NFASC-NTRK1, a kinase fused to a glue protein

Neurofascin is a cell adhesion protein that guides neurons. In the wild type protein, the “glue-like” self ssociations occur outside the neuron.  This is not always the case when the neurofascin gene, NFASC, is fused to the TrkA gene, NTRK1.  NFASC-NTRK1 was discovered in survey of 162 glioblastoma multiforme (GBM) tumors (Kim, 2012). The authors  found that 39 exons of neurotrophic tyrosine kinase receptor type 1 (NTRK1, encoding TrkA) are fused to 59 exons of the genes that are highly expressed in neuronal tissues: neurofascin (NFASC) and brevican (BCAN). The focus of this blog is on the product of the NFASC gene, the neurofascin.

The NFASC gene encodes an L1 family immunoglobulin cell adhesion molecule with multiple Ig and fibronectin domains. Protein expression data from genecards.org suggests that neurofascin is very much a nervous system protein under the control of nervous system transcription factors. Tissues not expressing neurofascin have been edited out for clarity.

Estimated NFASC protein expression in various human tissues

NFASC expression.

Diseases caused by variations in the NFASC gene are related to neuronal development and neuronal voltage gated sodium channels.

The Neurofascin gene

Like brevican/BCAN, the other fusion partner of NTRK1/TrkA, NFASC is located on chromosome 1.  Unlike BCAN, they are not neighbors in  the fragile site 1q23.1. Transcription of the NFASC gene is controlled by REST (neuronal), RCOR1 (neuronal), MAZ (blood and immune neuronal expression), NRF1 (neurite outgrowth and respiration in mitochondria), E2F1 (cell cycle control and tumor suppressor).

Ensembl chromosome 1 showing NFASC and NTRK1 gene location.

The chromosome location of the NFASC and NTRK1 genes, genecards.org.

 

Function of neurofascin, the NFASC  gene product

UniProt and  Genecards  unless otherwise noted)

  • part of the extracellular matrix facilitating neurite outgrowth.
  • neurite fasciculation, and organization of the axon initial segment (AIS) and nodes of Ranvier on axons during early development.
  • attaches voltage-gated Na+ (Nav) channels to cytoskeletal and scaffolding proteins including gliomedin, ankyrin 3 (ankyrin-G), and betaIV spectrin.
  • links the AIS extracellular matrix to the intracellular cytoskeleton.
  • Ig domains 1-4 form horse shoe like structures that are capable of forming multimers (Wei, 2012).
  • Ig domains 5-6 probably function as spaces between the horseshoe Ig repeats and the membrane to which the C-terminus may be anchored.
  • Monomers of the fusion protein would not bring Trk1 kinase domains in close proximity for autophosphorylation.  Polymers could conceivably achieve this.
Picture showing Protein domains of Neurofascin and TrkA. Inset shows monomers and polymers of Trk1 and Fn interactions

Monomers versus polymers of the fusion protein.

 

Comments on transfection study of Kim, 2012

EGRFviii (positive control) and NFASC-NTRK1 transfected NIH 3T3 cells are tumorigenic in nude mice.  The rate of tumor development was faster with NFASC-NTRK1 than the positive control.

  • The cells expressing the NTRK1 fusion gene appeared to have increased phosphorylation of TrkA.  Phosphorylated TrkA is kinase active.
  • Increased phosphorylation of AKT or ERK, down stream targets of TrkA was not seen.
  • Downstream signaling of the NTRK1 fusion gene product bypasses these signaling nodes.
Kim2014_NFASC-NTRK1

NFASC-NTRK1 drives tumor growth from Kim, 2014.

 

The results of Kim (2014) are very interesting and a good start in understanding the biology of kinase fusion proteins. It should be noted that phospholipase Cγ and Shc/RAS are downstream targets of activated TrkA.  Perhaps being fused to the C-terminus of neurofascin prevents some, but not all, of the downstream targets of TrkA from reaching the active site.

NFASC-NTRK1sum

Signaling of NGF/TrkA

 

Important information:

We have reviewed how an oligomer forming housekeeping protein (neurofascin) can become fused to a kinase normally requiring ligand induced dimerization for activation. The same report demonstrated that  a downstream enzyme is activated by this brain  cancer associated fusion kinase.

A TrkA directed kinase inhibitor, entrectinib, would be the logical way to control such a cancer.   We have shown this for a different lung cancer associated TrkA fusion protein that had metastasized to the brain.

There is an open NTRK fusion clinical trial that is actively enrolling any solid tumor patient with NTRK fusions (STARTRK-2).   For more information go to the NTRK trial website.

References:

Kim J, Lee Y, Cho HJ, Lee YE, An J, Cho GH, Ko YH, Joo KM, Nam DH. (2014) NTRK1 fusion in glioblastoma multiforme. PLoS One.9(3):e91940. PubMed

Wei CH, Ryu SE. (2012) Homophilic interaction of the L1 family of cell adhesion molecules. Exp Mol Med. 44(7):413-23.  PubMed

 

Leave a Reply