Page 127 - Molecular features of low-grade developmental brain tumours

DYSREGULATION OF MMP/TIMP IN SEGA: MODULATION BY MIR-320D IN VITRO
Introduction
Tuberous Sclerosis Complex (TSC) is an autosomal dominantly inherited neurocutaneous disorder affecting approximately 1 million individuals worldwide 1,2. It is caused by inactivating mutations in either the TSC1 or the TSC2 gene 3,4 resulting in constitutive activation of the mammalian target of rapamycin (mTOR) pathway which can affect cell growth and proliferation 5-8. Several brain abnormalities are observed in patients with TSC, including cortical tubers, subependymal nodules (SENs) and subependymal giant cell astrocytomas (SEGA) 9-11. SEGA are progressive low-grade tumours that develop in the first two decades of life in children and adolescents with TSC with a prevalence ranging from 5% to 25% 12-16. They are slow-growing tumours located near the foramen of Monro and extended growth can lead to the obstruction of the cerebrospinal fluid flow and acute hydrocephalus 16,17. Symptoms associated with growing SEGA include headaches, photophobia, diplopia, ataxia, or changes in seizure severity 18. SEGA are believed to develop from SENs and are characterized by distinctive cytomegalic cells, which display an immature neuroglial phenotype 19-22.
Genetically, there is evidence of second-hit inactivation of TSC1 or TSC2 in SEGA. However, in ~20% of SEGA these second-hit mutations are not observed, suggesting that additional molecular processes might be involved in SEGA growth 6,23,24. Several studies have performed transcriptional profiling of SEGA identifying differential expression of genes related to the immune system, MAPK family signaling cascades and extracellular matrix (ECM) organization 24-26. Dysregulation of ECM organization has also been seen in TSC cortical tubers with a specific role for matrix metalloproteinases (MMPs) and their endogenous inhibitors (TIMPs), suggesting that the ECM might play an important role in TSC 27-29. MMPs are calcium- dependent zinc-containing endopeptidases that are expressed with a pro-peptide that needs to be removed for activation 30. TIMPs are ‘wedge-like’-shaped molecules with four residues at N-terminal that form a ridge that can non-covalently bind to the active site of MMPs and thereby inactivate them 31. The MMP/TIMP proteolytic system is known to be involved in the degradation of ECM 32, tissue morphogenesis 33, cell migration 34, angiogenesis 35, blood-brain barrier (BBB) dysfunction 36, wound healing and inflammation 37. Differential expression of MMPs is found in many pathologies including cancer, where they affect proliferation and the metastasis of tumour cells 38-40.
Several MMP inhibitors exist that can regulate MMP overactivity. However, the currently available MMP inhibitors can have a variety of side-effects indicating the complexity of MMP regulation 41. MicroRNAs (miRNAs) are short non-coding RNAs which are 20 to 25 nucleotides long and are able to regulate the expression of protein-coding genes, including MMPs 42,43. They are involved in many physiological processes, such as differentiation, proliferation and development 44 and have been implicated in neurological disorders 45,46. Previous research showed several miRNAs to be differentially expressed in TSC cortical tubers and SEGA 26,28,29,47,48. In particular miRNAs have been shown to participate in MMP regulation at the posttranscriptional level in TSC tuber-derived astroglial cultures and glioma cell lines 28,49,50.
In our previous SEGA transcriptome study we identified differential expression of
125
5