Page 65 - Read Online

Page 270 de Kouchkovsky et al. J Transl Genet Genom 2021;5:265-77 https://dx.doi.org/10.20517/jtgg.2021.32

the R219 hotspot of the forkhead domain (enriched in t-SCNC) have thus been associated with increased
FOXA1 binding to non-canonical sites and activation of mesenchymal and neuroendocrine transcriptional
[30]
programs. Although R219 mutations are detected only in a small subset of t-SCNC , genome-wide
profiling of wild-type FOXA1-binding sites in patient-derived xenografts similarly shows reprogramming of
FOXA1 signaling in t-SCNC - with novel binding sites identified near regions of neuronal and stemness-
[47]
associated genes . Thus despite decreased transcriptional expression, persistent non-canonical FOXA1
signaling may still contribute to neuroendocrine differentiation in t-SCNC.

FOXA2 is another lineage-specific pioneer transcription factor implicated in the emergence of t-SCNC.
Unlike FOXA1, FOXA2 expression in the normal adult prostate is restricted to basal epithelial cells . Its
[48]
putative role in the development of t-SCNC was first suggested by the finding of FOXA2 upregulation
across various mouse models of t-SCNC [48-50] . Subsequent work has demonstrated that FOXA2 expression is
significantly enriched in t-SCNC patient samples, with strong staining by immunohistochemistry in up to
[51]
75% of t-SCNC tumors, compared with only 4% of adenocarcinoma samples . A more recent
transcriptional analysis of mCRPC biopsies has identified FOXA2 as one of the main master regulators
preferentially expressed in t-SCNC . Upregulation of FOXA2 in t-SCNC appears to be mediated in part by
[11]
the removal of repressive histone methylation marks in the FOXA2 promoter region . The resulting
[52]
increase in FOXA2 expression, in turn, has been shown to cooperate with hypoxia-inducible factor 1 alpha
(HIF1-α) to upregulate neuronal programs and specific HIF1 alpha targets genes (HES6, SOX9, and
KDM3A) under hypoxic conditions .
[53]

The ONECUT2 master regulator has surfaced in recent years as another key transcriptional regulator of
lineage plasticity and mediator of hypoxia-induced neuroendocrine differentiation [54,55] . Through pan-cancer
mRNA abundance analyses of poorly-differentiated neuroendocrine tumors, ONECUT2 was identified as
one of 9 transcription factors (along with ASCL1, INSM1, PROX1, SIX2, MYT1, and MYT1L) differentially
upregulated in neuroendocrine tumors compared to their non-neuroendocrine counterpart. In PC
specifically, ONECUT2 expression has been shown to be upregulated in prostatic adenocarcinoma
(compared with benign prostate tissue) and is associated with an increased risk of biochemical recurrence
[54]
following primary local therapy . ONECUT2 is further upregulated as PC progresses from primary
adenocarcinoma to mCRPC and from mCRPC-adenocarcinoma to t-SCNC, where the highest expression
[55]
levels are seen . In addition to promoting cell cycle-related transcriptional programs (including targets of
the E2F transcription factors), ONECUT2 also appears to be a key regulator of hypoxia-induced gene
expression and promotes angiogenesis and cellular proliferation under hypoxic conditions . Other work
[55]
has similarly identified ONECUT2 to be a key regulator of t-SCNC through downregulation of AR and
FOXA1 transcriptional program and upregulation of neuronal and stem-cell programs . The key
[54]
transcriptional features of t-SCNC are summarized in Table 2.

EPIGENETIC DRIVERS OF T-SCNC
The paucity of genomic differences observed across mCRPC-adenocarcinoma and t-SCNC underscores the
role of epigenetic changes in driving tumor phenotype and lineage plasticity. Indeed, targeted bisulfite
sequencing of CpG methylation status has shown a high degree of concordance between DNA methylation
and gene expression levels across t-SCNC and mCRPC-adenocarcinoma [3,56] . At the genome-wide level,
bisulfite sequencing demonstrates stark differences in the DNA methylation landscapes of t-SCNC and
mCRPC-adenocarcinoma tumor samples [29,57] . Along with DNA methylation, post-translational histone
modifications play an important role in regulating chromatin structure and have also been implicated in the
emergence of t-SCNC. Genome-wide sequencing of histone acetylation status thus clearly distinguishes
neuroendocrine and adenocarcinoma patient-derived xenografts, while genes upregulated in t-SCNC have