Anaplastic lymphoma kinase (ALK)
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Anaplastic lymphoma kinase (ALK) also known as ALK tyrosine kinase receptor or CD246 (cluster of differentiation 246) is an enzyme that in humans is encoded by the ALK gene.
ALK plays an important role in the development of the brain and exerts its effects on specific neurons in the nervous system.
The deduced amino acid sequences reveal that ALK is a novel receptor 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).
The ALK gene can be oncogenic in three ways – by forming a fusion gene with any of several other genes, by gaining additional gene copies or with mutations of the actual DNA code for the gene itself.
Anaplastic large-cell lymphoma
The 2;5 chromosomal translocation is associated with approximately 60% anaplastic large-cell lymphomas (ALCLs). The translocation creates a fusion gene consisting of the ALK (anaplastic lymphoma kinase) gene and the nucleophosmin (NPM) gene: the 3' half of ALK, derived from chromosome 2 and coding for the catalytic domain, is fused to the 5' portion of NPM from chromosome 5. The product of the NPM-ALK fusion gene is oncogenic. In a smaller fraction of ALCL patients, the 3' half of ALK is fused to the 5' sequence of TPM3 gene, encoding for tropomyosin 3. In rare cases, ALK is fused to other 5' fusion partners, such as TFG, ATIC, CLTC1, TPM4, MSN, ALO17, MYH9.
Adenocarcinoma of the lung
The EML4-ALK fusion gene is responsible for approximately 3-5% of non-small-cell lung cancer (NSCLC). The vast majority of cases are adenocarcinomas. The standard test used to detect this gene in tumor samples is fluorescence in situ hybridization (FISH) by a US FDA approved kit. Recently Roche Ventana obtained approval in China and European Union countries to test this mutation by immunohistochemistry. Other techniques like reverse-transcriptase PCR (RT-PCR) can also be used to detect lung cancers with an ALK gene fusion but not recommended. ALK lung cancers are found in patients of all ages, although on average these patients tend to be younger. ALK lung cancers are more common in light cigarette smokers or nonsmokers, but a significant number of patients with this disease are current or former cigarette smokers. EML4-ALK-rearrangement in NSCLC is exclusive and not found in EGFR- or KRAS-mutated tumors.
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