Familial craniofacial fibrous dysplasia: absence of linkage to GNAS1 and the gene for cherubism.

EDITOR—Fibrous dysplasia is a benign fibro-osseous condition that may involve one or more bones in the cranial and extracranial skeleton. There are four main types of fibrous dysplasia: monostotic in which only one bone is aVected; polyostotic in which multiple bones are aVected; McCune-Albright syndrome (MIM 174800), a polyostotic form accompanied by pigmentation lesions and endocrine dysfunction presenting as precocious puberty in females ; and a craniofacial form (CFD) in which only bones of the craniofacial complex are aVected. The majority of patients with CFD present in their second decade; the lesions are generally unilateral and tend to become static once skeletal maturity is reached. Fibrous dysplasia usually occurs with no family history, although CFD has been shown to be inherited in an autosomal dominant fashion. 8 Activating mutations within the guanine nucleotide binding protein gene (GNAS1) (MIM 139320), located on chromosome 20q13.2-13.3, have been identified in lesions from monostotic fibrous dysplasia, polyostotic fibrous dysplasia, 12 and McCuneAlbright syndrome. At present it is unknown whether this gene is mutated in the germline of patients with familial CFD. Another candidate gene for CFD is the gene responsible for cherubism (MIM 118400), recently localised to a 3 cM interval on the telomere of chromosome 4p. 15 Cherubism is a hereditary fibro-osseous lesion of the jaw, but in contrast to CFD the disease presents in the first decade of life and involves the mandible and maxilla bilaterally. However, like CFD, cherubism stabilises and largely resolves with skeletal maturity. As the gene for familial CFD has not been identified, we have studied a family with the disease, which has been under observation for over 20 years. 16 The family was interviewed and samples were collected with full informed consent of the patients involved and with local Ethical Review Board approval. DNA was extracted from peripheral venous blood samples using conventional methods. Polymorphic microsatellite markers were PCR amplified and electrophoresed through 4% denaturing polyacrylamide gels on a conventional gel system. One PCR primer was end labelled using T4 polynucleotide kinase and ãPATP and the gel was exposed to autoradiography film for 1-16 hours. Two point and multipoint linkage analyses were carried out using the MLINK suite of programs. A model was used in which disease alleles were given a frequency of 0.0001, the penetrance was set at 90%, and no sporadic cases were allowed. Ten polymorphic microsatellite markers from the telomeric region of 4p and two markers flanking the GNAS1 gene were used to generate haplotypes from these regions. The marker order and sex averaged genetic distances in cM within the chromosome 4 interval from the Genethon map are 4pter D4S2936, D4S3038 1.8 D4S1614—1.9 D4S3034, D4S412. Using additional information and mapping data from the current studies the following order has been reached 4pter D4S2936, D4S3038 PDE6B D4S3360 D4S1614 D4S43 D4S127 HD D4S3034, D4S412. The chromosome 20 Genethon map places the two markers D20S171 2.1 D20S173. The Genetic Location Database has ordered these two markers flanking GNAS1. Two point lod scores (table 1) confirmed that there is no strong evidence to suggest linkage to chromosome 4p16.3. While D20S171 produced a highly negative lod score, D20S173 produced a positive lod score which may indicate the presence of a recombination event. However, as the aVected subjects do not appear to inherit the same haplotype (fig 1), the result at D20S173 was probably because of an allele frequent in the population. Three point lod scores of –4.79 using D4S3360 and the HD trinucleotide repeat and –5.23 using D20S171 and D20S173 were obtained. The same somatic mutation (Arg to His or Cys) has been found in lesions from monostotic fibrous dysplasia, polyostotic fibrous dysplasia, and McCune-Albright syndrome. 11 13 This indicates that these disorders share the same pathogenesis, with the mutation occurring at diVerent stages of development, resulting in the varying extent of tissue involvement. The eVect of these activating mutations is to increase the cAMP levels with the follow on eVect of increasing Fos expression. Our data indicate that mutations in GNAS1 are not involved in the development of familial CFD, although of course it remains possible that the genetic cause is within another gene involved in the same regulatory pathway. Distinguishing clinically between cherubism and CFD may be diYcult if the skeletal abnormalities are localised to