Bengt Hallberg

Mechanistic insight into ALK receptor tyrosine kinase in human cancer biology

The burgeoning field of anaplastic lymphoma kinase (ALK) in cancer encompasses many cancer types, from very rare cancers to the more prevalent non-small-cell lung cancer (NSCLC). The common activation of ALK has led to the use of the ALK tyrosine kinase inhibitor (TKI) crizotinib in a range of patient populations and to the rapid development of second-generation drugs targeting ALK. In this Review, we discuss our current understanding of ALK function in human cancer and the implications for tumour treatment.

Appearance of the novel activating F1174S ALK mutation in neuroblastoma correlates with aggressive tumour progression and unresponsiveness to therapy

Mutations in the kinase domain of the ALK kinase have emerged recently as important players in the genetics of the childhood tumor neuroblastoma. Here, we report the appearance of a novel ALK mutation in neuroblastoma, correlating with aggressive tumor behavior. Analyses of genomic DNA from biopsy samples initially showed ALK sequence to be wild type. However, during disease progression, mutation of amino acid F1174 to a serine within the ALK kinase domain was observed, which correlated with aggressive neuroblastoma progression in the patient. We show that mutation of F1174 to serine generates a potent gain-of-function mutant, as observed in 2 independent systems. First, PC12 cell lines expressing ALK(F1174S) display ligand-independent activation of ALK and further downstream signaling activation. Second, analysis of ALK(F1174S) in Drosophila models confirms that the mutation mediates a strong, rough eye phenotype upon expression in the developing eye. Thus, we report a novel ALK(F1174S) mutation that displays ligand-independent activity in vivo, correlating with rapid and treatment-resistant tumor growth. The study also shows that initial screening in the first tumor biopsy of a patient may not be sufficient and that further molecular analysis, in particular in tumor progression and/or tumor relapse, is warranted for better understanding of the treatment of neuroblastoma patients.

Anaplastic Lymphoma Kinase (ALK) regulates initiation of transcription of MYCN in neuroblastoma cells

Neuroblastoma is a neural crest-derived embryonal tumour of the postganglionic sympathetic nervous system and a disease with several different chromosomal gains and losses, which include MYCN-amplified neuroblastoma on chromosome 2, deletions of parts of the chromosomes 1p and 11q, gain of parts of 17q and triploidy. Recently, activating mutations of the ALK (Anaplastic Lymphoma Kinase) RTK (Receptor Tyrosine Kinase) gene have been described in neuroblastoma. A meta-analysis of neuroblastoma cases revealed that ALK mutations (49 of 709 cases) in relation to genomic subtype were most frequently observed in MYCN amplified tumours (8.9%), correlating with a poor clinical outcome. MYCN proteins target proliferation and apoptotic pathways, and have an important role in the progression of neuroblastoma. Here, we show that both wild-type and gain-of-function mutants in ALK are able to stimulate transcription at the MYCN promoter and initiate mRNA transcription of the MYCN gene in both neuronal and neuroblastoma cell lines. Further, this stimulation of MYCN gene transcription and de novo MYCN protein expression is abrogated by specific ALK inhibitors, such as crizotinib (PF-2341066), NVP-TAE684, and by small interfering RNA to ALK resulting in a decrease in proliferation rate. Finally, co-transfection of ALK gain-of-function mutations together with MYCN leads to an increase in transformation potential. Taken together, our results indicate that ALK signalling regulates initiation of transcription of the MYCN gene providing a possible explanation for the poor clinical outcome observed when MYCN is amplified together with activated ALK.

The constitutive activity of the ALK mutated at positions F1174 or R1275 impairs receptor trafficking

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK), which is transiently expressed during development of the central and peripheral nervous system. ALK has been recently identified as a major neuroblastoma predisposition gene and activating mutations have also been identified in a subset of sporadic neuroblastoma tumors. Two hot spots of ALK mutations have been observed at positions F1174 and R1275. Here, we studied stably transfected cell lines expressing wild-type or F1174L- or R1275Q-mutated ALK in parallel with a neuroblastoma cell line (CLB-GE) in which the allele mutated at position F1174 is amplified. We observed that the mutated ALK variants were essentially intracellular and were largely retained in the reticulum/Golgi compartments. This localization was corroborated by a defect of N-linked glycosylation. Although the mutated receptors exhibited a constitutive activation, the minor pool of receptor addressed to the plasma membrane was much more tyrosine phosphorylated than the intracellular pool. The use of antagonist monoclonal antibodies suggested that the constitutive activity of the mutated receptors did not require the dimerization of the receptor, whereas adequate dimerization triggered by agonist monoclonal antibodies increased this activity. Finally, kinase inactivation of the mutated receptors restored maturation and cell-surface localization. Our results show that constitutive activation of ALK results in its impaired maturation and intracellular retention. Furthermore, they provide a rationale for the potential use of kinase inhibitors and antibodies in ALK-dependent tumors.

