Clinical and animal research findings in pycnodysostosis and gene mutations of cathepsin K from 1996 to 2011

Inherited diseases caused by mutations in cathepsin protease genes

Lysosomal cathepsins are proteolytic enzymes increasingly recognized as prognostic markers and potential therapeutic targets in a variety of diseases. In those conditions, the cathepsins are mostly overexpressed, thereby driving the respective pathogenic processes. Although less known, there are also diseases with a genetic deficiency of cathepsins. In fact, nowadays 6 of the 15 human proteases called 'cathepsins' have been linked to inherited syndromes. However, only three of these syndromes are typical lysosomal storage diseases, while the others are apparently caused by defective cleavage of specific protein substrates. Here, we will provide an introduction on lysosomal cathepsins, followed by a brief description of the clinical symptoms of the various genetic diseases. For each disease, we focus on the known mutations of which many have been only recently identified by modern genome sequencing approaches. We further discuss the effect of the respective mutation on protease structure and activity, the resulting pathogenesis, and possible therapeutic strategies.

Molecular analysis of the CTSK gene in a cohort of 33 Brazilian families with pycnodysostosis from a cluster in a Brazilian Northeast region

Background

Pycnodysostosis is an autosomal recessive skeletal dysplasia, the prevalence of which is estimated to be low (1 per million). Nevertheless, in recent years we have found 27 affected individuals from 22 families in Ceará State, a region of the Brazilian Northeast, giving a local prevalence of 3 per million. This local prevalence associated with a high parental consanguinity, suggesting a possible founder effect, prompted us to perform a molecular investigation of these families to test this hypothesis.

Methods

The CTSK gene was sequenced by the Sanger method in the patients and their parents. In addition to 18 families from Ceará, this study also included 15 families from other Brazilian regions. We also investigated the origin of each family from the birthplace of the parents and/or grandparents.

Results

We have studied 39 patients, including 33 probands and 6 sibs, from 33 families with pycnodysostosis and identified six mutations, five previously described (c.436G>C, c.580G>A, c.721C>T, c.830C>T and c.953G>A) and one novel frameshift (c.83dupT). This frameshift variant seems to have a single origin in Ceará State, since the haplotype study using the polymorphic markers D1S2344, D1S442, D1S498 and D1S2715 suggested a common origin. Most of the mutations were found in homozygosity in the patients from Ceará (83.3 %) while in other states the mutations were found in homozygosity in half of patients. We have also shown that most of the families currently living outside of Ceará have northeastern ancestors, suggesting a dispersion of these mutations from the Brazilian Northeast.

Conclusions

The high frequency of pycnodysostosis in Ceará State is the consequence of the high inbreeding in that region. Several mutations, probably introduced a long time ago in Ceará, must have spread due to consanguineous marriages and internal population migration. However, the novel mutation seems to have a single origin in Ceará, suggestive of a founder effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s40001-016-0228-7) contains supplementary material, which is available to authorized users.

From disease to treatment: from rare skeletal disorders to treatments for osteoporosis

During the past 15 years there has been an expansion of our knowledge of the cellular and molecular mechanisms regulating bone remodeling that identified new signaling pathways fundamental for bone renewal as well as previously unknown interactions between bone cells. Central for these developments have been studies of rare bone disorders. These findings, in turn, have led to new treatment paradigms for osteoporosis some of which are at late stages of clinical development. In this article, we review three rare skeletal disorders with case descriptions, pycnodysostosis and the craniotubular hyperostoses sclerosteosis and van Buchem disease that led to the development of cathepsin K and sclerostin inhibitors, respectively, for the treatment of osteoporosis.

