LMNA-NTRK1, weaving kinase domains into a fabric

Lamins A and C are alternatively spliced products of the LMNA gene. Lamins are considered nuclear intermediate filaments. Lamins are components of the nuclear lamina. The nuclear lamina lines the nucleoplasmic side of the nuclear membrane.

Diseases caused by variations in the LMNA gene

From Gene Genecards and/or UniProt unless otherwise noted:

  • Emery-Dreifuss muscular dystrophy 2 and 3.
  • Cardiomyopathy, dilated A1.
  • Lipodystrophy, familial, partial.
  • Limb girdle muscular dystrophy.
  • Charcot-Marie-Tooth disease (peripheral nervous system demyelinating disease).
  • Hutchington-Gilford Progeria.

Functions of the nuclear lamina framework

From Gene Genecards and/or UniProt unless otherwise noted:

  • Nuclear assembly.
  • Chromatin organization.
  • Nuclear membrane and telomere dynamics.
  • Normal development of peripheral nervous system.
  • Skeletal muscle and for muscle satellite cell proliferation.
  • Osteoblastogenesis and bone formation.
  • Adipose tissue distribution.
Estimated LMNA proetin expression various tissue.

Expression of lamin a, gencards.org

LMNA and NTRK the genes

Lamins A and C are alternatively spliced products of the LMNA gene.  The chromosomal location is very close to that of its fusion partner NTRK1. Genecards.org and uniprot.org contain detailed information regarding disease associated variations in the LMNA gene.

Ensemble chromosome 1 location of LMNA and NTRK1 genes.

Chromosomal location of LMNA and NTRK1

The proteins, focused on lamin A/C

protein domains of native lamin A/C and TrkA with inset picture showing lamin crystal structure with TrkA and membrane location of lamins.

Protein domains of native lamin A/C and TrkA

The amino (N) and carboxy ( C ) termini of lamin A are labeled as well as some phosphorylation sites (from Ihalainen, 2015). Also shown are the transmembrane and kinase domains of TrkA. Splice sites were determined by Sartore-Bianchi et. al. (2015).

TrkA kinase domain interaction with lamin oligomers.

Possibilities for achieving activation of the TrkA kinase domain via lamin oligomerization.

 

TrkA phosphorylation of downstream targets

Fig A shows Trk receoptor signaling pathway. Fig B is a western blot shwoing Fusion products LMNA-TRKA, TPM3-TRKA, along with PLCgama1, AKT, MAPK and loading control GAPDH.

Fig. 3D from Sartore-Bianchi, 2015.

Panel A,  TrkA signaling under normal circumstances.

Panel B
• The KM12 colon cancer cell line that contains a TPM3-TrkA fusion kinase. Both it and the LMNA-TrkA cells are activated by autophosphorylation.
• Phospholipase Cγ1 (PLCγ) is activated by TrkA phosphorylation
• Protein kinase B (coded for by the AKT gene) is activated by phosphorylation by Phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K). PI3K can be activated by phosphorylation by TrkA.
• Mitogen activated protein kinase (MAPK) is a very downstream target of activated TrkA. There are many intermediate effectors in this cascade.
• Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a housekeeping protein used to demonstrate equal protein loading.
The lamin-TrkA fusion protein activates traditional downstream enzymes wherever its location in the cell might be.

We know that downstream targets of TrkA are phosphorylated in tumors bearing the LMNA-NTRK1 mutation  (Sartore-Bianchi, 2015). We just don’t know how. The role of MAPK pathways in LMNA laminopathies has been reviewed elsewhere (Lee, 2016).

  • Whether the lamin-TrkA fusion makes its way into structures with wildtype lamin remains to be determined.
  • If such structures exist, would TrkA activity within them be influenced by mechanical stress?

Important information

We have reviewed how an oligomer forming housekeeping protein (lamin) can become fused to a kinase normally requiring ligand induced dimerization for activation (Sartore-Bianchi, 2015).  We have also reviewed a report in the literature demonstrating that downstream enzymes are activated by this colon cancer associated fusion kinase.  A TrkA directed kinase inhibitor would be the logical way to control such a cancer.

We have not covered the emerging opportunity of Trk inhibitors like entrectinib, in controlling such a fusion kinase in this blog. NTRK fusions are taking on a more important role, because of new clinical trials of investigation agents that can regulate TRK.

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:

Sartore-Bianchi A, Ardini E, Bosotti R, Amatu A, Valtorta E, Somaschini A, Raddrizzani L, Palmeri L, Banfi P, Bonazzina E, Misale S, Marrapese G, Leone A, Alzani R, Luo D, Hornby Z, Lim J, Veronese S, Vanzulli A, Bardelli A, Martignoni M, Davite C, Galvani A, Isacchi A, Siena S. (2015)Sensitivity to Entrectinib Associated With a Novel LMNA-NTRK1 Gene Fusion in Metastatic Colorectal Cancer. J Natl Cancer Inst. 2015 Nov 12;108(1).   PubMed

A free version of this paper is available in section Ignyta authored publications.

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