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CTSK
cathepsin K
CTSK is located on the long (q) arm of chromosome 1, at band 1q21.3. Arm ratio per GRCh38 - banding schematic.
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Overview
CTSK is located on chromosome 1 and encodes cathepsin K, a member of the papain-like cysteine protease family. This enzyme plays a central role in bone remodelling by degrading collagen and other proteins in the bone matrix. Under normal circumstances, osteoclasts - specialised bone-resorbing cells - secrete cathepsin K to break down old bone tissue, allowing new bone to form in a continuous cycle of renewal.
When both copies of CTSK carry pathogenic variants, the resulting loss of enzyme function prevents normal bone breakdown. This leads to pycnodysostosis, a rare autosomal recessive condition first described in the 1960s. The disorder affects skeletal development and density, with clinical features typically becoming apparent in early childhood. Understanding CTSK function has provided insights into bone metabolism and potential therapeutic targets for conditions affecting bone density.
What the gene does
Cathepsin K functions as a cysteine protease with exceptional activity against type I collagen, the primary structural protein in bone matrix. The enzyme is synthesised as an inactive precursor and undergoes processing to generate the mature, catalytically active form. Within the acidic environment of the resorption lacuna - a specialised compartment beneath osteoclasts - cathepsin K cleaves collagen fibres at multiple sites, facilitating efficient bone matrix degradation.
The protease demonstrates remarkable specificity for bone tissue, with highest expression levels in osteoclasts. Beyond collagen, cathepsin K degrades other matrix components including osteopontin and osteonectin, contributing to the comprehensive breakdown of mineralised tissue. The enzyme's activity is tightly regulated by pH, with optimal function occurring in the acidic microenvironment of active bone resorption. This localised activity ensures that bone breakdown occurs precisely where needed, maintaining skeletal integrity whilst allowing continuous remodelling throughout life. Dysregulation of cathepsin K activity has implications not only for rare genetic disorders but also for understanding common conditions affecting bone metabolism.
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Chromosome location
CTSK is located on the long arm of chromosome 1 at position 21.3, designated 1q21.3. The gene spans approximately 12 kilobases of genomic DNA and contains eight exons. This chromosomal region harbours several genes involved in skeletal development and connective tissue function, though CTSK is the primary gene in this locus associated with bone remodelling disorders. The relatively compact gene structure facilitates comprehensive genetic testing when pycnodysostosis is suspected clinically.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein. The cathepsin K protein comprises 329 amino acids, with the mature enzyme formed after removal of an N-terminal signal peptide and propeptide region. The catalytic domain contains the characteristic cysteine protease active site, which mediates peptide bond cleavage through a nucleophilic mechanism. Structural studies have revealed that cathepsin K adopts a compact globular fold typical of papain-like proteases, with the active site positioned in a cleft that accommodates collagen triple-helix structures. This architectural feature contributes to the enzyme's distinctive ability to cleave intact collagen fibres, a capability not shared by all cysteine proteases.
Key variants
Pathogenic variants in CTSK are distributed throughout the gene, with missense, nonsense, and frameshift variants all reported in individuals with pycnodysostosis. Many disease-causing variants affect the catalytic domain, disrupting enzyme activity or protein stability. Some populations exhibit founder variants, with specific mutations occurring at higher frequencies due to shared ancestry. Genetic testing typically sequences all coding exons and flanking intronic regions to identify biallelic pathogenic variants. The autosomal recessive inheritance pattern means that carriers - individuals with one pathogenic variant - generally remain asymptomatic, whilst affected individuals inherit variants from both parents.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.290del | p.Leu97fs | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.338del | p.Gly113fs | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.364C>T | p.Arg122Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.457A>T | p.Lys153Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.658A>T | p.Lys220Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.737_738del | p.Ser246fs | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.818del | p.Asn273fs | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.83dup | p.Trp29fs | Pathogenic | ★★☆☆ | Pyknodysostosis |
c.894G>A | p.Trp298Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Pyknodysostosis |
c.908G>A | p.Gly303Glu | Pathogenic/Likely pathogenic | ★★☆☆ | Fetal anomalies with a likely genetic cause |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Biallelic pathogenic variants in CTSK cause pycnodysostosis, a rare skeletal dysplasia affecting approximately 1 in 1.7 million births. The condition is characterised by generalised osteosclerosis (increased bone density), short stature, fragile bones prone to fracture despite their density, and distinctive craniofacial features including a prominent forehead and underdeveloped facial bones. Individuals may experience dental abnormalities, delayed closure of cranial sutures, and increased susceptibility to bone fractures, particularly of the long bones and clavicles. Skeletal abnormalities typically become evident in early childhood, though intellectual development is generally normal. Management focuses on symptomatic treatment and monitoring for complications, as there is currently no cure for the underlying genetic condition.
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Inheritance pattern
Conditions caused by pathogenic CTSK variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
CTSK appears on multiple NHS Genomic Medicine Service gene panels, reflecting its established role in inherited skeletal and metabolic disorders. The gene holds green (high evidence) classification on panels including Skeletal Dysplasia, Osteopetrosis, Lysosomal Storage Disorder, and Fetal Anomalies, amongst others. Green status indicates strong evidence linking CTSK variants to the associated phenotypes, supporting its inclusion in diagnostic testing pathways. CTSK is also listed on the Developmental Disorders Genotype-to-Phenotype (DDG2P) database and panels for rare syndromic craniosynostosis and undiagnosed metabolic disorders. This broad panel representation underscores the gene's importance in diagnosing rare bone disorders within the NHS, and genetic testing for CTSK may be offered when clinical features suggest pycnodysostosis or related skeletal dysplasias.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
How is pycnodysostosis inherited?
Pycnodysostosis follows an autosomal recessive inheritance pattern, meaning an affected individual inherits a pathogenic CTSK variant from each parent. Parents who carry one variant typically show no symptoms, whilst each child has a 25% chance of inheriting both variants and developing the condition.
Can carriers of CTSK variants experience any symptoms?
Carriers of a single pathogenic CTSK variant generally remain asymptomatic throughout life. The presence of one functional gene copy produces sufficient cathepsin K enzyme to maintain normal bone remodelling, so carriers do not develop features of pycnodysostosis.
Is genetic testing available for CTSK in the UK?
Yes, CTSK genetic testing is available through the NHS Genomic Medicine Service when clinical features suggest pycnodysostosis or related skeletal dysplasias. The gene's inclusion on multiple gene panels means testing may be arranged by specialist clinicians as part of a comprehensive diagnostic assessment.