Osteogenesis Imperfecta Type IX — Comprehensive Research Report

1. Disease Information

Overview

Osteogenesis Imperfecta Type IX (OI9; OI type 9) is a rare, autosomal recessive form of brittle bone disease caused by loss-of-function variants in the PPIB gene (peptidylprolyl isomerase B), which encodes cyclophilin B (CyPB), an endoplasmic reticulum–resident peptidyl-prolyl cis-trans isomerase. OI type IX was formally delineated as a molecular subtype when Van Dijk et al. (2009) demonstrated that homozygous null mutations in PPIB cause severe-to-lethal bone fragility (PMID: 19781681). It is distinguished from other recessive OI types affecting the prolyl 3-hydroxylation complex (types VII and VIII, caused by CRTAP and LEPRE1/P3H1 mutations, respectively) primarily by the absence of rhizomelia (proximal limb shortening) and by a relatively broader phenotypic spectrum ranging from perinatal lethality to moderate deforming disease.

Key Identifiers

Synonyms and Alternative Names

• Osteogenesis imperfecta, type 9
• OI9
• Osteogenesis imperfecta due to cyclophilin B deficiency
• PPIB-related osteogenesis imperfecta
• Cyclophilin B-deficient osteogenesis imperfecta
• Recessive OI type IX

Evidence Base

Information is derived from aggregated disease-level resources (OMIM, Orphanet), published molecular genetics studies, individual patient case reports, and animal model data from the Ppib-null mouse. Because OI type IX is extremely rare, the clinical literature consists primarily of small case series and individual family reports rather than large epidemiological cohorts.

2. Etiology

Disease Causal Factors

OI type IX is a monogenic disorder. It is caused exclusively by biallelic (homozygous or compound heterozygous) loss-of-function variants in PPIB (chromosome 15q22.31), the gene encoding cyclophilin B (CyPB). There are no established non-genetic causes. The disorder arises from deficiency of CyPB, which impairs post-translational modification of type I procollagen in the endoplasmic reticulum.

Genetic Risk Factors

• Causal gene: PPIB (HGNC:9239), chromosome 15q22.31
• Inheritance: Autosomal recessive — both copies of PPIB must be non-functional for disease to occur
• Carrier status: Heterozygous carriers are phenotypically unaffected
• Founder variants: A homozygous missense variant c.509G>A / p.Gly170Asp is specific to East Asian (predominantly Chinese) populations, with an allele frequency of ~0.000924 in gnomAD East Asian populations; estimated variant age ~65,160 years (PMID: 34659339)
• Consanguinity: Described in multiple consanguineous families given the rarity of null alleles

Pathogenic Variants Reported

Functional consequences: All reported variants result in absence or severe reduction of CyPB protein. Critically, CyPB loss is independent of its complex partners: in patient fibroblasts, P3H1 levels are substantially reduced (indicating P3H1 stability depends partially on CyPB), but CRTAP remains unaffected (PMID: 19781681).

Environmental Risk Factors

No environmental risk factors specifically modulate risk of OI type IX. As a Mendelian autosomal recessive disorder, risk is determined entirely by carrier status of parents. Advanced parental age is not a significant risk factor. Consanguinity increases the probability of biallelic inheritance in rare-allele families.

Protective Factors

No genetic protective factors have been identified that prevent disease expression once biallelic PPIB mutations are inherited. Modifier alleles affecting collagen processing could theoretically modulate severity but have not been systematically identified.

3. Phenotypes

OI type IX spans a spectrum from perinatal lethality to moderate deforming disease (PMID: 21282188). The majority of reported patients have severe-to-lethal phenotypes, but at least one molecularly confirmed patient without rhizomelia has had a moderate disease course.

Core Skeletal Phenotypes

Extraskeletal Phenotypes

Clinical Distinctive Features Vs. Other Recessive OI Types

A key distinguishing phenotypic feature is the absence of rhizomelia (proximal limb shortening) in OI type IX, contrasting with OI types VII (CRTAP) and VIII (LEPRE1/P3H1), which typically show rhizomelic shortening of upper extremities. As noted for the Native American patient: "There appears to be a trend toward rhizomelic shortening and less severe bowing of the extremities, as compared to patients with comparably severe OI caused by COL1A1 or COL1A2 mutation" (PMID: 27625864)—though one case did show prenatal rhizomelia, indicating phenotypic overlap.

Hearing Loss

Hearing loss management recommendations: "Patients with OI caused by PPIB mutation should have appropriate early and regular management of their hearing" (PMID: 27625864). Whether congenital sensorineural hearing loss is a true recurrent feature of OI type IX or represents a co-incidental finding remains uncertain given limited case numbers.

