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.