Cathepsin K Inhibition: A New Mechanism for the Treatment of Osteoporosis

Cathepsin K (CatK), a cysteine protease, is highly expressed by osteoclasts and very efficiently degrades type I collagen, the major component of the organic bone matrix. Robust genetic and pharmacological preclinical studies consistently demonstrate that CatK inhibition increases bone mass, improves bone microarchitecture and strength. Recent advances in the understanding of the molecular and cellular mechanisms involved in bone modeling and remodeling suggest that inhibition of CatK decreases bone resorption, but increases the number of cells of osteoclast lineage. This in turn maintains the signals for bone formation, and perhaps may even increase bone formation on some cortical surfaces. Several CatK inhibitors, including relacatib, balicatib, odanacatib and ONO-5334 had entered clinical development for metabolic bone disorders with increased bone resorption, such as postmenopausal osteoporosis. However, odanacatib (ODN) is the only candidate continuing in development. ODN is a highly selective oral CatK inhibitor dosed once-weekly in humans. In a Phase 2 clinical trial, postmenopausal women treated with ODN had sustained reductions of bone resorption markers, while bone formation markers returned to normal after an initial decline within the first 2 years on treatment. In turn areal bone mineral density increased continuously at both spine and hip for up to 5 years. ODN has also been demonstrated to improve bone mass in women with postmenopausal osteoporosis previously treated with alendronate and in men with osteoporosis. ODN is currently in a worldwide Phase 3 fracture outcome trial for the treatment of postmenopausal osteoporosis with interim results supporting its anti-fracture efficacy at the spine, hip and non-vertebral sites.

Pycnodysostosis presenting as atypical stridor

OBJECTIVES

Pycnodysostosis is a rare genetic disorder caused by a mutation of the cathepsin K gene involved in bone turnover. It is responsible, in particular, for a combination of dwarfism and bone fragility. Upper airway obstruction may be observed, but associated stridor has never been previously described.

MATERIALS AND METHODS

Single-centre retrospective study over a period of 15 years with review of the literature.

RESULTS

Three children (aged 2-18 months) were managed for stridor and obstructive sleep apnoea syndrome confirmed by polysomnography. Physical examination of these children revealed stridor with laryngomalacia, characteristic dysmorphic features and failure to thrive. Patient 1 presented typical laryngomalacia treated by surgical section of the aryepiglottic folds. Patient 2 presented upper airway obstruction with a narrow nasopharynx and long soft palate, treated by surgery and noninvasive ventilation. Patient 3 presented moderate laryngomalacia and nasal obstruction, treated by surgery and noninvasive ventilation.

CONCLUSION

The diagnosis of pycnodysostosis must be considered in the presence of atypical laryngomalacia associated with multifactorial upper airway obstruction, failure to thrive and dysmorphic syndrome. A genetics consultation is essential in these patients.

The role of cathepsin K in oral and maxillofacial disorders

Cathepsin K (CTSK) was thought to be a collagenase, specifically expressed by osteoclasts, and played an important role in bone resorption. However, more and more research found that CTSK was expressed in more extensive cells, tissues, and organs. It may not only participate in regulating human physiological activity, but also be closely related to a variety of disease. In this review, we highlight the relationship between CTSK and oral and maxillofacial disorders on the following three aspects: oral and maxillofacial abnormities in patients with pycnodysostosis caused by CTSK mutations, oral and maxillofacial abnormities in Ctsk(-/-) mice, and the role of CTSK in oral and maxillofacial diseases, including periodontitis, peri-implantitis, tooth movement, oral and maxillofacial tumor, root resorption, and periapical disease.

Pycnodysostosis with Special Emphasis on Dentofacial Characteristics

Pycnodysostosis is an autosomal recessive disorder that manifests as osteosclerosis of the skeleton due to the defective osteoclasts mediated bone turnover. The diagnosis of this disorder is established on the basis of its characteristic features and must be differentially diagnosed with other bone disorders. Dental surgeons should be aware of the limitations and possible adverse oral complications such as osteomyelitis of bone in these patients. This will guide them in planning realistic treatment goals. This paper reports the clinical and radiographic features of pycnodysostosis with the great emphasis on its dentofacial characteristics. The aim of this case report is to give an insight into the etiology, pathogenesis, and differential diagnosis of this disorder and to prepare the dentists and maxillofacial surgeons to overcome the challenges in treating these patients.

