The Impact of Germline Alterations in Appendiceal Adenocarcinoma

Abstract

Purpose: More than 10% of assessed patients with appendiceal adenocarcinoma have a pathogenic (P) or likely pathogenic (LP) germline variant, including genes implicated in heritable gastrointestinal cancer syndromes, such as Lynch syndrome. We defined the clinical and molecular impact of heritable alterations in appendiceal adenocarcinoma to evaluate the need for dedicated appendiceal screening and prevention strategies in patients with LP/P germline variants.

Experimental design: We performed an integrated germline and somatic molecular analysis for patients with confirmed appendiceal adenocarcinoma. Patients underwent paired tumor-normal sequencing for up to 90 hereditary cancer risk genes and 505 genes for somatic mutation profiling. We defined the cooccurrence of LP/P germline variants and second-hit pathogenic somatic alterations. The associations between germline variants and patient clinicopathologic features were also evaluated.

Results: Twenty-five of 237 patients (10.5%) carried pathogenic or likely pathogenic germline variants in cancer susceptibility genes. Clinicopathologic characteristics and appendiceal adenocarcinoma-specific survival were similar in patients with or without germline variants. Most (92%, N = 23/25) patients with germline variants demonstrated no second-hit somatic alterations, including loss of heterozygosity. Two patients with a germline APC I1307K low-penetrance founder variant exhibited secondary somatic pathogenic alterations in APC. However, only one patient tumor exhibited APC-mediated WNT signaling dysregulation: a plausible consequence of multiple somatic APC mutations with no germline variant contribution. Four patients had germline variants in PMS2 or MSH2 associated with Lynch syndrome, yet their cancers were microsatellite-stable.

Conclusions: Germline variants are likely incidental without a contributory driver role in appendiceal adenocarcinoma. Appendiceal adenocarcinoma screening in patients with germline variants is not clearly merited.

Figure 1:. Germline mutation prevalence in patients with appendiceal adenocarcinoma with co-occurring somatic mutation and copy number status.

A, Germline mutations in the 237 cohort patients organized by predicted mutation penetrance. Percentage in parentheses indicates the percent of patients with the variant out of the total assessed patients. *The specific detected variants are associated with low (APC p.I1307K), recessive (FH c.1431_1433dupAAA (p.K477dup)), and uncertain (CHEK2 p.I157T) penetrance. B, Germline mutation penetrance with somatic and copy number alterations in respective genes. *Indicates a patient with two different germline mutations.

Figure 2:. APC germline and somatic alterations in two patients with appendiceal adenocarcinoma.

The APC gene is shown with annotated functional domains and the pathogenic inactivating germline (top) and somatic (bottom) alterations for each patient. For patient 1, two proposed variations are indicated where somatic alterations occur trans (left arrow) or one in syn (right arrow) with the germline APC alteration. Patient 2 exhibited pathogenic loss of heterozygosity (LOH) in the non-germline APC allele. Tumor immunohistochemical staining for β-Catenin reveals nuclear and cytoplasmic (APC loss) versus membranous (APC intact) β-Catenin localization for Patient 1 and 2, respectively. Abbreviations include “chromosome” (Chr.)