Page 193 - scheppingen
P. 193

Summary
Tuberous sclerosis complex (TSC) is a genetic disease with an incidence between 1 in 6,000 and 1 in 11,000 births presenting with multiple neurological symptoms including epilepsy, mental retardation, and autism. Loss-of-function mutations in either TSC1 or TSC2 lead to constitutive mTOR activation, resulting in an abnormal development of the cerebral cortex with alterations in cortical lamination, cell size and axon and dendrite growth and multiple focal brain structural abnormalities called tubers. Epilepsy is the most prevalent and severe manifestation of TSC, occurring in 70 to 80% of all patients, often already starting early in life. A dysregulated inflammatory response is present in various pathologies of the central nervous system, and increasing evidence shows an important role of brain inflammation in the pathophysiology of epilepsy. Astrocytes are important mediators of inflammatory processes in the brain and play an important role in several neurological disorders, including epilepsy. Indeed, abnormal activation of vari- ous inflammatory pathways has been observed in astrocytes in brain lesions associated with TSC. Increasing evidence supports the involvement of microRNAs in the regulation of astrocyte-mediated inflammatory response. In this thesis, we aimed to investigate the molecular mechanisms of pathology in TSC, in order to find potential targets and novel therapeutic strategies. Additionally, we aimed to evaluate possible functional modula- tions of inflammatory pathways using in vitro astrocyte cultures.
In chapter 2, we aimed to define distinct histological patterns within tubers and to correlate these findings with clinical data. Based on the proportion of calcifications, dysmorphic neurons and giant cells we proposed a new histopathological classification system for cortical tubers in TSC, designated tuber type A, B, and C. Subsequently, we examined the complexity of the molecular signaling network in TSC by evaluating the coding and non-coding transcriptional landscape of TSC cortical tubers using mRNA and small RNA sequencing, which is described in chapter 3. We observed increased expres- sion of genes associated with inflammatory, innate and adaptive immune responses, and a down-regulation of genes associated with neurogenesis and glutamate receptor signal- ing. MicroRNAs represented the largest class of over-expressed small non-coding RNA species in tubers. Amongst these over-expressed miRNAs in TSC, in particular the miR34 family (miR34a, miR34b and miR34c) was overexpressed, of which we validated the func- tional role of miR34b in stimulating neurite outgrowth.
In chapter 4, we evaluated a possible relationship between the expression the β1, β1i, β5, and β5i subunits of the (immuno)proteasome, a multisubunit enzyme com- plex involved in protein degradation, and the clinical course of epilepsy in TSC and focal cortical dysplasia (FCD). Increased expression of these subunits were detected in dys- morphic neurons, balloon/giant cells, and reactive astrocytes, which positively correlated with seizure frequency. Increased expression could be attenuated by treatment with the mTOR inhibitor rapamycin in FCD-derived cell cultures.
In chapter 5, we found increased expression of miR21, miR146a and miR155 in TSC cortical tubers and subependymal giant cell astrocytomas, predominantly in dys- morphic neurons, giant cells and reactive astrocytes. Expression of these miRNAs could be induced by IL-1β stimulation of cultured glial cells. Overexpression of miR146a or miR155 resulted in anti- or pro-inflammatory effects respectively under inflammatory
SUMMARY
 191
eight



























































































   191   192   193   194   195