A subcranial Le Fort III advancement with distraction osteogenesis as a clinical strategy to approach pycnodysostosis with midface retrusion and exorbitism

Pycnodysostosis is a rare autosomal recessive skeletal disorder involving a constellation of craniofacial manifestations including midface retrusion. We report the case of a 13-year-old girl with pycnodysostosis who presented with exorbitism, midface retrusion, malocclusion, and obstructive sleep apnea. Here, we describe the successful use of subcranial Le Fort III advancement using distraction osteogenesis with internal Kawamoto distracters. After a latency of 5 days, distraction for 10 days, and consolidation for 12 weeks, her midface was advanced by 10 mm with slight overcorrection at the occlusion level. At 2 years postoperatively, the patient had complete remission of her sleep apnea, resolution of her exorbitism, and amelioration of her class III malocclusion to class I. To the best of our knowledge, this is the first report of a successful subcranial Le Fort III midface advancement with distraction osteogenesis for craniofacial reconstruction of a pycnodysostosis. Our report highlights the surgical options that have been described for this craniofacial deformity and presents a novel and expedient approach for patients with pycnodysostosis presenting with exorbitism, midface retrusion, and/or sleep apnea.

Atypical femur fractures in a patient with pycnodysostosis: a case report

Pycnodysostosis is a rare autosomal recessive disease due to a mutation in the gene for the enzyme Cathepsin K. It is characterized by short stature, craniofacial dysmorphias, osteosclerosis, and brittle bones. There are only a few reports in the literature describing surgical interventions for long bone fractures in pycnodysostosis patients, most of which describe intramedullary nail treatment of isolated long bone fractures. We describe a case in which a pregnant female with pycnodysostosis presented with a shaft fracture of the left femur following minor trauma and a history of increasing thigh pain. Radiographs obtained in the emergency room also revealed an impending subtrochanteric fracture of the contralateral side. The acute left femoral shaft fracture was treated with an adolescent-sized intramedullary nail; it was decided to defer surgery on the contralateral side until after pregnancy. Three months later, the patient had the contralateral femur prophylactically fixated with a plate and screws. One year after the index surgery, both methods demonstrated satisfactory healing both clinically and radiographically. Although we recommend use of an intramedullary nail for long bone fractures in patients with pycnodysostosis, a plate can be utilized if health conditions or skeletal morphology precludes use of a nail.

Dental Abnormalities Caused by Novel Compound Heterozygous CTSK Mutations

Cathepsin K (CTSK) is an important protease responsible for degrading type I collagen, osteopontin, and other bone matrix proteins. The mutations in the CTSK gene can cause pycnodysostosis (OMIM 265800), a rare autosomal recessive bone dysplasia. Patients with pycnodysostosis have been reported to present specific dental abnormalities; however, whether these dental abnormalities are related to dysfunctional CTSK has never been reported. Here we investigated the histologic changes of cementum and alveolar bone in a pycnodysostosis patient, caused by novel compound heterozygous mutations in the CTSK gene (c.87 G>A p.W29X and c.848 A>G p.Y283C). The most impressive manifestations in tooth were extensive periradicular high-density clumps with unclear periodontal space by orthopantomography examination and micro-computed tomography scanning analysis. Hematoxylin/eosin and toluidine blue staining and atomic force microscopy analysis showed that the cementum became significantly thickened, softened, and full of cementocytes. The disorganized bone structure was the main character of alveolar bone. The p.W29X mutation may represent the loss-of-function allele with an earlier termination codon in the precursor CTSK polypeptide. Residue Y283 is highly conserved among papain-like cysteine proteases. Three-dimensional structure modeling analysis found that the loss of the hydroxybenzene residue in the Y283C mutation would interrupt the hydrogen network and possibly affect the self-cleavage of the CTSK enzyme. Furthermore, p.Y283C mutation did not affect the mRNA and protein levels of overexpressed CTSK in COS-7 system but did reduce CTSK enzyme activity. In conclusion, the histologic and ultrastructural changes of cementum and alveolar bone might be affected by CTSK mutation via reduction of its enzyme activity (clinical trial registration: ChiCTR-TNC-10000876).

