Detection of GRM1 gene rearrangements in chondromyxoid fibroma: a comparison of fluorescence in-situ hybridisation, RNA sequencing and immunohistochemical analysis.

AIMS: Chondromyxoid fibroma (CMF) is a rare, benign bone tumour which arises primarily in young adults and is occasionally diagnostically challenging. Glutamate metabotropic receptor 1 (GRM1) gene encodes a metabotropic glutamate receptor and was recently shown to be up-regulated in chondromyxoid fibroma through gene fusion and promoter swapping. The aim of this study was to interrogate cases of CMF for the presence of GRM1 gene rearrangements, gene fusions and GRM1 protein overexpression. METHODS AND RESULTS: Selected cases were subjected to testing by fluorescent in-situ hybridisation (FISH) with a GRM1 break-apart probe, a targeted RNA sequencing method and immunohistochemical study with an antibody to GRM1 protein. Two cases were subjected to whole transcriptomic sequencing. In 13 of 13 cases, GRM1 protein overexpression was detected by immunohistochemistry using the GRM1 antibody. Of the 12 cases successfully tested by FISH, nine of 12 showed GRM1 rearrangements by break-apart probe assay. Targeted RNA sequencing analysis did not detect gene fusions in any of the eight cases tested, but there was an increase in GRM1 mRNA expression in all eight cases. Two cases subjected to whole transcriptomic sequencing (WTS) showed elevated GRM1 expression and no gene fusions. CONCLUSION: GRM1 gene rearrangements can be detected using FISH break-apart probes in approximately 75% of cases, and immunohistochemical detection of GRM1 protein over-expression is a sensitive diagnostic method. The gene fusion was not detected by targeted RNA sequencing, due most probably to the complexity of fusion mechanism, and is not yet a reliable method for confirming a diagnosis of CMF in the clinical setting.

Preferential electrical coupling regulates neocortical lineage-dependent microcircuit assembly.

Radial glial cells are the primary neural progenitor cells in the developing neocortex. Consecutive asymmetric divisions of individual radial glial progenitor cells produce a number of sister excitatory neurons that migrate along the elongated radial glial fibre, resulting in the formation of ontogenetic columns. Moreover, sister excitatory neurons in ontogenetic columns preferentially develop specific chemical synapses with each other rather than with nearby non-siblings. Although these findings provide crucial insight into the emergence of functional columns in the neocortex, little is known about the basis of this lineage-dependent assembly of excitatory neuron microcircuits at single-cell resolution. Here we show that transient electrical coupling between radially aligned sister excitatory neurons regulates the subsequent formation of specific chemical synapses in the neocortex. Multiple-electrode whole-cell recordings showed that sister excitatory neurons preferentially form strong electrical coupling with each other rather than with adjacent non-sister excitatory neurons during early postnatal stages. This preferential coupling allows selective electrical communication between sister excitatory neurons, promoting their action potential generation and synchronous firing. Interestingly, although this electrical communication largely disappears before the appearance of chemical synapses, blockade of the electrical communication impairs the subsequent formation of specific chemical synapses between sister excitatory neurons in ontogenetic columns. These results suggest a strong link between lineage-dependent transient electrical coupling and the assembly of precise excitatory neuron microcircuits in the neocortex.

Slitrks as emerging candidate genes involved in neuropsychiatric disorders.

Slitrks are a family of structurally related transmembrane proteins belonging to the leucine-rich repeat (LRR) superfamily. Six family members exist (Slitrk1-6) and all are highly expressed in the central nervous system (CNS). Slitrks have been implicated in mediating basic neuronal processes, ranging from neurite outgrowth and dendritic elaboration to neuronal survival. Recent studies in humans and genetic mouse models have led to the identification of Slitrks as candidate genes that might be involved in the development of neuropsychiatric conditions, such as obsessive compulsive spectrum disorders and schizophrenia. Although these system-level approaches have suggested that Slitrks play prominent roles in CNS development, key questions remain regarding the molecular mechanisms through which they mediate neuronal signaling and connectivity.