Rhinencephalon changes in tuberous sclerosis complex

Tsc2 shapes olfactory bulb granule cell molecular and morphological characteristics

Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by mutations that inactivate TSC1 or TSC2. Hamartin and tuberin are encoded by TSC1 and TSC2 which form a GTPase activating protein heteromer that inhibits the Rheb GTPase from activating a growth promoting protein kinase called mammalian target of rapamycin (mTOR). Growths and lesions occur in the ventricular-subventricular zone (V-SVZ), cortex, olfactory tract, and olfactory bulbs (OB) in TSC. A leading hypothesis is that mutations in inhibitory neural progenitor cells cause brain growths in TSC. OB granule cells (GCs) are GABAergic inhibitory neurons that are generated through infancy by inhibitory progenitor cells along the V-SVZ. Removal of Tsc1 from mouse OB GCs creates cellular phenotypes seen in TSC lesions. However, the role of Tsc2 in OB GC maturation requires clarification. Here, it is demonstrated that conditional loss of Tsc2 alters GC development. A mosaic model of TSC was created by performing neonatal CRE recombinase electroporation into inhibitory V-SVZ progenitors yielded clusters of ectopic cytomegalic neurons with hyperactive mTOR complex 1 (mTORC1) in homozygous Tsc2 mutant but not heterozygous or wild type mice. Similarly, homozygous Tsc2 mutant GC morphology was altered at postnatal days 30 and 60. Tsc2 mutant GCs had hypertrophic dendritic arbors that were established by postnatal day 30. In contrast, loss of Tsc2 from mature GCs had negligible effects on mTORC1, soma size, and dendrite arborization. OB transcriptome profiling revealed a network of significantly differentially expressed genes following loss of Tsc2 during development that altered neural circuitry. These results demonstrate that Tsc2 has a critical role in regulating neural development and shapes inhibitory GC molecular and morphological characteristics.

Olfactory bulb enlargement in neurofibromatosis type 1: report of a novel finding

Neurofibromatosis type 1 (NF1) is a genetic autosomal dominant disease caused by mutation of the protein neurofibromin, a regulator of cell growth. The most frequent intracranial findings are unidentified bright objects (UBOs), thickening of the corpus callosum, sphenoid wing dysplasia, cerebral vasculopathy, optic and non-optic pilocytic astrocytomas, and plexiform neurofibromas. We report two cases of NF1 patients with asymptomatic olfactory bulbs (OBs) enlargement depicted with Magnetic Resonance Imaging (MRI). To the best of our knowledge, this finding has not been reported in the scientific literature so far. We hypothesize that olfactory bulbs enlargement may have a pathogenetic nature like that of the UBOs as in one of our patients there was spontaneous regression during follow-up. The olfactory bulbs enlargement expands the broad neuroradiological spectrum of finding of NF1. More reports are required to better understand incidence, pathogenesis, and clinical behavior of olfactory bulbs enlargement in NF1 patients.

The Neurodevelopmental Pathogenesis of Tuberous Sclerosis Complex (TSC)

Tuberous sclerosis complex (TSC) is a model disorder for understanding brain development because the genes that cause TSC are known, many downstream molecular pathways have been identified, and the resulting perturbations of cellular events are established. TSC, therefore, provides an intellectual framework to understand the molecular and biochemical pathways that orchestrate normal brain development. The TSC1 and TSC2 genes encode Hamartin and Tuberin which form a GTPase activating protein (GAP) complex. Inactivating mutations in TSC genes (TSC1/TSC2) cause sustained Ras homologue enriched in brain (RHEB) activation of the mammalian isoform of the target of rapamycin complex 1 (mTORC1). TOR is a protein kinase that regulates cell size in many organisms throughout nature. mTORC1 inhibits catabolic processes including autophagy and activates anabolic processes including mRNA translation. mTORC1 regulation is achieved through two main upstream mechanisms. The first mechanism is regulation by growth factor signaling. The second mechanism is regulation by amino acids. Gene mutations that cause too much or too little mTORC1 activity lead to a spectrum of neuroanatomical changes ranging from altered brain size (micro and macrocephaly) to cortical malformations to Type I neoplasias. Because somatic mutations often underlie these changes, the timing, and location of mutation results in focal brain malformations. These mutations, therefore, provide gain-of-function and loss-of-function changes that are a powerful tool to assess the events that have gone awry during development and to determine their functional physiological consequences. Knowledge about the TSC-mTORC1 pathway has allowed scientists to predict which upstream and downstream mutations should cause commensurate neuroanatomical changes. Indeed, many of these predictions have now been clinically validated. A description of clinical imaging and histochemical findings is provided in relation to laboratory models of TSC that will allow the reader to appreciate how human pathology can provide an understanding of the fundamental mechanisms of development.

Cerebellar lesions as potential predictors of neurobehavioural phenotype in tuberous sclerosis complex

AIM

To improve the genetic, clinical, and neuroradiological characterization of cerebellar involvement in tuberous sclerosis complex (TSC) and determine whether cerebellar lesions could be a reliable biomarker of neurological impairment.

METHOD

This retrospective cohort study, held at two tertiary paediatric university centres, was conducted on patients with a confirmed diagnosis of TSC who underwent brain magnetic resonance imaging between October 2009 and May 2016. The study population consisted of 112 patients with TSC (median age 10y; range 5mo-38y; 61 females, 51 males).

RESULTS

The results from multivariable statistical analysis indicated that cerebellar involvement (34 out of 112 patients, none carrying a TSC1 mutation) was the most powerful predictor of supratentorial cortical tuber load; however, cerebellar involvement was not the best predictor of clinical phenotype when supratentorial tuber load and TSC2 mutations were taken into consideration. The association between cerebellar lesions and a more severe clinical and neuroradiological phenotype was statistically significant and may be due to its strong association with TSC2 mutations and higher cortical tuber load.

INTERPRETATION

Cerebellar involvement is not the best predictor of neurobehavioural outcome, including TSC-related autism, after adjusting for TSC2 and the number of cortical tubers. Its role in the TSC clinical phenotype needs to be investigated further.

WHAT THIS PAPER ADDS

Cerebellar involvement is a powerful predictor of supratentorial cortical involvement and a potential biomarker of disease severity. Cerebellar lesions significantly correlate with a more severe clinical and neuroradiological phenotype. Cerebellar involvement is not the best predictor of neurobehavioural outcome.