Molecular and clinical analysis in a series of patients with Pyknodysostosis reveals some uncommon phenotypic findings

Pyknodysostosis is a rare autosomal recessive skeletal dysplasia characterized by short stature, deformity of the skull, osteosclerosis, hypoplasia of the clavicle, and bone fragility. Radiographs show increased bone density, osteosclerosis, and acroosteolysis of the terminal phalanges. The pycnodysostosis gene is located on chromosome 1q21 and encodes an enzyme called Cathepsin K. Cathepsin K is a cysteine protease lysosomal protein associated with the degradation of bone and cartilage. In the current study, the authors described the clinical, radiological and molecular features of a group of six Mexican patients, including two familial and two sporadic cases, with Pyknodysostosis. One of the patients presented hypoacusia, an unusual finding in this disease.

Current research on pycnodysostosis

Pycnodysostosis is a rare autosomal recessive disorder caused by an inactivating mutation in cathepsin K (CTSK) and characterized by dysmorphic facial features, a short stature, acroosteolysis, osteosclerosis with increased bone fragility, and delayed closure of cranial sutures. Patients usually present with short stature or dysmorphic features the Pediatric Endocrinology or Genetics clinics, with atypical fractures to the orthopedics clinics or hematological abnormalities to the hematology clinics. However, under-diagnosis or misdiagnosis of this condition is a major issue. Pycnodysostosis is not a life threatening condition, but craniosynostosis, frequent fractures, respiratory-sleep problems, and dental problems may cause significant morbidity. Although no specific treatment for this disorder has been described, patients should be followed for complications and treated accordingly. A specific treatment for the disorder must be established in the future to prevent complications and improve quality of life for patients in the current era of advanced molecular research.

Cathepsin K analysis in a pycnodysostosis cohort: demographic, genotypic and phenotypic features

Background

To characterize cathepsin K (CTSK) mutations in a group of patients with pycnodysostosis, who presented with either short stature or atypical fractures to pediatric endocrinology or dysmorphic features to pediatric genetics clinics.

Methods

Seven exons and exon/intron boundaries of CTSK gene for the children and their families were amplified with PCR and sequenced. Sixteen patients from 14 families with pycnodysostosis, presenting with typical dysmorphic features, short stature, frequent fractures and osteosclerosis, were included in the study.

Results

We identified five missense mutations (M1I, I249T, L7P, D80Y and D169N), one nonsense mutation (R312X) and one 301 bp insertion in intron 7, which is revealed as Alu sequence; among them, only L7P and I249 were described previously. The mutations were homozygous in all cases, and the families mostly originated from the region where consanguineous marriage rate is the highest. Patients with M1I mutation had fractures, at younger ages than the other pycnodysostosis cases in our cohort which were most probably related to the severity of mutation, since M1I initiates the translation, and mutation might lead to the complete absence of the protein. The typical finding of pycnodysostosis, acroosteolysis, could not be detected in two patients, although other patients carrying the same mutations had acroosteolysis. Additionally, none of the previously described hot spot mutations were seen in our cohort; indeed, L7P and R312X were the most frequently detected mutations.

Conclusions

We described a large cohort of pycnodysostosis patients with genetic and phenotypic features, and, first Alu sequence insertion in pycnodysostosis.

Sclerosing bone dysplasias with involvement of the craniofacial skeleton

In this review we provide a complete overview of the existing sclerosing bone dysplasias with craniofacial involvement. Clinical presentation, disease course, the craniofacial symptoms, genetic transmission pattern and pathophysiology are discussed. There is an emphasis on radiologic features with a large collection of CT and MRI images. In previous reviews the craniofacial area of the sclerosing bone dysplasias was underexposed. However, craniofacial symptoms are often the first symptoms to address a physician. The embryology of the skull and skull base is explained and illustrated for a better understanding of the affected areas.