The kinase ALK stimulates the kinase ERK5 to promote the expression of the oncogene MYCN in neuroblastoma

Targeting the kinase ERK5 may disrupt the activation of an oncogenic transcription factor in a subset of neuroblastoma patients. A New Target in Neuroblastoma Neuroblastoma is a common and aggressive pediatric cancer caused by various molecular abnormalites. Similar to other cancers, poor prognosis correlates with increased abundance or activation of the cell surface receptor tyrosine kinase ALK or increased abundance of the transcription factor MYCN. An ALK inhibitor used in the clinic is not wholly effective, and there are no therapies to directly target MYCN. Umapathy et al. found that ALK stimulated the expression of the gene encoding MYCN through a pathway involving several kinases in patient tumor cells. Targeting one of these kinases, ERK5, decreased the abundance of MYCN and suppressed proliferation in ALK-positive neuroblastoma cells in culture. Combined inhibition of ALK and ERK5 was more effective than the ALK inhibitor alone in limiting tumor growth in a mouse model. Thus, ERK5 represents a new target for treating ALK-driven cancers. Anaplastic lymphoma kinase (ALK) is an important molecular target in neuroblastoma. Although tyrosine kinase inhibitors abrogating ALK activity are currently in clinical use for the treatment of ALK-positive (ALK+) disease, monotherapy with ALK tyrosine kinase inhibitors may not be an adequate solution for ALK+ neuroblastoma patients. Increased expression of the gene encoding the transcription factor MYCN is common in neuroblastomas and correlates with poor prognosis. We found that the kinase ERK5 [also known as big mitogen-activated protein kinase (MAPK) 1 (BMK1)] is activated by ALK through a pathway mediated by phosphoinositide 3-kinase (PI3K), AKT, MAPK kinase kinase 3 (MEKK3), and MAPK kinase 5 (MEK5). ALK-induced transcription of MYCN and stimulation of cell proliferation required ERK5. Pharmacological or RNA interference–mediated inhibition of ERK5 suppressed the proliferation of neuroblastoma cells in culture and enhanced the antitumor efficacy of the ALK inhibitor crizotinib in both cells and xenograft models. Together, our results indicate that ERK5 mediates ALK-induced transcription of MYCN and proliferation of neuroblastoma, suggesting that targeting both ERK5 and ALK may be beneficial in neuroblastoma patients.

Anaplastic lymphoma kinase activates the small GTPase Rap1 via the Rap1-specific GEF C3G in both neuroblastoma and PC12 cells

Many different types of cancer originate from aberrant signaling from the anaplastic lymphoma kinase (ALK) receptor tyrosine kinase (RTK), arising through different translocation events and overexpression. Further, activating point mutations in the ALK domain have been recently reported in neuroblastoma. To characterize signaling in the context of the full-length receptor, we have examined whether ALK is able to activate Rap1 and contribute to differentiation/proliferation processes. We show that ALK activates Rap1 via the Rap1-specific guanine-nucleotide exchange factor C3G, which binds in a constitutive complex with CrkL to activated ALK. The activation of the C3G/Rap1 pathway results in neurite outgrowth of PC12 cells, which is inhibited by either overexpression of Rap1GAP or siRNA-mediated knockdown of Rap1 itself or the guanine nucleotide exchange factor C3G. Significantly, this pathway also appears to function in the regulation of proliferation of neuroblastoma cells such as SK-N-SH and SH-SY5Y, because abrogation of Rap1 activity by Rap1-specific siRNA or overexpression of Rap1GAP reduces cellular growth. These results suggest that ALK activation of Rap1 may contribute to cell proliferation and oncogenesis of neuroblastoma driven by gain-of-function mutant ALK receptors.

Exoenzyme S of Pseudomonas aeruginosa is not able to induce apoptosis when cells express activated proteins, such as Ras or protein kinase B/Akt.

Intracellular targeting of the Pseudomonas aeruginosa toxins, such as exoenzyme S (ExoS), cause cell death, as well as morphological and physiological changes in various tissue culture cells and animal models. In this report we have investigated the mechanism behind ExoS‐mediated cell death. In order to address this issue, we have used cell lines expressing activated forms of various components of the Ras signalling pathway in order to evaluate the importance of the Ras pathway for viability and survival upon ExoS infection. Here we show that activated Ras is able to protect cells against cell death, regardless of whether it has been ADP‐ribosylated by ExoS. Further, an activated form of protein kinase B (PKB)/Akt also leads to decreased level of cell death in response to ExoS infection, indicating that an important ExoS survival target is located upstream of Raf‐1 and PKB/Akt. Moreover, we show that ExoS infection inhibits phosphorylation of FOXO3a, and induces caspase‐3 activity, which are hallmarks for induction of cell death. In conclusion, we suggest that Ras proteins are an important cellular target for the P. aeruginosa toxin ExoS, which induces cell death during pathogenesis as a means of defending the bacterium against eukaryotic phagocytosis.

