The prognostic and predictive value of homologous recombination deficiency in gastrointestinal cancer.

The homologous recombination (HR) system repairs DNA double-strand breaks produced by the DNA damage response, which is a complex signaling pathway consisting of the key proteins BRCA1/2 and other DNA repair proteins, such as the ATM, PALB2, BARD1, RAD51, and Fanconi anemia proteins. Mutations and epigenetic alterations in HR-related genes may lead to homologous recombination deficiency (HRD), resulting in genomic instability and contributing to the development of certain solid tumors. The biological significance and molecular mechanism of BRCA1/2 mutation-related HRD are well understood, but the relationships of other HR-related genes and their variant forms with HRD have not been sufficiently studied. These genes exhibit multiple forms of variation, including one or more HR genes, germline or somatic mutations, monoallelic or biallelic variants, and not all variants present HRD. Therefore, HRD is usually defined as HR-related gene variation, but recent studies have shown that defining it as the combined score of loss of heterozygosity, LST and TAI, known as the HRD score, can more accurately assess genomic instability. In patients with HRD, platinum-based therapy and poly ADP-ribose polymerase enzyme inhibitor (PARPi) have been shown to perform well in ovarian, breast, and prostate cancers. For gastrointestinal cancer (GI cancer), HRD has been relatively well studied in pancreatic cancer, but its role in other cancers has rarely been reported. Herein, we review the pathogenesis and predictive value of HRD, including the use of platinum drugs, PARPi, and immunotherapy, in digestive system tumors.