Protein profiling for hepatocellular carcinoma biomarker discovery in West African subjects
Keywords: Biomarkers
Hepatitis
Hepatocellular carcinoms
Proteomics
Proteins
Liver cirrhosis

Background: Hepatocellular Carcinoma (HCC) is the third most common cause of cancer related death worldwide and is often diagnosed by measuring serum Alpha-fetoprotein (AFP); a stand-alone biomarker with limited diagnostic proficiency. To compensate for this, AFP is commonly used in conjunction with high performance imaging and radiological methods. However, as the burden of HCC is predominantly in the developing world where such technologies are not readily available, it is imperative that efforts are made to pursue the discovery of novel, high performance, easy to measure and robust biomarkers. With the aim of improving on the diagnostic ability of AFP, our project focuses on the study of plasma proteins as identified by Mass Spectrometry (MS) in order to investigate differences seen in the respective proteomes of controls and subjects with liver cirrhosis (LC) and HCC.
Methods: Matrix Assisted Laser Desorption Ionization Time-of-Flight MS (MALDI-TOF MS) was first attempted on weak cation exchange (WCX) fractionated plasma in a pilot selection of forty subjects. On the main case-control group, quantitative MS analysis using liquid chromatography electro spray ionization quadrupole time-of-flight (LC-ESI Q-TOF) was conducted on 339 subjects using a pooled expression profiling approach. Enzyme-linked immunosorbent assays (ELISA) and 1 and 2Dimentional electrophoresis methods were performed to validate and detail candidate protein levels and modification patters in individual and pooled subjects. The human plasma used for the MS based protein discovery experiments was collected as part of a five year Liver Cancer Case-control Study (Gambia, West Africa). A smaller set of samples from subjects who formed a spectrum of non-liver disease controls, LC and HCC were obtained from the Jos University Teaching Hospital (JUTH) in Nigeria and ELISA and gel electrophoresis assays conducted on them to confirm the trends and differences seen in the Gambian subject set.
Results: Bioinformatic evaluation of MALDI-TOF data highlighted peak masses 2444m/z, 2583m/z and 2559m/z to have high diagnostic abilities based on area under curve (AUC) statistics of >0.75. Of these polypeptide fragments, one was identified as the plasma glycoprotein, alpha chain fibrinogen. Results from the large-scale label free discovery experiments indicated twenty-six proteins to be differentially expressed between the three subject groups. These prospective markers include proteins previously linked to HCC as well as novel candidates, namely glutathione peroxidase 3, serum amyloid p, carboxypeptidase N and complement factors I and H which have not been implicated in the context of HCC diagnostics. Direct measurement of Hemopexin (HPX), alpha-1-antitrypsin (α1AT), apolipoprotein A1 (Apo A1) and complement component 3 (CC3) levels confirmed their change in abundance in LC and HCC versus control patients. Further biochemical characterization of glycosylated HPX isolated from glycoprotein enriched plasma sample pools showed evidence of isoelectric point shifts, indicating differential glycosylation patterns in high mannose structures of HPX which may be disease stage linked. The direct measurements of HPX, α1AT, Apo A1 & CC3 conducted on the independent Nigerian subject group also confirmed much of the trends reported from the Gambia Liver Cancer Study (GLCS) plasma.
Conclusions: The independently validated, significant changes in the quantitative expression of ApoA1, α1AT, CC3 and HPX could be exploited for development into high-performance affordable assays, usable in the diagnosis and monitoring of HCC and LC patients. The unique signatures observed for most of these proteins, from liver disease free controls to LC and HCC suggest their involvement in independent pathways. As such, combining some or all of these four markers within a diagnostic panel could offer a much-needed boost in robustness and accuracy for AFP. The differences in the processing and molecular weight separation of these proteins also offers a novel inroad into biomarker identification. These suggested disease specific signatures could with further study offer highly specific biomarkers able to discern the key stages that predispose individuals to hepatocarcinogenesis.
Impact: This is the first MS based discovery and extensive validation study on West African subjects whose primary cause of HCC are the Hepatitis B Virus (HBV) and fungal toxins.