Intragenic Anaplastic Lymphoma Kinase (ALK) Rearrangements: Translocations as a Novel Mechanism of ALK Activation in Neuroblastoma Tumors

Anaplastic lymphoma kinase (ALK) has been demonstrated to be deregulated in sporadic as well as in familiar cases of neuroblastoma (NB). Whereas ALK‐fusion proteins are common in lymphoma and lung cancer, there are few reports of ALK rearrangements in NB indicating that ALK mainly exerts its oncogenic capacity via activating mutations and/or overexpression in this tumor type. In this study, 332 NB tumors and 13 cell lines were screened by high resolution single nucleotide polymorphism microarray. Gain of 2p was detected in 23% (60/332) of primary tumors and 46% (6/13) of cell lines, while breakpoints at the ALK locus were detected in four primary tumors and two cell lines. These were further analyzed by next generation sequencing and a targeted enrichment approach. Samples with both ALK and MYCN amplification displayed complex genomic rearrangements with multiple breakpoints within the amplicon. None of the translocations characterized in primary NB tumors are likely to result in a chimeric protein. However, immunohistochemical analysis reveals high levels of phosphorylated ALK in these samples despite lack of initial exons, possibly due to alternative transcription initiation sites. Both ALK proteins predicted to arise from such alterations and from the abnormal ALK exon 4–11 deletion observed in the CLB‐BAR cell line show strong activation of downstream targets STAT3 and extracellular signal‐regulated kinase (ERK) when expressed in PC12 cells. Taken together, our data indicate a novel, although rare, mechanism of ALK activation with implications for NB tumorigenesis.

Brigatinib, an anaplastic lymphoma kinase inhibitor, abrogates activity and growth in ALK-positive neuroblastoma cells, Drosophila and mice

Anaplastic lymphoma kinase (ALK) is a tyrosine kinase receptor which has been implicated in numerous solid and hematologic cancers. ALK mutations are reported in about 5-7% of neuroblastoma cases but the ALK-positive percentage increases significantly in the relapsed patient population. Crizotinib, the first clinically approved ALK inhibitor for the treatment of ALK-positive lung cancer has had less dramatic responses in neuroblastoma. Here we investigate the efficacy of a second-generation ALK inhibitor, brigatinib, in a neuroblastoma setting. Employing neuroblastoma cell lines, mouse xenograft and Drosophila melanogaster model systems expressing different constitutively active ALK variants, we show clear and efficient inhibition of ALK activity by brigatinib. Similar abrogation of ALK activity was observed in vitro employing a set of different constitutively active ALK variants in biochemical assays. These results suggest that brigatinib is an effective inhibitor of ALK kinase activity in ALK addicted neuroblastoma that should be considered as a potential future therapeutic option for ALK-positive neuroblastoma patients alone or in combination with other treatments.

The ALK inhibitor PF-06463922 is effective as a single agent in neuroblastoma driven by expression of ALK and MYCN.

ABSTRACT The first-in-class inhibitor of ALK, c-MET and ROS1, crizotinib (Xalkori), has shown remarkable clinical efficacy in treatment of ALK-positive non-small cell lung cancer. However, in neuroblastoma, activating mutations in the ALK kinase domain are typically refractory to crizotinib treatment, highlighting the need for more potent inhibitors. The next-generation ALK inhibitor PF-06463922 is predicted to exhibit increased affinity for ALK mutants prevalent in neuroblastoma. We examined PF-06463922 activity in ALK-driven neuroblastoma models in vitro and in vivo. In vitro kinase assays and cell-based experiments examining ALK mutations of increasing potency show that PF-06463922 is an effective inhibitor of ALK with greater activity towards ALK neuroblastoma mutants. In contrast to crizotinib, single agent administration of PF-06463922 caused dramatic tumor inhibition in both subcutaneous and orthotopic xenografts as well as a mouse model of high-risk neuroblastoma driven by Th-ALKF1174L/MYCN. Taken together, our results suggest PF-06463922 is a potent inhibitor of crizotinib-resistant ALK mutations, and highlights an important new treatment option for neuroblastoma patients. Summary: Our results suggest that PF-06463922 is a potent inhibitor of crizotinib-resistant ALK mutations, and highlights an important new treatment option for neuroblastoma patients.