Questioning a Li-Fraumeni Syndrome diagnosis: Characterization of a commonly observed germline TP53 variant, p.Arg156His

10612 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