Exome sequencing identifies CTSK mutations in patients originally diagnosed as intermediate osteopetrosis

Autosomal Recessive Osteopetrosis is a genetic disorder characterized by increased bone density due to lack of resorption by the osteoclasts. Genetic studies have widely unraveled the molecular basis of the most severe forms, while cases of intermediate severity are more difficult to characterize, probably because of a large heterogeneity. Here, we describe the use of exome sequencing in the molecular diagnosis of 2 siblings initially thought to be affected by "intermediate osteopetrosis", which identified a homozygous mutation in the CTSK gene. Prompted by this finding, we tested by Sanger sequencing 25 additional patients addressed to us for recessive osteopetrosis and found CTSK mutations in 4 of them. In retrospect, their clinical and radiographic features were found to be compatible with, but not typical for, Pycnodysostosis. We sought to identify modifier genes that might have played a role in the clinical manifestation of the disease in these patients, but our results were not informative. In conclusion, we underline the difficulties of differential diagnosis in some patients whose clinical appearance does not fit the classical malignant or benign picture and recommend that CTSK gene be included in the molecular diagnosis of high bone density conditions.

Sclerotic lesions of the spine: MRI assessment

Sclerotic (T2 dark) lesions of the spine are infrequent and, as a result, these are often missed or misdiagnosed. Plain films may not be always available during magnetic resonance imaging (MRI) readout. Knowledge of such lesions and their imaging appearances on MRI evaluation is essential for a reader. Additionally, a systematic approach is important to accurately diagnose these lesions. In this article we discuss the various causes of spinal sclerotic lesions, describe their MRI characteristics with relevant case examples, and outline a systematic approach to their evaluation.

Novel CTSK mutation resulting in an entire exon 2 skipping in a Thai girl with pycnodysostosis

Pycnodysostosis is a rare autosomal recessive skeletal dysplasia characterized by osteosclerosis, short stature, acro-osteolysis of the distal phalanges, bone fragility and skull deformities. Mutations in the cathepsin K (CTSK) gene, which encodes a lysosomal cysteine protease highly expressed in osteoclasts, have been found to be responsible for the disease. We identified a Thai girl with pycnodysostosis. Her parents were first cousins. Polymerase chain reaction sequencing of the entire coding regions of CTSK of the proband's complementary DNA revealed that the whole exon 2 was skipped. We subsequently amplified exon 2 using genomic DNA, which showed that the patient was homozygous for a c.120G>A mutation. The mutation was located at the last nucleotide of exon 2. Its presence was confirmed by restriction enzyme analysis using BanI. The skipping of exon 2 eliminates the normal start codon. The mutation has never been previously reported, thus the current report expands the CTSK mutational spectrum.

Pycnodysostosis with novel gene mutation and severe obstructive sleep apnoea: management of a complex case

Pycnodysostosis is a rare genetic disease. Impaired osteoclastic function is the basis for typical phenotypic features and bone fragility. The main differential diagnosis is osteopetrosis, also associated with altered bone remodelling, but with a more severe prognosis. We describe the case of an 8-year-old boy who presented life-threatening obstructive sleep apnoea successfully managed with non-invasive ventilation. Haematological overlap phenotype included anaemia and altered bone marrow, more common in osteopetrosis. Molecular analysis of the CTSK gene revealed a mutation not previously described in the literature.

[Pycnodysostosis: a rare disease with frequent fractures]

Pycnodysostosis is a rare disease caused by a dysfunction of the osteoclasts due to a mutation in the cathepsin K gene. We present a case of a young adult patient with the above mentioned syndrome, who suffered an atypical fracture of the tibia after a low energy fall. Some bone changes that could have predisposed the fracture were observed when examined in the Emergency Department. Not long afterwards he suffered the same type of fracture in another tibia. Due to the conditions typical of the pycnodysostosis, the above mentioned fracture required an unconventional approach for this mid-shaft tibial fracture (osteosynthesis plate), combined with a longer consolidation time. The case was finally resolved satisfactorily.

