The current study examined sera from 160
colon cancer patients and 60 normal individuals to determine whether antibody to mutated
p21 ras protein was present. Studies focused on the
aspartic acid substitution at
amino acid position 12 (denoted D12), one of the most common mutations in
colon adenocarcinoma.
IgA antibodies directed against mutated p21 ras-D12
protein were detected in 51 (32%) of 160
colon cancer patients, but only in 1 (2.5%) of 40 normal individuals. The greater incidence of antibody in
cancer patients provides presumptive evidence that immunization to the
ras proteins occurred as a result of the
malignancy. Examination of sera for antibody reactivity to wild-type
p21 ras protein (denoted
p21 ras-G12) as well as
p21 ras proteins bearing the D12, V12, S12, or L61 mutations showed that antibody detected was largely to normal segments of the
p21 ras protein.
Epitope mapping, using
peptide neutralization assays with mutated or normal ras
peptides as competitors, demonstrated that in 10 (67%) of 15 sera examined the antibody reactivity to p21 ras-G12
protein was neutralized by
peptides near the carboxyl terminus of
p21 ras protein, but not by
peptides spanning the specific point mutation region. Antibody reactivities correlated with peripheral blood lymphocyte count, but did not correlate with patient age, sex, histology, stage,
tumor locus,
lymph node metastasis, or serum
carcinoembryonic antigen.