RSPO2 gene rearrangement: a powerful driver of β-catenin activation in liver tumours

Abstract

ObjectiveWe aimed at the identification of genetic alterations that may functionally substitute for CTNNB1 mutation in ß-catenin-activated hepatocellular adenomas (HCAs) and hepatocellular carcinoma (HCC).DesignLarge cohorts of HCA (n=185) and HCC (n=468) were classified using immunohistochemistry. The mutational status of the CTNNB1 gene was determined in ß-catenin-activated HCA (b-HCA) and HCC with at least moderate nuclear CTNNB1 accumulation. Ultra-deep sequencing was used to characterise CTNNB1wild-type and ß-catenin-activated HCA and HCC. Expression profiling of HCA subtypes was performed.ResultsA roof plate-specific spondin 2 (RSPO2) gene rearrangement resulting from a 46.4 kb microdeletion on chromosome 8q23.1 was detected as a new morphomolecular driver of β-catenin-activated HCA. RSPO2 fusion positive HCA displayed upregulation of RSPO2 protein, nuclear accumulation of β-catenin and transcriptional activation of β-catenin-target genes indicating activation of Wingless-Type MMTV Integration Site Family (WNT) signalling. Architectural and cytological atypia as well as interstitial invasion indicated malignant transformation in one of the RSPO2 rearranged b-HCAs. The RSPO2 gene rearrangement was also observed in three β-catenin-activated HCCs developing in context of chronic liver disease. Mutations of the human telomerase reverse transcriptase promoter—known to drive malignant transformation of CTNNB1-mutated HCA—seem to be dispensable for RSPO2 rearranged HCA and HCC.ConclusionThe RSPO2 gene rearrangement leads to oncogenic activation of the WNT signalling pathway in HCA and HCC, represents an alternative mechanism for the development of b-HCA and may drive malignant transformation without additional TERT promoter mutation.

RSPO2 abnormal transcripts result from read-through in liver tumours with high ß-catenin activation and CTNNB1 mutations

“…All abnormal transcripts fused RSPO2 exon 2 or 3 to an adjacent RNA transcribed sequence located 58 813–58 922 nucleotides upstream (chr8:108,141,620–108,141,729, hg38) (figure 1B). RSPO2 exon 2 boundaries were similar to that described by Longerich and collaborators,1 whereas the upstream sequence was variable. In two of the tumours with abnormal RSPO2 transcript, we performed whole-genome sequencing and we observed no focal deletions at the DNA level.…”

“…Thomas Longerich and collaborators1 recently published an intriguing observation in hepatocellular adenoma (HCA) showing an activation of the Wnt/ß-catenin signalling pathway without CTNNB1 or APC mutations. The authors identified a recurrent deletion leading to a fusion between a short interspersed nuclear element (SINE) sequence and RSPO2 gene in three HCAs and three hepatocellular carcinomas (HCCs) all activated for ß-catenin, including one tumour with CTNNB1 mutations and five tumours without APC or CTNNB1 mutations.…”

“…Misinterpretation of read-throughs and deletion can occur frequently in targeted resequencing of poor-quality nucleic acid samples. Indeed, Longerich and colleagues1 performed genomic analyses in DNA and RNA extracted from formalin-fixed paraffin-embedded (FFPE) that can produced an artefact of sequencing or sequences difficult to interpret 3. Also, high sensitivity of the targeted resequencing kit method using chimeric primers could induce false-positive identification of fusion genes corresponding to read-throughs.…”

Hepatocellular adenomas: one step beyond

“…Such a heterogeneity was already known for IHCAs, in which several genes might be involved, with variable frequencies. Moreover, RSPO2 is shown by Longerich et al 1 to be also involved in a small proportion of hepatocellular carcinomas, which is not surprising in light of the major role played by the Wnt/beta-catenin pathway in liver carcinogenesis. These results reinforce the connections already demonstrated between the b-HCA subgroup and HCC.…”

“…In Gut , Longerich et al describe the identification of a new gene, RSPO2 , involved in the pathogenesis of hepatocellular adenomas (HCA) and of a small fraction of hepatocellular carcinomas (HCC), through the activation of the Wnt/beta-catenin pathway, already known to be involved in the pathogenesis of a significant proportion of liver tumours 1. These results reduce the percentage of so-called ‘unclassified’ HCA, underline the tight relationship of some HCA with HCC and add another piece to the puzzle of the extraordinary clinical and biological diversity of HCA.…”