Page 33 - Read Online

Armengol et al. Hepatoma Res 2021;7:50 https://dx.doi.org/10.20517/2394-5079.2021.19 Page 3 of 12

Comprehensive genomic profiling analyses also revealed additional genes affected by mutations,
amplification, or loss at very low frequency [29,30] . Recently, a global dysregulation of RNA editing, an
epigenetic mechanism that confers specific nucleotide changes on RNA transcripts without altering the
sequence of genomic DNA, has been reported in HB accompanied by a specific hyperediting of BLCAP
(bladder cancer-associated protein), a gene with tumor suppressor functions . These genes represent
[27]
potentially actionable targets to be further evaluated in view of a precision medicine approach for HB
patients.

TRANSCRIPTOMIC AND EPIGENOMIC STUDIES
Besides activating CTNNB1 mutation, which is found in the majority of HB cases, these tumors have a low
mutation burden, suggesting that the molecular determinants of the clinical heterogeneity of HB should not
be sought among their genetic aberrations. Instead, several papers have associated the different clinical
behavior of HB with differences in their transcriptome. Using gene expression arrays, a transcriptomic
study of 24 HBs clearly revealed two main prognostic subclasses of tumors, which were named C1 and
[11]
C2 . Both transcriptomic subtypes had similar percentages of CTNNB1 mutation; however, they displayed
a strong difference in their gene expression profile in terms of stem-cell/progenitor-like and proliferating
markers as well as in hepatic function and Wnt/β-catenin target related genes. The differences in the
transcriptome of C1 and C2 tumors were also linked to tumor histology, because, when a simplified
pathological parameter called “main epithelial component” was taken into account, C2 tumors displayed
predominantly immature histotypes (i.e., embryonal and crowded fetal), whereas C1 tumors showed a
predominantly well-differentiated fetal histotype, which resembled early and late stages of liver
development, respectively. More importantly, the two transcriptomic subtypes were shown to be associated
with clinical prognostic factors (i.e., metastasis and vascular invasion) and patient outcome. Specifically,
children harboring C2 tumors showed a significantly reduced probability of event-free survival as compared
with patients with C1 tumors. Out of these findings, a 16-gene signature discriminating C1 and C2
subclasses was defined in an attempt to translate the molecular findings to the clinical setting . Of note, the
[11]
dichotomy of HB samples was also found in a subsequent microRNA (miR) array analysis of 49 HB samples
that identified Cm1 and Cm2 subgroups by determining a specific four-miR signature . In a recent study,
[31]
using a large tri-national cohort of surgical samples from 174 HB patients, the prognostic prediction of the
16-gene signature was validated and proposed for inclusion in a new stratification system in combination
with clinical factors with the aim of improving risk-adapted management of HB patients . Interestingly,
[26]
another recent study has demonstrated the prognostic prediction of the 16-gene signature in 35 pre-treated
HB samples from diagnostic biopsies .
[15]

[11]
Almost 10 years after the identification of the C1 and C2 subclasses , three independent studies suggested
the presence of a third transcriptomic subclass of HBs. Using gene expression arrays, Sumazin et al.
[16]
defined HB1, HB2, and HB3 with distinct NFE2L2 and Wnt-signaling activity and LIN28B, HMGA2,
SALL4, and AFP gene expression. By performing RNA-sequencing in 25 HBs, besides C1 tumors,
Hooks et al. identified two different transcriptomic C2 tumors called C2A and C2B and a discriminating
[32]
4-gene signature comprising HSD17B6, ITAG6, TOP2A, and VIM genes. In line with this study, Carrillo-
[27]
Reixach et al. studied the transcriptome of 32 HBs using RNA-seq and also identified two different C2
tumors, which they named: C2-Pure, to reflect its high similarity with the 16-gene expression profile of C2
tumors, and C2-B, which had a distinct gene expression profile and represented about a quarter of the C2
tumors. To capture the transcriptomic complexity of HB and be able to discriminate the three
transcriptomic subtypes, the authors updated the 16-gene signature by including VIM expression in a new
[27]
algorithm .