Background: Germline pathogenic variants (PV) in the TP53 gene are associated with Li-Fraumeni syndrome (LFS). Increased use of multi-gene panel testing has identified TP53 variants suspected to have reduced penetrance, a departure from the significantly increased cancer risks expected with LFS, posing challenges in variant classification and clinical management. TP53 c.467G>A (p.Arg156His), R156H, is a variant with over 250 observations across multiple laboratories with prior discordant classification. R156H was recently downgraded by laboratories from likely pathogenic (VLP) to variant of uncertain significance (VUS). Characterization of R156H to clarify clinical significance has the potential to impact care for many. Methods: R156H carriers were identified through the Li-Fraumeni & TP53: Understanding & Progress (LiFT UP) study (https://liftupstudy.org) from 2019-2022. Clinical data were collected and reviewed for phenotypic characterization and to determine whether LFS Classic and/or Chompret criteria were met, and to assign Li-Fraumeni spectrum classification (Kratz et al.). Results: Proband/family characteristics are in the Table. Twenty R156H carriers were identified in 11 families. Seventeen carriers had a personal history of cancer; 8 had a LFS core cancer. All core cancers were breast except for an astrocytoma and a pediatric sarcoma. Two breast cancer cases also carried a BRCA2 PV. The average age at first cancer diagnosis was 40.5 years (range 6-71). No families met Classic LFS criteria. One proband met Chompret criteria due to breast cancer <31 years (who also carried a BRCA2 PV). Ten families were classified as attenuated LFS per Kratz et al. Conclusions: Most families in our case series did not meet LFS criteria, raising the question of appropriate clinical management and the willingness to de-escalate LFS surveillance. Our series supports the downgrade to VUS, but the presence of a few non-breast LFS core cancers warrants work to determine if R156H is a reduced penetrance PV. Recruitment of these families is ongoing through the LiFT UP study to perform segregation analyses and cancer risk estimations. This work may guide evaluation of other variants presenting similar challenges.

ASTRO Astrocytoma, BR Breast, CO Colon, CX Cervix, GB Gall Bladder, HD Hodgkin’s Disease, LG Lung, LK Leukemia, LYM Lymphoma, MNG Meningioma OMEL Ocular Melanoma, OV Ovary, PAN Pancreas, PNET Primitive Neuroectodermal Tumor, PR Prostate, RCC Renal, SARC Sarcoma, THY Thyroid, UT Uterine.