and G133F were almost completely inactive, whereas G133A displayed about 70% of wild-type activity. Sequence analysis revealed that G133 on 17ß-HSD3 is located in a motif highly conserved in 17ß-HSDs and other short-chain dehydrogenase/reductase (SDR) enzymes. A homology model of 17ß-HSD3 predicted that arginine or any other bulky residue at position 133 causes steric hindrance of cofactor NADPH binding, whereas substrate binding seems to be unaffected. The results indicate an essential role of G133 in the arrangement of the cofactor binding pocket, thus explaining the loss-of-function of 17ß-HSD3 mutant G133R in the patients investigated.