Benign fibro-osseous lesions (BFOLs) frequently display overlapping histological features. The differentiation of fibrous dysplasia (FD) from other BFOLs can be difficult, even for experienced orthopedic pathologists. Accurately distinguishing FD from other BFOLs may have significant clinical and treatment implications. A somatic mutation in gene GNAS encoding the ␣ subunit of the G protein (Gs␣) involving the codon corresponding to Arg 201 has been identified in FD and is specifically absent in other BFOLs. We have developed a quantitative assay by pyrosequencing that has a detection sensitivity of 95%. The test allows the identification of the two most common types of mutation (Arg¡His and Arg¡Cys) in a single reaction, with the ability to analyze other rare mutations. Of the 24 FD cases in this series, 23 (96%) were positive for GNAS/Gs␣ mutation. Nineteen of 23 positive cases exhibited a G¡A mutation (Arg¡His), whereas four had a C¡T mutation (Arg¡Cys). One of three BFOL, not otherwise specified cases was positive for G¡A mutation. None of the osteofibrous dysplasia, ossifying fibromas, or other bone lesions were positive for this mutation. Our experience is that pyrosequencing is an easy and accurate quantification method for Gs␣ mutation detection in fibrous dysplasia. Mutation analysis of the Gs␣ by pyrosequencing has significant potential for improving discrimination between FD and other BFOLs in problematic cases. Fibrous dysplasia is a benign fibro-osseous lesion (BFOL) of the medullary cavity, which may involve one or more bones.1 Because fibrous dysplasia (FD) and other BFOLs frequently display overlapping histological features, differentiation of FD from a BFOL can be difficult, even for experienced orthopedic pathologists, particularly on small biopsy samples.2,3 A somatic mutation at codon 201 of the ␣ subunit of G protein (Gs␣), encoded by the GNAS gene, has been identified in FD (monostotic and polyostotic forms) and in multiple endocrinopathies of McCune-Albright syndrome, 4 but is specifically absent in other BFOLs, such as osteofibrous dysplasia.3,5,6 Point mutations result in replacement of Arg (CGT) at position 201 with, most commonly, His (CAT) or Cys (TGT) 4 ; and in rare cases, with Ser (AGT), 7 Leu (CTT), 8 or Gly (GGT).
9The mutated Gs␣ is constitutively activated, leading to cAMP activation and downstream physiological responses in affected tissues, including bone, skin, endocrine glands, and other tissues. 4,10,11 Because of the mosaic nature of this mutation, the distribution of mutant cells and the ratio of mutant to normal cells in a lesion or affected individual may be reflected in the severity of phenotypic expression.10,11 Interestingly, the percentage of mutant stem cells decreases as a lesion ages resulting in "normalization" of FD of bone.12 Addressing these issues require an extremely sensitive and accurate quantitative assay for Arg201 mutation detection.Many detection assays have been developed to identify the Arg201 mutations. The two most widely used methods are direct ...