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Rationale: Pycnodysostosis is a rare autosomal recessive skeletal dysplasia caused by a mutation in the cathepsin K encoded by cathepsin K gene (CTSK). Medullary thyroid carcinoma (MTC) is also a relatively rare type of primary thyroid carcinoma., Patient Concerns: A 31-year-old woman presenting a short stature and a palpable nodule in the front of her neck that had gradually increased in size during the last 2 years was referred to our department. She has experienced multiple fractures at lower limbs in the last 2 decades., Diagnoses: The patient's clinical examination revealed short stature, underweight, a prominent forehead, stubby fingers, and a fixed nodule in the right thyroid lobe. Intraoral examination revealed multiple clinically malposed and missing teeth, as well as chronic periodontitis with a narrow and grooved palate. Radiographic examination revealed typical widely separated cranial sutures and an open anterior/posterior fontanel with an obtuse gonial angle, acroosteolysis, and osteosclerosis with narrowed medullary cavities. Ultrasonography of the thyroid gland showed a marked hypoechoic solid nodule in the right lobe in which tumor cell clusters were confirmed by ultrasound-guided fine needle aspiration biopsy and was suspected to be MTC. Laboratory tests revealed dramatically elevated serum calcitonin >2000 pg/L (reference range: 0-5 pg/L) and carcinoembryonic antigen (CEA) 134.37 ng/mL (reference range: 0-5 ng/mL). Genotypic screening revealed compound heterozygous mutations in the CTSK gene (c.158delA, P.Asn53Thr/c.C830T, P.Ala277Val) but no mutation associated with the familial forms of MTC., Interventions: The patient underwent a total thyroidectomy with right-sided functional neck dissection., Outcomes: CEA and serum calcitonin decreased significantly postthyroidectomy, and no further fracture has been reported by the patient so far., Lessons: The present study is the first to report a rare case of the coexistence of pycnodysostosis with a compound CTSK gene mutation and sporadic MTC. Radiological techniques and gene analysis play key roles in the definitive diagnosis., (Copyright © 2017 The Authors. Published by Wolters Kluwer Health, Inc. All rights reserved.)

Rationale: Pycnodysostosis is a rare autosomal recessive skeletal dysplasia characterized by short stature, craniofacial dysmorphism, acro-osteolysis, osteosclerosis, and brittle bone with poor healing. Pycnodysostosis results from the deficient activity of cathepsin K, a lysosomal cysteine protease that is encoded by CTSK., Patient Concerns: We report a Korean adult patient with pycnodysostosis and atypical femur fracture whose diagnosis was confirmed by next-generation sequencing (NGS) of candidate genes. A 41-year-old female patient was presented with a left femur fracture after falling down. Underlying sclerotic bone disease was suspected as a radiographic skeletal survey showed thickened cortical bones, and the total body bone density was increased (T score was 5.3, and Z score was 4.9)., Diagnoses: We performed candidate gene sequencing of various sclerotic bone diseases for the differential molecular diagnosis of underlying sclerosing bone disease. Two heterozygous variants of CTSK were detected. One was a frameshift variant in exon 5, c.426delT (p.Phe142Leufs*19), which was previously reported, and the other was a novel missense variant in exon 6, c.755G>A (p.Ser252Asn). Sanger sequencing of CTSK confirmed the 2 heterozygous variants and thus the patient was diagnosed with pycnodysostosis., Interventions: The patient had emergency surgery for subtrochantic femoral fracture., Outcomes: After 4 months of surgery, the patient had almost a full range of hip and knee movements and radiographs show the substantial bridging callus across the fracture., Lessons: Candidate gene sequencing could be a useful diagnostic tool for the genetically heterogeneous skeletal dysplasia group, especially in cases with a mild or atypical clinical phenotype.

Pycnodysostosis (OMIM 265800) is an uncommon hereditary disorder characterized by osteosclerosis of the skeleton, short stature, and bone fragility. The syndrome was first described by Maroteaux and Lamy (1962). Facial dysmorphology, hypoplasia of the mandible,dysplasia of the skull, bones with delayed closure of the cranial sutures, clavicular dysplasia, acroosteolysis or partial aplasia of the terminal phalanges, and abnormal tooth eruption have also been reported (Gelb et al., 1995). An autosomal recessive mode of inheritance has been also suggested and the locus of the disease was initially mapped to human chromosome 1q21 by genetic linkage (Bernard et al., 1980). Since then, several mutations on unrelated patients and consanguineous families have been identified in the cathepsin K gene (CTSK), affecting osteoclast function.Only two previous reports have demonstrated the presence of craniosynostosis in patients with pycnodysostosis(Fleming et al., 2007; Osimani et al., 2010). The purpose of this case report is to describe the craniofacial and dental features of a 12-year-old boy with pycnodysostosisand an uncommon association with craniosynosotosis.