USP8 Somatic Mutations in Cushing’s Disease and Silent Corticotropinomas

Background: Somatic mutations in the ubiquitin-specific peptidase 8 (USP8) gene have been described in Cushing’s disease (CD). These mutations increase proopiomelanocortin transcription resulting in ACTH production and seem to correlate with somatostatin receptor type 5 (SST5) expression. Aims: Screen USP8 in patients with corticotropinomas and correlate USP8 mutational status with SST5 expression in CD. Methods: Tumor DNA was extracted and then exon 14 amplified by PCR. SST5 was assessed by immunohistochemistry (clone UMB4) and quantified multiplying the percentage of positive cells (0,0%; <10%,1;10-50%, 2; 51-80%, 3; >80%, 4) and intensity (mild, 1; moderate, 2; intense, 3), giving a score (IRS) from 0-12 with ≥ 6 considered high. Results: Among 59 patients, 38 had CD and 21 silent corticotropinomas. In CD, 13 (34.2%) patients had pathogenic mutations (6 had p.Ser719del; 5 had p.Pro720Arg and 2 had p.Pro720Gln). In the mutated CD group, all were women and had median age of 34.5 years (20-46). Median ACTH was 64.7pg/mL [(34.8-330.0), normal <46], urinary free cortisol (UFC) 435.0μg/24h [(87.0-1386.0), normal <100], cortisol after overnight 1mg dexamethasone suppression test (ODST) 17.4μg/dL [(5.0-48.7), normal <1.8], salivary cortisol (SC) 8.1μg/dL [(1.0-15.5), normal <0.35]. Median largest tumor size was 0.9 cm (0-1.9), ki-67 1.7 (0.2-10.0) and IRS 12 (1-12). In wild-type CD group, 19 (76.0%) were women and had median age was 35.0 years old (14-62). Median ACTH was 59.7 (39.0-137.0), UFC 305.8 (77.0-1302.0), cortisol after ODST 23.6 (10.0-33.3), SC 0.67 (0.27-1.28). Median largest tumor diameter 0.7cm (0-3.3), ki-67 1.8 (0.2-10) and IRS 4 (0-12). SC was higher in mutated group compared to wild-type (p=0.001) as well as IRS (p=0.009). In silent corticotropinomas, 2 (9.5%) had pathogenic mutations (1 p.Ser718Pro and 1 p.Pro720Arg): male, 36 years old, 3.2 cm tumor, Ki-67 4%, IRS 6; and female, 52 years old, 3.4 cm tumor, Ki-67 2.5%, IRS 12, respectively. One tumor had a variant not reported as pathogenic (p.Thr739Ala): male, 46 years old, 3.7 cm tumor, Ki-67 0.5%, IRS 0. USP8-wild-type silent corticotropinomas had IRS 0-2. Conclusion: One third of CD patients presented with somatic USP8 mutation. Similar to another study, about 10% of silent corticotropinomas also presented somatic USP8 mutation. Expression of SST5 was high in USP8-mutated CD and higher than wild-type group.