Page 77 - Read Online

Page 6 of 26 Skorupan et al. J Cancer Metastasis Treat 2023;9:5 https://dx.doi.org/10.20517/2394-4722.2022.106

The genomics of ASCP have been assessed by several groups and are summarized in Figure 2. An initial
study by Brody et al. analyzed samples from 8 ASCP patients and identified KRAS codon 12 mutations in all
[53]
samples, as well as frequent DPC4 and TP53 alterations . Subsequently, others also demonstrated KRAS
mutation in 100% of 33 combined tumor samples, making the prevalence of this mutation even higher than
that seen in PDAC [60,61] . In addition, mutation of TP53, the second most frequently mutated gene in PDAC,
[62]
was found in 88% of 17 ASCP cases . Frequent amplification of the MYC oncogene was also noted .
[61]
Through the use of laser capture microdissection to separately isolate adenomatous and squamous
components of ASCP, it was shown that both histotypes have similar genomic changes, consistent with both
components deriving from a common progenitor . This was also seen in an ASCP arising from intraductal
[61]
[63]
papillary mucinous neoplasm . In summary, the genomics of both the squamous and glandular
components of ASCP closely resemble what is typical for PDAC. Novel mutations unique to ASCP have
been difficult to identify. Liu et al. reported frequent somatic mutations in the UPF1 RNA surveillance gene
[64]
of ASCP tumors and proposed this could be a unique genetic driver of ASCP ; however, three succeeding
studies in other ASCP cohorts were unable to replicate these findings [39,61,65] . It was later noted that 45% of
UPF1 mutations purportedly present in ASCPs were identical to normal genetic variants, and functional
studies of these mutations identified no pathogenic characteristics . Taken together, these findings
[66]
suggested that ASCP and PDAC are genetically similar and may represent the same tumor type.

[35]
ASCP has numerous similarities to the basal/squamous transcriptomic subtype of PDAC . Characteristic
gene programs found in basal-type PDAC include MYC pathway activation, upregulated expression of
transcription factor TP63ΔN and target genes, and activated EGF and TGF-β signaling. The basal/squamous
subtype was also associated with epigenetic downregulation of genes involved in pancreatic endodermal cell
fate determination, such as GATA6. Hayashi et al. extended these observations in their integrated analysis of
the histologic, genomic, and transcriptional characteristics of PDAC . A new classification system was
[39]
developed, identifying PDAC tumors with > 30% keratinization or immunohistochemical labeling with
squamous markers p63 or CK5/6 as having squamous differentiation (SD), while those tumors with some,
but ≤ 30%, of cancer cells having these characteristics were termed PDAC with squamoid features (SF).
Overall, 15.7% of 123 exocrine pancreatic cancer cases showed SF or SD, while 5.7% of cases met the
pathologic definition for ASCP. SF/SD tumor samples, whether pathologically defined as ASCP or not,
overlaid with the basal transcriptomic subtype of PDAC, meaning that as many as 10% of PDAC cases
contain squamous components and have a transcriptomic profile indistinguishable from ASCP. MYC
amplification was much more frequent in SF/SD/ASCP tumors, and MYC copy number was highest in areas
of SF/SD within tumors heterogeneous for squamous and glandular components, although it was noted that
overexpression of MYC in glandular-type PDAC did not induce squamous histology. MYC is a major
instigator of metastasis in PDAC. MYC expression has also been shown to drive glycolysis and to stimulate
recruitment of immunosuppressive cell types to the tumor microenvironment [67-71] . Of note, SF/SD tumors
had a higher likelihood of mutation in a chromatin modifying gene than tumors lacking SF/SD, and the
presence of these changes in conjunction with MYC amplification or overexpression were associated with
expression of squamous differentiation markers. It is important to note that mutations in chromatin
modifying genes did not exceed 50% of SF/SD cases, meaning that other genetic or epigenetic changes that
lead to squamous marker expression must exist .
[39]

Epigenetic changes cause squamous trans-differentiation and a more aggressive phenotype
The squamous program is initiated in pancreatic cancer as in other squamous tumors by upregulation of
transcription factor TP63. Specifically, expression of the ΔN isoform of TP63 (TP63ΔN) is sufficient to drive
the squamous differentiation program in PDAC; forced expression of Tp63ΔN in mice resulted in more
aggressive tumors that grew faster, were more motile and invasive and metastasized more frequently [72,73] .