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Page 6 of 12 David. J Cancer Metastasis Treat 2022;8:32 https://dx.doi.org/10.20517/2394-4722.2022.71

[23]
most common tumor suppressors TP53 and CDKN2A were even more common than in classical tumors .
Given the sensitivity of these tumor suppressors to oncogene activation, this may be linked to KRAS gains
observed in basal-like tumors. Squamous subtype tumors (a classification aligned with basal-like) also
exhibit loss of the histone demethylase KDM6A [17,38] . Importantly, pancreas-specific knockout of Kdm6a in a
KRAS G12D -driven mouse model of PDA leads to the formation of squamous-like PDA, suggesting a causal
[38]
relationship between KDM6A alterations and tumor subtype .
In summary, evidence has been presented suggesting that cell of origin, microenvironment, and genetic
alterations all play a role in determining PDA transcriptional subtypes. Cases in which, within the same
patient, tumor progression is accompanied by gains in KRAS signaling and acquisition of basal-like
properties would argue against a role for the cell of origin in determining subtype in that patient, though it
does not rule out a role for the cell of origin in globally influencing PDA transcriptional phenotypes. Studies
in mice or humans that follow the evolution of transcriptional subtypes throughout tumor progression
should provide an idea about the generality of classical-to-basal transitions during tumor evolution.

TRANSCRIPTIONAL CONTROL OF TUMOR SUBTYPES
While the mechanisms that might allow the interconversion of PDA subtypes demand more study, several
TFs have been implicated in the maintenance of each subtype. With some important exceptions, the TFs
that control the two subtypes are generally lineage-specific TFs that control transcriptional enhancers
specifying each differentiation state. According to the study of these TFs and their target genes, it has
become clear that while they cooperate to maintain subtype identity, many of these TFs control distinct
aspects of the biological properties of each subtype. Therefore, these key TFs provide important insights
into the specific biology of each PDA subtype.

TF determinants of classical PDA
As discussed above, frequent GATA6 amplifications have been identified in PDA [39,40] , suggesting that it
functions as a PDA lineage survival oncogene . The fact that these amplifications preferentially occur in
[41]
[23]
classical PDA makes it an obvious candidate for determining the classical state . Indeed, the expression of
GATA6 is a robust discriminator between basal and classical PDA [25,42] . Like lineage survival oncogenes in
other tissues, GATA6 inhibits EMT in PDA cell lines, likely through direct transcriptional control of E-
cadherin and activation of other epithelial-promoting TFs such as FOXA1 and FOXA2 . Later studies
[43]
suggested that GATA6 stands as a guardian against the acquisition of basal-like characteristics [43,44] .
Pancreas-specific deletion of GATA6 upon tumor initiation by mutant KRAS accelerated tumor formation
but failed to result in tumors of squamous histology . On the contrary, deletion of GATA6 in established
[43]
PanIN lesions resulted in invasive tumors with increased squamous differentiation, determined by the
expression of basal markers KRT5 and KRT14 . The tumors resulting from “late” GATA6 knockout were
[45]
not only more proliferative and chemoresistant, but also displayed evidence of increased ability to evade
adaptive immunity, most notably through downregulation of class I MHC molecules. Consistent with this,
human tumors with low GATA6 expression exhibited significantly fewer infiltrating CD8+ T cells than high
GATA6 tumors. These results suggest that transcriptional subtypes may dictate PDA immune phenotype,
with implications for immunotherapy that require follow-up.

GATA6 is a key endodermal TF that plays a crucial role in pancreatic development [46-48] , and it cooperates
with additional endodermal TFs to maintain the classical subtype. In addition to FOXA1/A2, GATA6
cooperates with HNF4A and HNF1A, both of which are also developmentally important endodermal TFs
whose loci are frequently methylated and silenced in squamous/basal-like PDA [17,45] . HNF4A was also shown
to directly contribute to the expression of genes associated with the classical phenotype, and like Gata6,