Brad R. Rosenberg

Detection of a Recurrent DNAJB1-PRKACA Chimeric Transcript in Fibrolamellar Hepatocellular Carcinoma

Oncogenic Suspect Exposed It can be difficult logistically to study the genomics of rare variants of common cancers. Nevertheless, Honeyman et al. (p. 1010) studied fibrolamellar hepatocellular carcinoma (FL-HCC), a rare and poorly understood liver tumor that affects adolescents and young adults and for which there is no effective treatment. FL-HCCs from 15 patients all expressed a chimeric RNA transcript and protein containing sequences from a molecular chaperone fused in frame with sequences from the catalytic domain of protein kinase A. The chimeric protein retained kinase activity in vitro. Such recurrent gene fusions in cancer may signal a role in pathogenesis and provide an opportunity for therapeutic intervention. A rare form of liver cancer affecting young adults expresses a chimeric kinase that may contribute to pathogenesis. Fibrolamellar hepatocellular carcinoma (FL-HCC) is a rare liver tumor affecting adolescents and young adults with no history of primary liver disease or cirrhosis. We identified a chimeric transcript that is expressed in FL-HCC but not in adjacent normal liver and that arises as the result of a ~400-kilobase deletion on chromosome 19. The chimeric RNA is predicted to code for a protein containing the amino-terminal domain of DNAJB1, a homolog of the molecular chaperone DNAJ, fused in frame with PRKACA, the catalytic domain of protein kinase A. Immunoprecipitation and Western blot analyses confirmed that the chimeric protein is expressed in tumor tissue, and a cell culture assay indicated that it retains kinase activity. Evidence supporting the presence of the DNAJB1-PRKACA chimeric transcript in 100% of the FL-HCCs examined (15/15) suggests that this genetic alteration contributes to tumor pathogenesis.

Transcriptome-wide sequencing reveals numerous APOBEC1 mRNA editing targets in transcript 3′ UTRs

Apolipoprotein B–editing enzyme, catalytic polypeptide-1 (APOBEC1) is a cytidine deaminase initially identified by its activity in converting a specific cytidine (C) to uridine (U) in apolipoprotein B (apoB) mRNA transcripts in the small intestine. Editing results in the translation of a truncated apoB isoform with distinct functions in lipid transport. To address the possibility that APOBEC1 edits additional mRNAs, we developed a transcriptome-wide comparative RNA sequencing (RNA-Seq) screen. We identified and validated 32 previously undescribed mRNA targets of APOBEC1 editing, all of which are located in AU-rich segments of transcript 3′ untranslated regions (3′ UTRs). Further analysis established several characteristic sequence features of editing targets, which were predictive for the identification of additional APOBEC1 substrates. The transcriptomics approach to RNA editing presented here dramatically expands the list of APOBEC1 mRNA editing targets and reveals a novel cellular mechanism for the modification of transcript 3′ UTRs.

Characterizing immune repertoires by high throughput sequencing: strategies and applications

As the key cellular effectors of adaptive immunity, T and B lymphocytes utilize specialized receptors to recognize, respond to, and neutralize a diverse array of extrinsic threats. These receptors (immunoglobulins in B lymphocytes, T cell receptors in T lymphocytes) are incredibly variable, the products of specialized genetic diversification mechanisms that generate complex lymphocyte repertoires with extensive collections of antigen specificities. Recent advances in high throughput sequencing (HTS) technologies have transformed our ability to examine antigen receptor repertoires at single nucleotide, and more recently, single cell, resolution. Here we review current approaches to examining antigen receptor repertoires by HTS, and discuss inherent biological and technical challenges. We further describe emerging applications of this powerful methodology for exploring the adaptive immune system.

Beyond SHM and CSR: AID and Related Cytidine Deaminases in the Host Response to Viral Infection

As the primary effector of immunoglobulin somatic hypermutation (SHM) and class switch recombination (CSR), activation-induced cytidine deaminase (AID) serves an important function in the adaptive immune response. Recent advances have demonstrated that AID and a group of closely related cytidine deaminases, the APOBEC3 proteins, also act in the innate host response to viral infection. Antiviral activity was first attributed to APOBEC3G as a potent inhibitor of HIV. It is now apparent that the targets of the APOBEC3 proteins extend beyond HIV, with family members acting against a wide variety of viruses as well as host-encoded retrotransposable genetic elements. Although it appears to function through a different mechanism, AID also possesses antiviral properties. Independent of its antibody diversification functions, AID protects against transformation by Abelson murine leukemia virus (Ab-MLV), an oncogenic retrovirus. Additionally, AID has been implicated in the host response to other pathogenic viruses. These emerging roles for the AID/APOBEC cytidine deaminases in viral infection suggest an intriguing evolutionary connection of innate and adaptive immune mechanisms.

Diverse functions for DNA and RNA editing in the immune system

Polynucleotide DNA and RNA editing enzymes alter nucleic acid sequences and can thereby modify encoded informational content. Two major families of polynucleotide editing enzymes, the AID/APOBEC cytidine deaminases (which catalyze the deamination of cytidine to uridine) and the adenosine deaminases acting on RNA (ADARs, which catalyze the deamination of adenosine to inosine), function in a variety of host defense mechanisms. These enzymes act in innate and adaptive immune pathways, with both host and pathogen targets. DNA editing by the cytidine deaminase AID mediates immunoglobulin somatic hypermutation and class switch recombination, providing the antibody response with the flexibility and diversity to defend against an almost limitless array of varied and rapidly adapting pathogenic challenges. Other cytidine deaminases (APOBEC3) restrict retroviral infection by editing viral retrogenomes. Adenosine deaminases (ADARs) shape innate immune responses by modifying host transcripts that encode immune effectors and their regulators. Here we review current knowledge of polynucleotide DNA and RNA editors with a focus on these and other functions they serve in the immune system.

