Isoaspartylation appears to trigger small cell lung cancer-associated autoimmunity against neuronal protein ELAVL4

Mechanisms of immune tolerance breakdown in paraneoplastic neurological syndromes

Paraneoplastic neurological syndromes (PNS) are rare autoimmune disorders triggered by the presence of a cancer. The autoimmunity is herein directed against proteins expressed both in the tumor and in the nervous system, namely the onconeural antigens, against which are directed specific autoantibodies, each of them characterizing a neurological syndrome. The mechanisms of the immune tolerance breakdown in PNS leading to the production of specific autoantibodies directed against the nervous system and leading to the immune attack begins to be explained. Each syndrome is associated with a specific histo-molecular subtype of tumor suggesting a link between the PNS genesis and oncogenesis. The expression of the onconeural antigen by these tumors is insufficient to explain the immune tolerance breakdown. In some PNS tumors, alterations of the antigen have been identified: mutations, gene copy number variation and overexpression of transcript and protein. But in others PNS, no such molecular alterations of the onconeural antigens have been demonstrated. In these cases, other mechanisms of neoantigen generation that may be involved remain to be deciphered. Cancer outcomes of PNS tumors are also characterized by the high frequency of lymph node metastasis at diagnosis. At the primary tumor site, the antitumor immune reaction seems to be particularly intense and characterized by a prominence of B-cell and Ig-secreting plasma cells that may generate the autoantibody secretion. The immune control mechanisms leading to such organization of the immune attack are not known to date. Renewed research efforts are thus needed to better understand the mechanism of immune tolerance breakdown in each PNS and determine potential targets to meet the therapeutic challenges posed by these rare disorders.

Breaking tolerance: autoantibodies can target protein posttranslational modifications

Autoantibodies (AAb) are an immunological resource ripe for exploitation in cancer detection and treatment. Key to this translation is a better understanding of the self-epitope that AAb target in tumor tissue, but do not bind to in normal tissue. Posttranslational modifications (PTMs) on self-proteins are known to break tolerance in many autoimmune diseases and have also recently been described in cancer. This scope of possible autoantigens is quite broad and new high-dimensional and -throughput technologies to probe this repertoire will be necessary to fully exploit their potential. Here, we discuss the strengths and weaknesses of existing high-throughput platforms to detect AAb, review the current methods for characterizing immunogenic PTMs, describe the main challenges to identifying disease-relevant antigens and suggest the properties of future technologies that may be able to address these challenges. We conclude that exploiting the evolutionary power of the immune system to distinguish between self and nonself has great potential to be translated into antibody-based clinical applications.

Posttranslational modifications induce autoantibodies with risk prediction capability in patients with small cell lung cancer

Small cell lung cancer (SCLC) elicits the generation of autoantibodies that result in unique paraneoplastic neurological syndromes. The mechanistic basis for the formation of such autoantibodies is largely unknown but is key to understanding their etiology. We developed a high-dimensional technique that enables detection of autoantibodies in complex with native antigens directly from patient plasma. Here, we used our platform to screen 1009 human plasma samples for 3600 autoantibody-antigen complexes, finding that plasma from patients with SCLC harbors, on average, fourfold higher disease-specific autoantibody signals compared with plasma from patients with other cancers. Across three independent SCLC cohorts, we identified a set of common but previously unknown autoantibodies that are produced in response to both intracellular and extracellular tumor antigens. We further characterized several disease-specific posttranslational modifications within extracellular proteins targeted by these autoantibodies, including citrullination, isoaspartylation, and cancer-specific glycosylation. Because most patients with SCLC have metastatic disease at diagnosis, we queried whether these autoantibodies could be used for SCLC early detection. We created a risk prediction model using five autoantibodies with an average area under the curve of 0.84 for the three cohorts that improved to 0.96 by incorporating cigarette smoke consumption in pack years. Together, our findings provide an innovative approach to identify circulating autoantibodies in SCLC with mechanistic insight into disease-specific immunogenicity and clinical utility.

RNA binding protein HuD promotes autophagy and tumor stress survival by suppressing mTORC1 activity and augmenting ARL6IP1 levels

Background

Neuronal-origin HuD (ELAVL4) is an RNA binding protein overexpressed in neuroblastoma (NB) and certain other cancers. The RNA targets of this RNA binding protein in neuroblastoma cells and their role in promoting cancer survival have been unexplored. In the study of modulators of mTORC1 activity under the conditions of optimal cell growth and starvation, the role of HuD and its two substrates were studied.

Methods

RNA immunoprecipitation/sequencing (RIP-SEQ) coupled with quantitative real-time PCR were used to identify substrates of HuD in NB cells. Validation of the two RNA targets of HuD was via reverse capture of HuD by synthetic RNA oligoes from cell lysates and binding of RNA to recombinant forms of HuD in the cell and outside of the cell. Further analysis was via RNA transcriptome analysis of HuD silencing in the test cells.

Results

In response to stress, HuD was found to dampen mTORC1 activity and allow the cell to upregulate its autophagy levels by suppressing mTORC1 activity. Among mRNA substrates regulated cell-wide by HuD, GRB-10 and ARL6IP1 were found to carry out critical functions for survival of the cells under stress. GRB-10 was involved in blocking mTORC1 activity by disrupting Raptor-mTOR kinase interaction. Reduced mTORC1 activity allowed lifting of autophagy levels in the cells required for increased survival. In addition, ARL6IP1, an apoptotic regulator in the ER membrane, was found to promote cell survival by negative regulation of apoptosis. As a therapeutic target, knockdown of HuD in two xenograft models of NB led to a block in tumor growth, confirming its importance for viability of the tumor cells. Cell-wide RNA messages of these two HuD substrates and HuD and mTORC1 marker of activity significantly correlated in NB patient populations and in mouse xenografts.

Conclusions

HuD is seen as a novel means of promoting stress survival in this cancer type by downregulating mTORC1 activity and negatively regulating apoptosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-02203-2.

Data on isoaspartylation of neuronal ELAVL proteins

This article contains experimental data examining the propensity of neuronal ELAVL proteins to become isoaspartylated. The data are related to the article “Isoaspartylation appears to trigger small cell lung cancer-associated autoimmunity against neuronal protein ELAVL4” (M.A. Pulido, M.K. DerHartunian, Z. Qin, E.M. Chung, D.S. Kang, A.W. Woodham, J.A. Tsou, R. Klooster, O. Akbari, L. Wang, W.M. Kast, S.V. Liu, J.J.G.M. Verschuuren, D.W. Aswad, I.A. Laird-Offringa, 2016) [1], in which it was reported that the N-terminal region of recombinant human ELAVL4 protein, incubated under physiological conditions, acquires a type of highly immunogenic protein damage. Here, we present Western blot analysis data generated by using an affinity-purified polyclonal rabbit antibody (raised against an N-terminal ELAVL4 isoaspartyl-converted peptide) to probe recombinant protein fragments of the other three members of the ELAVL family: the highly homologous neuronal ELAVL2 (HuB) and ELAVL3 (HuC), and the much less homologous ubiquitously expressed ELAVL1 (HuR).