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 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.
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.
Gene rearrangements and overexpression in other tumours
Xalkori (crizotinib), produced by Pfizer, was approved by the FDA for treatment of late stage lung cancer on August 26, 2011. Early results of an initial Phase I trial with 82 patients with ALK induced lung cancer showed an overall response rate of 57%, a disease control rate at 8 weeks of 87% and progression free survival at 6 months of 72%.
Ceritinib was approved by the FDA in April 2014 for the treatment of patients with anaplastic lymphoma kinase (ALK)-positive metastatic non-small cell lung cancer (NSCLC) who have progressed on or are intolerant to crizotinib.
Entrectinib (RXDX-101) is a selective tyrosine kinase inhibitor developed by Ignyta, Inc., with specificity, at low nanomolar concentrations, for all of three Trk proteins (encoded by the three NTRK genes, respectively) as well as the ROS1, and ALK receptor tyrosine kinases. An open label, multicenter, global phase 2 clinical trial called STARTRK-2 is currently underway to test the drug in patients with ROS1/NTRK/ALK gene rearrangements.
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