Profiling of Cross-Functional Peptidases Regulated Circulating Peptides in BRCA1 Mutant Breast Cancer

Women
with inherited BRCA1 mutations are more
likely to develop breast cancer (BC); however, not every carrier will
progress to BC. The aim of this study was to identify and characterize
circulating peptides that correlate with BC patients carrying BRCA1 mutations. Circulating peptides were enriched using
our well-designed nanoporous silica thin films (NanoTraps) and profiled
by mass spectrometry to identify among four clinical groups. To determine
the corresponding proteolytic processes and their sites of activity,
purified candidate peptidases and synthesized substrates were assayed
to verify the processes predicted by the MERPOS database. Proteolytic
processes were validated using patient serum samples. The peptides,
KNG1K438‑R457 and C 3fS1304‑R1320, were identified as putative peptide candidates to differentiate BRCA1 mutant BC from sporadic BC and cancer-free BRCA1 mutant carriers. Kallikrein-2 (KLK2) is the major
peptidase that cleaves KNG1K438‑R457 from kininogen-1,
and its expressions and activities were also found to be dependent
on BRCA1 status. We further determined that KNG1K438‑R457 is cleaved at its C-terminal arginine by carboxypeptidase
N1 (CPN1). Increased KLK2 activity, with decreased CPN1 activity,
results in the accumulation of KNG1K438‑R457 in BRCA1-associated BC. Our work outlined a useful strategy
for determining the peptide–petidase relationship and thus
establishing a biological mechanism for changes in the peptidome in BRCA1-associated BC.