α11β1 integrin-mediated MMP-13-dependent collagen lattice contraction by fibroblasts: evidence for integrin-coordinated collagen proteolysis

We have previously determined that integrin α11β1 is required on mouse periodontal ligament (PDL) fibroblasts to generate the force needed for incisor eruption. As part of the phenotype of α11(-/-) mice, the incisor PDL (iPDL) is thickened, due to disturbed matrix remodeling. To determine the molecular mechanism behind the disturbed matrix dynamics in the PDL we crossed α11(-/-) mice with the Immortomouse and isolated immortalized iPDL cells. Microarray analysis of iPDL cells cultured inside a 3D collagen gel demonstrated downregulated expression of a number of genes in α11-deficient iPDL cells, including matrix metalloproteinase-13 (MMP-13) and cathepsin K. α11(-/-) iPDL cells in vitro displayed disturbed interactions with collagen I during contraction of attached and floating collagen lattices and furthermore displayed reduced MMP-13 protein expression levels. The MMP-13 specific inhibitor WAY 170523 and the Cathepsin K Inhibitor II both blocked part of the α11 integrin-mediated collagen remodeling. In summary, our data demonstrate that in iPDL fibroblasts the mechanical strain generated by α11β1 integrin regulates molecules involved in collagen matrix dynamics. The positive regulation of α11β1-dependent matrix remodeling, involving MMP-13 and cathepsin K, might also occur in other types of fibroblasts and be an important regulatory mechanism for coordinated extracellular and intracellular collagen turnover in tissue homeostasis.

A novel mutation (R122Q) in the cathepsin K gene in a Chinese child with Pyknodysostosis

BACKGROUND

Pyknodysostosis (OMIM 265800) is a rare, autosomal recessive sclerosing skeletal dysplasia as a consequence of the diminished capacity of osteoclasts to degrade organic bone matrix. Pyknodysostosis is caused by mutation in the cathepsin K (CTSK) gene. Up to date, 34 different CTSK mutations have been identified in patients with Pyknodysostosis; however, only one mutation was previously identified in a Chinese patient. The objective of this study was to characterize the clinical manifestations and features of Pyknodysostosis and identify the mutation of the causative gene in a Chinese family with Pyknodysostosis.

METHODS

We investigated a non-consanguineous Chinese family in which an 11-year-old child was affected with Pyknodysostosis. Altogether, 203 persons, including the affected individual, his parents and 200 healthy donors, were recruited and genomic DNA was extracted. All 8 exons of the CTSK gene, including the exon-intron boundaries, were amplified and sequenced directly.

RESULTS

The proband displayed a novel homozygous missense mutation c.365G>A in exon 4 of the CTSK gene. This mutation leads to the substitution of the arginine at position 122 by glutamine (R122Q) in cathepsin K. The parents were heterozygous for this gene mutation, and the mutation was not found in the 200 unrelated controls.

CONCLUSION

Our study suggests that the novel missense mutation c.365G>A (R122Q) in exon 4 of CTSK gene was responsible for Pyknodysostosis in the Chinese family.

A child with bone fractures and dysmorphic features: remember of pycnodysostosis and craniosynostosis

Accidental bony injuries are common in children. Children may also present with bony injuries following non-accidental injuries. Pathological fractures, though extremely rare, are an important entity and constitute fractures that occur in abnormal bones, usually after minor trauma. Pycnodysostosis is a rare skeletal dysplasia characterised by a clinical phenotype that includes short stature, skull deformities, osteosclerosis, acroosteolysis and bone fragility. Often the disease is diagnosed at an early age as a result of the investigation of short stature. However, the diagnosis is sometimes delayed and must be considered in any child with a history of recurrent or multiple bone fractures and dysmorphic features. The purpose of this report is to describe the clinical, radiological and genetic issues of a 9-year-old girl with a long history of multiple bone fractures. She had been subjected to safeguarding investigations previously and was identified to have dysmorphic features diagnosed as pycnodysostosis associated with craniosynostosis.