Discordance between germline genetic findings and abnormal tumor immunohistochemistry staining of mismatch repair proteins in individuals with suspected Lynch syndrome.

Background and Aims: Tumor immunohistochemical staining (IHC) of DNA mismatch repair (MMR) proteins is often used to guide germline genetic testing and variant classification for patients with suspected Lynch syndrome. This analysis examined the spectrum of germline findings in a cohort of individuals showing abnormal tumor IHC. Methods: We assessed individuals with reported abnormal IHC findings and referred for testing with a six-gene syndrome-specific panel (n=703). Pathogenic variants (PVs) and variants of uncertain significance (VUS) in MMR genes were designated expected/unexpected relative to IHC results. Results: The PV positive rate was 23.2% (163/703; 95% confidence interval [CI], 20.1%-26.5%); 8.0% (13/163; 95% CI, 4.3%-13.3%) of PV carriers had a PV in an unexpected MMR gene. Overall, 121 individuals carried VUS in MMR genes expected to be mutated based on IHC results. Based on independent evidence, in 47.1% (57/121; 95% CI, 38.0%-56.4%) of these individuals the VUSs were later reclassified as benign and in 14.0% (17/121; 95% CI, 8.4%-21.5%) of these individuals the VUSs were reclassified as pathogenic. Conclusions: Among patients with abnormal IHC findings, IHC-guided single-gene genetic testing may miss 8% of individuals with Lynch syndrome. In addition, in patients with VUS identified in MMR genes predicted to be mutated by IHC, extreme caution must be taken when the IHC results are considered in variant classification.

High risk of breast cancer in women with biallelic pathogenic variants in CHEK2.

PURPOSE: Compared to breast cancer risk genes such as BRCA2, ATM, PALB2, and NBN, no defined phenotype is currently associated with biallelic pathogenic variants (PVs) in CHEK2. This study compared the prevalence of breast and other cancers in women with monoallelic and biallelic CHEK2 PVs. METHODS: CHEK2 PV carriers were identified through commercial hereditary cancer panel testing (09/2013-07/2019). We compared cancer histories of 6473 monoallelic carriers to 31 biallelic carriers. Breast cancer risks were estimated using multivariate logistic regression and are reported as odds ratios (OR) with 95% confidence intervals (CI). RESULTS: Breast cancer frequency was higher among biallelic CHEK2 PV carriers (80.6%, 25/31) than monoallelic carriers (41.2%, 2668/6473; p < 0.0001). Biallelic carriers were more likely to be diagnosed at or before age 50 (61.3%, 19/31) and to have a second breast cancer diagnosis (22.6%, 7/31) compared to monoallelic carriers (23.9%, 1548/6473; p < 0.0001 and 8.1%, 523/6473; p = 0.0107, respectively). Proportionally more biallelic carriers also had any cancer diagnosis and > 1 primary diagnosis. Compared to women with no PVs, biallelic PV carriers had a higher risk of developing ductal invasive breast cancer (OR 8.69, 95% CI 3.69-20.47) and ductal carcinoma in situ (OR 4.98, 95% CI 2.00-12.35) than monoallelic carriers (OR 2.02, 95% CI 1.90-2.15 and OR 1.82, 95% CI 1.66-2.00, respectively). CONCLUSIONS: These data suggest that biallelic CHEK2 PV carriers have a higher risk for breast cancer, are more likely to be diagnosed younger, and to have multiple primary breast cancers compared to monoallelic carriers. Biallelic carriers also appear to have a higher risk of cancer overall. Therefore, more aggressive management may be appropriate for women with biallelic PVs in CHEK2 compared with current recommendations for monoallelic carriers.

Increased Identification of Candidates for High-Risk Breast Cancer Screening Through Expanded Genetic Testing.

PURPOSE: Breast MRI screening is recommended for women with a >20% lifetime risk for breast cancer on the basis of estimates derived from risk models dependent largely on family history. Alternatively, a >20% lifetime risk can be established through genetic testing of BRCA1 and BRCA2, as well as a growing selection of other genes associated with inherited breast cancer risk. The aim of this study was to quantify the impact of testing for genes other than BRCA1/2 and the extent to which mutation carriers in these genes would have been identified as candidates for enhanced screening on the basis of family history alone. METHODS: Women were tested with a 25-gene hereditary cancer panel including BRCA1/2 and 7 additional genes known to be associated with a >20% lifetime risk for breast cancer (ATM, CHEK2, PALB2, TP53, PTEN, CDH1, and STK11). Women found to carry pathogenic variants (PVs) were evaluated with the Claus model to assess whether they would have been found to be at >20% lifetime risk on the basis of family history. RESULTS: In total, 9,751 PVs in the selected breast cancer risk genes were identified in 9,641 women. BRCA1/2 accounted for 59.1% of the PVs, and 38.8% were in ATM, CHEK2, or PALB2. Only 24.7% of all women with PVs found in any gene reached the >20% lifetime risk threshold using the Claus model. CONCLUSIONS: Expanding genetic testing beyond BRCA1/2 significantly increases the number of women who are candidates for breast MRI and other risk reduction measures, most of whom would not have been identified through family history assessment.