Introduction:
Tuberculosis (TB) is in the top 10 global causes of death, with an estimated 10 million new cases, 1.2 million deaths among HIV-negative individuals and 251,000 deaths among HIV-positive individuals in 2018 (World Health Organization (WHO), 2019). The global burden of TB is unequally distributed, disproportionately affecting low- and middle-income countries particularly in Africa and Asia. One of the barriers to the effective treatment of TB are the adverse drug reactions experienced by patients on anti-TB medications, with one of the most common being drug-induced liver injury (DILI) (Saukkonen et al. , 2006). Three of the four first line drugs used in the treatment of TB - isoniazid, rifampicin and pyrazinamide - are potentially hepatotoxic (Girling, 1977). The estimate of the incidence of DILI in individuals undergoing anti-TB treatment for active TB varies from 2 to 33% depending on the cohort studied, drug regimen used, monitoring and reporting practices (Saukkonen et al. , 2006; Tostmann et al. , 2008; Ramappa and Aithal, 2013). Individuals who experience DILI often need to stop treatment and, if clinically indicated, recommence once liver function tests (LFTs) return to normal. However, for some individuals re-exposure to the same drugs leads to reoccurrence of DILI (Metushi, Uetrecht and Phillips, 2016), and for others liver injury progresses even after treatment has stopped (Hassan et al. , 2015). Therefore, there is an unmet clinical need for new tools to improve the safety of this essential antimicrobial treatment.
Diagnosis of DILI relies on LFTs, with alanine aminotransferase (ALT) activity considered one of the gold standards for determining liver injury. The DILI Expert Working Group defines DILI as ≥ 3 x upper limit of normal (ULN) of ALT in the presence of symptoms, or ≥ 5 x ULN ALT in the absence of symptoms (Aithal et al. , 2011). Although ALT is currently the gold standard for determining DILI there are issues associated with its use. ALT is not specific to the liver and can provide false positive results, with elevations in ALT occurring after muscular damage following exercise (Pettersson et al. , 2008) or subsequent to a myocardial infarction (LaDue and Wroblewski, 1955). Furthermore, elevations in ALT are not specific to DILI (Senior, 2012) and can occur due to metabolic perturbations (Hanley et al. , 2004; Sattar et al. , 2004). In paracetamol-overdose DILI there is a delay between insult to the liver and rise in ALT (Antoine and Dear, 2016), meaning ALT is not optimal as a biomarker of DILI in this context. To address these challenges recent work has identified novel biomarkers capable of diagnosing, and in some cases predicting DILI.
MicroRNAs (miRNAs) are small non-coding RNAs which regulate post-transcriptional gene expression. MiR-122 is a 22-nucleotide microRNA which is highly expressed in, and highly specific for, the liver, with little to no expression in other tissues. In liver injury, miR-122 is released from necrotic hepatocytes, resulting in elevated miR-122 concentrations in the bloodstream (Wang et al. , 2009). Cytokeratin-18 (K18) is an intermediate filament protein responsible for maintaining the cytoskeletal structure in the liver and other epithelial cells and is reported to make up 5% of the liver’s total protein content (Adebayo, Mookerjee and Jalan, 2012). K18 is a mechanistic biomarker of liver injury, providing information on the pattern of cell death. In apoptosis, the release of a caspase-cleaved form of K18 (cc-K18) is an early event during cellular structural rearrangement (Caulín, Salvesen and Oshima, 1997). Whereas, in necrosis, the full-length form of K18 (FL-K18) is passively released upon cell death (Caulín, Salvesen and Oshima, 1997). MiR-122 and K18 are able to predict DILI in patients who overdose on paracetamol earlier than standard LFTs (Antoine et al. , 2013; Dear et al. , 2018) and provides enhanced hepatic specificity over other biomarkers (Vliegenthartet al. , 2015). Both novel biomarkers have regulatory support from the US FDA as biomarkers for DILI (Food and Drug Administration Centre for Drug Evaluation and Research, 2016), although to date their development has been largely limited to testing in Western populations.
The aim of this study was to explore the properties of miR-122 and K18 in relevant European and African patients with mycobacterial infection (with and without HIV co-infection). Specifically, our aims were to determine whether the infective disease process and routine management with anti-TB medicines affect these biomarkers in the absence of DILI and to characterise how miR-122 and K18 change in relation to ALT and in cases of DILI.