Introduction
Hereditary Non-Polyposis Colorectal Cancer (HNPCC) is an autosomal
dominant inherited condition characterized by an increased
susceptibility to colorectal cancer (CRC) and other associated tumors
and defined by the Amsterdam I and II clinical criteria (Vasen, Mecklin,
Khan, & Lynch, 1991; Vasen, Watson, Mecklin, & Lynch, 1999). An
important fraction of these families is known as Lynch Syndrome and is
caused by germline inactivating mutations in the mismatch repair (MMR)
genes, which results in microsatellite instability (MSI) in the tumors.
However, it is estimated that almost half of the families that fulfill
the Amsterdam criteria do not present any defects in the MMR genes. For
this reason, the term Familial Colorectal Cancer Type X (FCCTX) emerged
to designate such group of HNPCC families with microsatellite stable,
MMR-proficient tumors, for whom the genetic basis underlying their
predisposition to CRC and other related cancers remains to be elucidated
(Lindor, 2009). Although the arrival of Next Generation Sequencing (NGS)
has allowed the identification of new CRC predisposition genes
(Esteban-Jurado et al., 2016; Evans, Green, & Woods, 2018; Garre et
al., 2011, 2015; Martín-Morales et al., 2017; Nieminen et al., 2014;
Schulz et al., 2014; Seguí et al., 2015; Smith et al., 2013; Valle,
2017), most FCCTX cases remain unexplained. In fact, FCCTX comprises a
heterogeneous group of families that presumably includes different
genetic syndromes involving high or moderate-penetrance mutations in
novel cancer-predisposing genes, but that could also result from a
combination of low-penetrance mutations in different genes (Zetner &
Bisgaard, 2017). Therefore, identifying the genetic cause of the
increased cancer susceptibility in FCCTX families is still a challenge.
Tyrosine phosphorylation is a covalent post-transcriptional modification
that is essential for signal transduction, regulating many processes in
all eukaryotic cells (Hunter, 2009). Thus, perturbations in tyrosine
phosphorylation are involved in many human diseases (Singh et al.,
2017). Given that this modification is coordinately regulated by protein
tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs),
genetic and epigenetic alterations in genes encoding PTKs and PTPs can
result in changes to the equilibrium of kinase-phosphatase activity that
might have a deleterious effect and produce abnormal cell proliferation,
which could ultimately lead to cancer (Julien, Dubé, Hardy, & Tremblay,
2011). In fact, many PTKs have been long known to act as oncogenes in
cancer initiation and progression, while most PTPs have been proven to
act as tumor suppressors, reversing the negative effects of PTKs (Julien
et al., 2011).
Wang et al. (Z. Wang et al., 2004) were the first to discover that PTPs
are frequently mutated in CRC, and it was later discovered that PTPs are
somatically mutated in many other cancers, playing particularly
important roles in colon and endometrium cancers (S. Zhao, Sedwick, &
Wang, 2015). Among all PTP genes, PTPRT is the most frequently
mutated in human cancers, with somatic mutations identified in 11% of
colon, 11% of esophagus, 10% of lung, 9% of stomach, 8% of
endometrium, 6% of bladder and 6% of head and neck cancers, as well as
in a smaller fraction of leukemia, breast, ovary, liver, pancreas and
prostate tumors (S. Zhao et al., 2015). Noteworthy, these somatic
mutations have been proven to act as driver mutations, leading to cancer
initiation and progression (L.-E. Wang et al., 2013). Another mechanism
leading to the loss of PTPRT function is the frequent hypermethylation
of its promoter, which has been recently reported in colorectal
(Laczmanska et al., 2013) and head and neck tumors (Peyser et al.,
2016).
PTPRT or PTPρ (Protein Tyrosine Phosphatase Receptor Type T or Rho)
belongs to the type IIB subfamily of classical receptor PTPs (class I).
Besco et al. showed that PTPRT interacts with adherence junction
components through its extracellular region and that most tumor-derived
mutations located in this domain impair cell-cell adhesion (Besco, Hooft
van Huijsduijnen, Frostholm, & Rotter, 2006). On the other hand, the
cytoplasmatic segment of this subfamily consists of a cadherin-like
juxtamembrane domain and two phosphatase domains: D1 and D2. It is
generally believed that the membrane-proximal PTP domain (D1) is
responsible for the tyrosine phosphatase activity per se, whereas the
second is a pseudo-phosphatase domain (D2) that has no phosphatase
activity (Tonks, 2006). However, many tumor-derived mutations are
located in the second catalytic domain (Z. Wang et al., 2004),
suggesting that this domain has an important structural function or
harbors a still unknown enzymatic activity. As a matter of fact, both
catalytic domains have been proven to be essential for the correct
function of the protein, and it has been suggested that D2 may be
important for the regulation of the phosphatase activity (X. Zhang et
al., 2007).
Regarding its substrates, two main proteins have been reported to be
modified by PTPRT, STAT3 and paxillin, both of which are well-known
oncogenes that are inactivated upon dephosphorylation by PTPRT (X. Zhang
et al., 2007; Y. Zhao et al., 2010) (Supplementary Figure S1). PTPRT
specifically dephosphorylates STAT3’s residue Y705 (X. Zhang et al.,
2007), whose phosphorylation is key for STAT3 activation (Darnell,
2005). It has been shown that pY705 STAT3 is up-regulated in a variety
of human cancers, playing an oncogenic role in tumor development
(Darnell, 2005; Lui et al., 2014; P. Zhang et al., 2011).
PTPRT-dependent inactivation of STAT3 has been reported to reduce the
expression of two of STAT3 target genes (Bcl-XL and SOCS3 )
in CRC cells, proving PTPRT’s role as an inhibitor of the IL6-JAK-STAT3
pathway (X. Zhang et al., 2007). On the other hand, PTPRT
dephosphorylates paxillin’s residue Y88, whose phosphorylation has been
also proven to be crucial for colorectal tumorigenesis (Y. Zhao et al.,
2017, 2010). PTPRT-mediated inactivation of paxillin results in the
decreased phosphorylation of its substrates (such as p130CAS, SHP2 and
Akt), hence inhibiting the PI3K-Akt pathway (Y. Zhao et al., 2017,
2010).
With the aim of identifying the genetic cause underlying FCCTX,
whole-exome sequencing was performed in a group of 13 Amsterdam-positive
MMR-proficient HNPCC families. The present study describes a truncating
germline mutation in the PTPRT gene that was identified in one of
these families.