The 2;5 chromosomal translocation occurs in most anaplastic large-cell non-Hodgkin's lymphomas arising from activated T lymphocytes. This rearrangement fuses the NPM nucleolar phosphoprotein gene on chromosome 5q35 to a protein tyrosine kinase gene, ALK, on chromosome 2p23. In the predicted hybrid protein, the amino terminus of nucleophosmin (NPM) is linked to the catalytic domain of anaplastic lymphoma kinase (ALK). Expressed in the small intestine, testis, and brain but not in normal lymphoid cells, ALK shows greatest sequence similarity to the insulin receptor subfamily of kinases. Unscheduled expression of the truncated ALK may contribute to malignant transformation in these lymphomas (Morris, 1994).
The 2;5 chromosomal translocation is frequently associated with anaplastic large cell lymphomas (ALCLs). The translocation creates a fusion gene consisting of the alk (anaplastic lymphoma kinase) gene and the nucelophosmin (npm) gene: the 3' half of alk derived from chromosome 2 is fused to the 5' portion of npm from chromosome 5. A recent study shows that the product of the npm-alk fusion gene is oncogenic. To help understand how the npm-alk oncogene transform cells, it is important to investigate the normal biological function of the alk gene product, ALK. cDNAs for both the human and mouse ALK proteins have been cloned. The deduced amino acid sequences reveal that ALK is a novel receptor protein-tyrosine kinase having a putative transmembrane domain and an extracellular domain. These sequences are absent in the product of the transforming npm-alk gene. ALK shows the greatest sequence similarity to LTK (leukocyte tyrosine kinase) whose biological function is presently unknown. RNA blot hybridization analysis of various tissues reveals that the alk mRNA is dominantly detected in the brain and spinal cord. Immunoblotting with anti-ALK antibody shows that ALK is highly expressed in the neonatal brain. Furthermore, RNA in situ hybridization analysis shows that the alk mRNA is dominantly expressed in neurons in specific regions of the nervous system such as the thalamus, mid-brain, olfactory bulb, and ganglia of embryonic and neonatal mice. These data suggest that ALK plays an important role(s) in the development of the brain and exerts its effects on specific neurons in the nervous system (Iwahara, 1997).
Anaplastic Lymphoma Kinase (ALK) was originally identified as a member of the insulin receptor subfamily of receptor tyrosine kinases that acquires transforming capability when truncated and fused to nucleophosmin (NPM) in the t(2;5) chromosomal rearrangement associated with non-Hodgkin's lymphoma, but further insights into its normal structure and function are lacking. A full-length normal human ALK cDNA and its product have been characterized, and the pattern of expression of its murine homologue has been determined in embryonic and adult tissues as a first step toward the functional assessment of the receptor. Analysis of the 6226 bp ALK cDNA identified an open reading frame encoding a 1620-amino acid (aa) protein of predicted mass approximately 177 kDa that is most closely related to leukocyte tyrosine kinase (LTK), the two exhibiting 57% aa identity and 71% similarity over their region of overlap. Biochemical analysis demonstrates that the approximately 177 kDa ALK polypeptide core undergoes co-translational N-linked glycosylation, emerging in its mature form as a 200 kDa single chain receptor. Surface labeling studies indicate that the 200 kDa glycoprotein is exposed at the cell membrane, consistent with the prediction that ALK serves as the receptor for an unidentified ligand(s). In situ hybridization studies reveal Alk expression beginning on embryonic day 11 and persisting into the neonatal and adult periods of development. Alk transcripts are confined to the nervous system and included several thalamic and hypothalamic nuclei; the trigeminal, facial, and acoustic cranial ganglia; the anterior horns of the spinal cord in the region of the developing motor neurons; the sympathetic chain, and the ganglion cells of the gut. Thus, ALK is a novel orphan receptor tyrosine kinase that appears to play an important role in the normal development and function of the nervous system (Morris, 1997).
Anaplastic lymphoma kinase (ALK) is a receptor-type protein tyrosine kinase that is expressed preferentially in neurons of the central and peripheral nervous systems at late embryonic stages. To elucidate the role of ALK in neurons, an agonist monoclonal antibody (mAb) was developed against the extracellular domain of ALK. Here, mAb16-39 is shown to elicit tyrosine phosphorylation of endogenously expressed ALK in human neuroblastoma (SK-N-SH) cells. Stimulation of these cells with mAb16-39 markedly induces the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), Shc, and c-Cbl and also their interaction with ALK and activation of ERK1/2. Furthermore, continuous incubation with mAb16-39 induces the cell growth and neurite outgrowth of SK-N-SH cells. These responses are completely blocked by MEK inhibitor PD98059 but not by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, indicating an essential role of the mitogen-activated protein kinase (MAP kinase) signaling cascade in ALK-mediated growth and differentiation of neurons (Motegi, 2004).
During synapse formation, specialized subcellular structures develop at synaptic junctions in a tightly regulated fashion. Cross-signalling initiated by ephrins, Wnts and transforming growth factor-beta family members between presynaptic and postsynaptic termini are proposed to govern synapse formation. It is not well understood how multiple signals are integrated and regulated by developing synaptic termini to control synaptic differentiation. FSN-1 is novel F-box protein that is required in presynaptic neurons for the restriction and/or maturation of synapses in Caenorhabditis elegans. Many F-box proteins are target recognition subunits of SCF (Skp, Cullin, F-box) ubiquitin-ligase complexes. fsn-1 functions in the same pathway as rpm-1, a gene encoding a large protein with RING finger domains. FSN-1 physically associates with RPM-1 and the C. elegans homologues of SKP1 and Cullin to form a new type of SCF complex at presynaptic periactive zones. Evidence is provided that T10H9.2, which encodes the C. elegans receptor tyrosine kinase ALK (anaplastic lymphoma kinase), may be a target or a downstream effector through which FSN-1 stabilizes synapse formation. This neuron-specific, SCF-like complex therefore provides a localized signal to attenuate presynaptic differentiation (Liao, 2004).
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