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Page 996 Juhlin. Cancer Drug Resist 2020;3:992-1000 I http://dx.doi.org/10.20517/cdr.2020.66

the development of this undifferentiated tumor type - although only the TSC1 and DAXX mutations were
carried along to the metastatic ATC component.

From a clonality perspective, a staggering five-fold increase in mutational burden was noted for the PDTC
component compared to the other tumors sequenced, including the ATC. The vast majority of these PDTC
mutations were found to be sub-clonal, and they were not carried on to the ATC component [Figure 1].
The authors then employed a computerized clonality analysis; in which impactful mutations in cancer
associated genes and whole-genome copy number data were merged to create phylogenetic clusters for
[27]
each of the different tumor components . By doing so, the authors were able to highlight a main, truncal
clone that branched off into five major sub-clones. The mutational and copy number backbone in the main
clones of the primary ATC and both metastatic samples were identified as small sub-clones present already
in the well-differentiated FTC - strongly suggesting that the tumor components were genetically linked.
These findings also open up possibilities to identify future markers of potentially lethal sub-clones in cases
of WDTC to detect cases at risk for future dedifferentiation, which would be highly warranted from a
clinical standpoint. It should be noted however, that non-functional and/or non-coding mutations were
not included when assessing clonality, which was deemed impossible given the sheer data volume. The
authors were thus not able to detail the clonal compositions using passenger alterations, which could have
been useful in separating chained clonal distribution patterns of driver gene mutations with associated co-
mutations from the de novo driver gene mutations occurring in the downstream tumor components.

The potential role of aberrant DNA repair in thyroid cancer dedifferentiation
[26]
Paulsson et al. identified a truncating, heterozygous MSH2 mutation in the FTC that was carried along
to all subsequent tumors analyzed. Notably, the MSH2 gene locus displayed copy-neutral LOH but retained
MSH2 protein expression in the FTC, although the downstream tumor components were negative for
MSH2 expression. Thus, the authors speculate that the copy neutral LOH must have preceded the MSH2
mutation in the FTC, followed by a second genetic event in the PDTC and ATC components leading to
abolished MSH2 expression - perhaps by promoter hypermethylation of the wildtype allele. All tumor
components exhibited MSI, with the lowest score in the FTC, and analysis of the mutational composition
showed a substantial accumulation of C>T transitions compared to A>G transitions and transversions for
all tumor components - a known hallmark for deficient MMR. Several significant signatures were correlated
to deficient MMR when correlating the genetic data to established COSMIC mutational signatures. By in silico
gene ontology analyses of impactful coding mutations in the mother clone (from which all major sub-
[26]
clones in the tumor components emerged), Paulsson et al. identified “TP53 associated transcription of
DNA repair genes” as the main mutated signaling pathway. In all, these data suggest that the mutational
inactivation of MSH2 gave rise to an MMR deficient profile for all tumor components.

In addition to the truncating MSH2 mutation, a missense ATM mutation was observed in the adjacent
PDTC component. ATM encodes a kinase protein that regulates signaling pathways involved in DNA repair
[28]
and overall genome stability through regulation of HRR, including TP53 associated signaling networks .
Interestingly, as the PDTC exhibited a hypermutator phenotype compared to all other tumor components,
one cannot exclude a synergic effect of combined MSH2 and ATM mutations driving the hypermutability.
Of note, ATM mutations have been found recurrently in ATCs . Even so, as both ATM and TP53 gene
[14]
mutations are recurrent events also in HRR competent cancer types, there was not enough evidence
to suggest that the ATC sample studied was indeed HRR deficient. Furthermore, when analyzing rare
constitutional variants in cancer-associated genes by interrogating the normal thyroid sample, the authors
detected a homozygous MUTYH variant (rs3219468) with a minor allele frequency of < 0.04% in the
general population. Interestingly, MUTYH is involved in DNA repair, especially response to oxidative DNA
damage . Whether this rare variant in any way influenced the development of multiple thyroid tumors in
[29]
this patient is not known.