THU235 Usefulness Of Digital Droplet Polymerase Chain Reaction (ddPCR) For Molecular Diagnosis Of McCune-Albright Syndrome In DNA From Peripheral Blood Leukocytes

Disclosure: A.G. de Faria: None. L.R. Montenegro: None. A.A. Jorge: None. R.S. Jallad: None. R.M. Martin: None. M.C. Fragoso: None. A.P. Canton: None. C.E. Seraphim: None. F.R. Tinano: None. N.C. Pinto: None. B.B. Mendonca: None. A. Latronico: None. V.N. Brito: None. Background: McCune-Albright Syndrome (MAS) is a rare congenital disorder caused by post-zygotic activating mutations in GNAS gene. Due to the mosaic pattern of this disease, mutation abundance is frequently low in several tissues, including blood cells. The emergence of digital droplet Polymerase Chain Reaction (ddPCR) is a breakthrough technology of quantitative PCR useful for a targeted detection and quantification of rare events. Objective: This study aims to identify the GNAS mutation in peripheral blood leukocytes of patients with MAS using ddPCR. Patients and Methods: Thirty-four patients (25 female and 9 male) with typical (classic triad) or atypical (one or two features) MAS were included. DNA was extracted from peripheral blood and was analyzed through ddPCR method. Two ddPCR mutation detection assays were used, each one targeting the p.R201C or p.R201H mutations. For each assay, the Taqman PCR system was used with two fluorescence probes: the first labeled with HEX, targeting de (WT allele), and the second labeled 6-FAM, targeting the (mutated allele). The mixture (reagents + DNA) was divided into 20,000 nanodrops, which was placed on a PCR plate and amplified by PCR. After amplification by PCR, the fluorescence of each droplet was read on the 200 Droplet Digital PCR system (Bio-Rad), being classified as positive (drop containing the target sequence) or negative. Results were analyzed using Quantasoft software (Bio-Rad) to determine the total amount of positive droplets in the original sample. The results were reported in fractional abundance (FA), corresponding to the percentage of mutated alleles within the total alleles. Results: Among all patients, the most common MAS feature was osteofibrous dysplasia (OFD) (73.5%), followed by peripheral precocious puberty (PPP) (64.7%), and café-au-lait skin spots (52.9%). Acromegaly or hyperthyroidism was diagnosed in 6 and 4 patients, respectively. Classical MAS triad, 2 clinical features, or 1 clinical feature (PPP or OFD) were present in 15, 9, and 10 patients, respectively. GNAS mutations were identified in 12/34 (35.3%) MAS patients: the p.R201C in 5 and the p.R201H in 7. Among them, 8 patients presented typical MAS and 4 patients with two MAS clinical features, respectively. Considering only typical MAS group, the positive molecular test rate was 53.3% (8/15). GNAS mutations were not identified in patients with only one MAS feature. There was a positive correlation between a positive molecular test and the number of MAS clinical features (p = 0.01). Median FA was 1.45%, ranged from 0.25% to 8.4%. There was no association between FA rate and the number of MAS clinical features (p=1.0). Conclusion: ddPCR representeds a more sensitive tool for identifying GNAS mutations in peripheral blood leukocytes, even with low abundance, in patients with classical and atypical MAS. Presentation: Thursday, June 15, 2023