Study population
The whole exome was studied in 13 FCCTX families recruited at the
Genetic Counseling Unit of Hospital Clínico San Carlos (Madrid, Spain).
All of the families fulfilled the Amsterdam I or II clinical criteria
for HNPCC (Vasen et al., 1991, 1999) and presented MMR-proficient tumors
with neither MSI nor lack of expression of MMR proteins. In addition,
none of them carried germline mutations in the MMR genes. Other family
members, whether healthy or affected, were also recruited for
segregation studies. Formalin-fixed paraffin-embedded (FFPE) tumor
blocks from the probands and/or their cancer-affected relatives were
obtained whenever available. Information on personal and family cancer
history was obtained, and cancer diagnoses were confirmed by medical and
pathology records. The study was approved by the Institutional Review
Board of Hospital Clínico San Carlos, and a written informed consent was
signed by each participant.
An independent series of 473 genetically unexplained MMR-proficient
familial and/or early-onset non-polyposis CRC unrelated patients
recruited at the Institut Català d’Oncologia (IDIBELL, Barcelona,
Spain), was included for validation purposes (Belhadj et al., 2019).
While 443 fulfilled the Amsterdam or Bethesda (Umar et al., 2004)
criteria at the time of referral to the genetic counseling, 30 did not
but were included in the study based on a clinical referral for
non-polyposis CRC and absence of MMR deficiency. Healthy individuals
with no cancer family history were recruited from the Blood Bank of
Hospital Clínico San Carlos (Madrid) and used as controls. FFPE tumor
blocks obtained from sporadic CRC patients were used as CRC controls,
while FFPE blocks containing non-tumor colon tissue were used as healthy
colon controls. All of these control subjects had previously signed an
informed consent.