Transcriptomic characterization of fibrolamellar hepatocellular carcinoma

Significance Fibrolamellar hepatocellular carcinoma (FLHCC) is a rare pediatric liver cancer. A deletion of ∼400 kb in one copy of chromosome 19 results in a chimeric protein, an activated protein kinase A. No other deletions, amplifications, mutations, or structural variants were found. This strongly implicates the chimera as the driving mutation. This paper examines gene expression in FLHCC. The results establish FLHCC as a single disease distinct from other cancers, including hepatocellular carcinoma. The results help explain some of the known pathophysiology: the collagen fibers that give fibrolamellar its name and the gynecomastia reported in young male patients. Finally, this work identifies oncogenes whose expression is increased and that may serve as targets for therapeutic intervention. Fibrolamellar hepatocellular carcinoma (FLHCC) tumors all carry a deletion of ∼400 kb in chromosome 19, resulting in a fusion of the genes for the heat shock protein, DNAJ (Hsp40) homolog, subfamily B, member 1, DNAJB1, and the catalytic subunit of protein kinase A, PRKACA. The resulting chimeric transcript produces a fusion protein that retains kinase activity. No other recurrent genomic alterations have been identified. Here we characterize the molecular pathogenesis of FLHCC with transcriptome sequencing (RNA sequencing). Differential expression (tumor vs. adjacent normal tissue) was detected for more than 3,500 genes (log2 fold change ≥1, false discovery rate ≤0.01), many of which were distinct from those found in hepatocellular carcinoma. Expression of several known oncogenes, such as ErbB2 and Aurora Kinase A, was increased in tumor samples. These and other dysregulated genes may serve as potential targets for therapeutic intervention.

Human Genetic Determinants of Viral Diseases

Much progress has been made in the identification of specific human gene variants that contribute to enhanced susceptibility or resistance to viral diseases. Herein we review multiple discoveries made with genome-wide or candidate gene approaches that have revealed significant insights into virus-host interactions. Genetic factors that have been identified include genes encoding virus receptors, receptor-modifying enzymes, and a wide variety of innate and adaptive immunity-related proteins. We discuss a range of pathogenic viruses, including influenza virus, respiratory syncytial virus, human immunodeficiency virus, human T cell leukemia virus, human papilloma virus, hepatitis B and C viruses, herpes simplex virus, norovirus, rotavirus, parvovirus, and Epstein-Barr virus. Understanding the genetic underpinnings that affect infectious disease outcomes should allow tailored treatment and prevention approaches in the future.

Identifying mRNA Editing Deaminase Targets by RNA-Seq

RNA editing deaminases act on a variety of targets in different organisms. A number of such enzymes have been shown to act on mRNA, with the resultant nucleotide changes modifying a transcripts information content. Though the deaminase activity of mRNA editing enzymes is readily demonstrated in vitro, identifying their physiological targets has proved challenging. Recent advances in ultra high-throughput sequencing technologies have allowed for whole transcriptome sequencing and expression profiling (RNA-Seq). We have developed a system to identify novel mRNA editing deamination targets based on comparative analysis of RNA-Seq data. The efficacy and utility of this approach is demonstrated for APOBEC1, a cytidine deaminase with a known and well-characterized mRNA editing target in the mammalian small intestine.

Circulating Plasmacytoid Dendritic Cells in Acutely Infected Patients with Hepatitis C Virus Genotype 4 are Normal in Number and Phenotype

The incidence of hepatitis C virus (HCV) genotype 4 infection in Egypt provides a unique opportunity to study the innate immune response to symptomatic acute HCV infection. We investigated whether plasmacytoid dendritic cells (pDCs) are activated as a result of HCV infection. We demonstrate that, even during symptomatic acute infection, circulating pDCs maintained a similar precursor frequency and resting phenotype, compared with pDCs in healthy individuals. Moreover, stimulation with a Toll-like receptor 9 agonist resulted in an intact inflammatory response. These data support the growing consensus that pDCs are not directly activated by HCV and therefore are viable targets for immunotherapy throughout HCV infection.

Genetic variation at IFNL4 influences extrahepatic interferon-stimulated gene expression in chronic HCV patients

Polymorphisms at IFNL4 strongly influence spontaneous resolution and interferon therapeutic response in hepatitis C virus (HCV) infection. In chronic HCV, unfavorable alleles are associated with elevated interferon (IFN)-stimulated gene (ISG) expression in the liver, but extrahepatic effects are less well characterized. We used RNA sequencing (RNA-Seq) to examine whether IFNL4 genetic variation (rs368234815) modulates ISG expression in peripheral blood mononuclear cells (PBMC) during chronic HCV infection. ISG expression was elevated in unstimulated PBMC homozygous for the unfavorable ΔG IFNL4 variant; expression following IFN-α stimulation was comparable across genotypes. These findings suggest that lambda interferons may have broader systemic effects during HCV infection.