Molecular genetic diagnostics of tuberous sclerosis complex in Bulgaria: six novel mutations in the TSC1 and TSC2 genes

Molecular-Genetic Characteristics and Genotype-Phenotype Correlations in Bulgarian Patients with Tuberous Sclerosis Complex

Objective

The aim of the study was to determine the molecular-genetic characteristics of the autosomal dominant systematic disorder Tuberous Sclerosis Complex (TSC1 and TSC2) in Bulgarian patients and to derive some genotype-phenotype correlations.

Material and Methods

In total 42 patients/families with suspected clinical diagnosis of TSC were analyzed. We used direct sequencing and MLPA for the TSC1 and TSC2 gene analysis.

Results

In 38 families (90.5%) we confirmed the suspected clinical diagnosis – 15 with TSC1 (35.7%) and 23 (54.8%) with TSC2. In 4 families (9.5%) pathogenic variants were not found. In all 38 patients with proven diagnosis of TSC, we found 38 different mutations, 15 of which (39%) were detected for the first time by our research group. The mutation “hotspots“ in TSC1 gene are exons 9, 15, 17 and 18, where 73% of the TSC1 mutations are localized, while the TSC2 gene mutation “hotspots“ are exons 13 and 34, with 22% of the mutations situated there. In the TSC2 patients the common clinical findings include subcortical tubers, epilepsy with generalized tonic-clonic seizures, subependymal giant cell astrocytoma, facial angiofibromas, ungual fibromas, cardiac rhabdomyomas and renal angiomyolipomas, while in the TSC1 patients typically cortical tubers, cortical dysplasia and subependymal nodules were registered. In patients with aggressive frameshift and nonsense TSC1 and TSC2 mutations commonly hypomelanotic macules, cortical and subcortical tubers, cortical dysplasia, epilepsy with different types of seizures were found. Renal angiomyolipomas and cysts were detected mainly in patients with large deletions. Shagreen patches and intellectual disability were typically registered in equal degree in patients with frameshift, nonsense and missense mutations.

Conclusion

Although some genotype-phenotype correlations were derived, there is a great inter- and intrafamilial clinical variability in TSC, so it is impossible to predict the course of the disease on the basis of the detected molecular defect. The obtained results helped us to develop a diagnostic algorithm for proper molecular-genetic diagnostics which permits adequate genetic counseling, prophylaxis and treatment in the affected TSC families.

A novel TSC1 frameshift mutation c.1550_1551del causes tuberous sclerosis complex by aberrant splicing and nonsense-mediated mRNA degradation (NMD) simultaneously in a Chinese family

Background

Tuberous sclerosis complex (TSC), belongs to autosomal dominant genetic disorder, which affects multiple organ systems in the body, including the skin, brain, lungs, kidneys, liver, and eyes. Mutations in TSC1 or TSC2 was proved to be associated with these conditions.

Methods

Gene‐panel Sequence of NGS was used to detect the mutation in a Chinese family. The research further investigates whether aberrant splicing and nonsense‐mediated mRNA degradation (NMD) could serve as a mechanism cause by TSC1 mutation. MINI‐Gene assay apply by pcMINI‐TSC1wt/mut plasmids delivered in HeLa and 293T cell lines. Recombinant plasmids expressing wild‐type and mutant‐type EGFP‐TSC1 were constructed and transiently transfected into human embryonic kidney cells 293T by lipofectamine. Real‐time PCR and Western Blot were performed to analyze the expression of mRNAs and proteins of EGFP‐TSC1 and NMD factor UPF1.

Results

The gene test verified a novel heterozygous TSC1 frameshift mutation (TSC1 c.1550_1551del) in the proband and her mother. From MINI‐Gene assay, the agarose gel showed that both the mutant and wild‐type mRNA possess two main bands, indicating two splicing modes, named band A and B, respectively. The mutation c.1550_1551del has not produced new splicing site, but there is a selective splicing in varying degree significantly after mutation. On the contrary, function validation assay showed that cells transfected with the mutant TSC1 plasmids expressed significantly lower TSC1 in mRNAs and proteins levels, compared with the wild‐type TSC1 plasmid transfection. A translation inhibitor cycloheximide and small interfering RNA of UPF1 (siRNA‐UPF1) increased mRNA or protein expression of TSC1 significantly in cells transfected with the mutant plasmids.

Conclusion

Our study demonstrated that the novel TSC1 frameshift mutation (TSC1 c.1550_1551del) trigger aberrant splicing and NMD simultaneously, causing decrease of hamartin, then, leading to tuberous sclerosis complex formation.