Onset and Severity

• Onset: Prenatal in severe/lethal cases (fractures visible on ultrasound in 2nd trimester); perinatal in lethal form; early postnatal in survivors
• Severity spectrum: Perinatal lethality (Sillence type II equivalent) to severe deforming (Sillence type III equivalent) to moderate
• Progression: Progressive deformity with growth; multiple fractures throughout childhood and potentially adulthood

4. Genetic / Molecular Information

Causal Gene

• Gene symbol: PPIB
• Gene full name: Peptidylprolyl isomerase B (cyclophilin B)
• HGNC ID: hgnc:9239
• OMIM Gene: #123841
• Chromosome location: 15q22.31
• Protein: Cyclophilin B (CyPB), 21 kDa
• Alternative gene names: CYP-S1, CYPB, HEL-S-39, SCYLP

Variant Classification

All reported disease-causing variants are classified pathogenic or likely pathogenic (ACMG/AMP criteria). The founder variant p.Gly170Asp received PM3, PM1 (change at pro-isomerase domain), and PP3 (REVEL score 0.94) evidence codes (PMID: 34659339).

Variant types reported: - Nonsense (PTC) - Frameshift (deletion) - Splice-site - Missense (affecting pro-isomerase domain)

All result in loss of function. Biochemical studies consistently reveal undetectable CyPB protein in patient fibroblasts from families with frameshift/nonsense mutations (PMID: 19781681).

Allele Frequency

• The c.509G>A / p.Gly170Asp Chinese founder allele frequency in gnomAD: 0.000924 in East Asian populations; absent in non-East Asian populations (PMID: 34659339)
• Other pathogenic alleles are extremely rare, below gnomAD detection thresholds or absent

Somatic vs. Germline

OI type IX is exclusively a germline disorder. Somatic mosaicism is theoretically possible but has not been documented in published literature.

Modifier Genes

No confirmed modifier genes have been described for OI type IX specifically. In the broader OI context, variants in BMP1, PLOD2, IFITM5, and other collagen-modifying genes have been considered modifiers, but these have not been specifically studied in PPIB-null disease.

Chromosomal Abnormalities

Not applicable; OI type IX is a point mutation / small insertion-deletion disorder without large-scale chromosomal changes.

5. Environmental Information

Environmental Factors

No specific environmental toxins, radiation, or occupational exposures are known to cause or exacerbate the molecular defect in OI type IX. As with all OI, secondary fracture risk may be modulated by environmental factors including fall risk, activity level, and calcium/vitamin D nutrition, but these do not alter the underlying genetic defect.

Lifestyle Factors

• Calcium and vitamin D supplementation are standard supportive measures for all OI patients
• Exercise programs (especially swimming, low-impact activities) are recommended to build muscle strength and improve mobility
• Contact sports are generally contraindicated

Infectious Agents

Respiratory infections are a significant complication (not cause) in patients with severe thoracic involvement. The Native American patient with lethal OI IX died at age 16 months from pneumonia (PMID: 27625864). Recurrent respiratory infections are a major source of morbidity in the severe phenotype.

6. Mechanism / Pathophysiology

The Prolyl 3-Hydroxylation Complex

The central pathophysiological mechanism in OI type IX is deficiency of cyclophilin B (CyPB), disrupting multiple post-translational modification steps of type I collagen in the endoplasmic reticulum.

CyPB (PPIB) functions as a component of a heterotrimeric complex with: 1. P3H1 (prolyl 3-hydroxylase 1, encoded by LEPRE1/P3H1) — the enzymatic hydroxylase 2. CRTAP (cartilage-associated protein) — structural scaffolding

This complex performs the 3-hydroxylation of proline at position 986 (P986) in the α1(I) chain of type I collagen — a modification essential for normal collagen fibril structure and bone mechanical properties.

Molecular functions of CyPB: - Peptidyl-prolyl cis-trans isomerase (PPIase) activity: CyPB is proposed to be the major PPIase catalyzing the rate-limiting step in collagen triple-helix formation, catalyzing cis-to-trans isomerization of proline imidic peptide bonds (proline constitutes ~20% of collagen residues) - Chaperone function: CyPB participates in ER chaperone complexes with BiP, GRP94, PDI, and calreticulin - Complex stabilization: CyPB is required for proper P3H1 retention/stability in cells; loss of CyPB leads to substantial reduction of P3H1 levels in patient fibroblasts

Collagen Modification Defects

Prolyl-3 hydroxylation: In PPIB-null patient fibroblasts, P986 3-hydroxylation is severely reduced (to ~33% of controls in OI IX, compared to 93–100% in controls). Paradoxically, this is less severe than in OI VII (CRTAP null, ~16%) and OI VIII (P3H1 null, ~22%), suggesting CyPB influences complex function but P3H1 retains partial activity without CyPB (PMID: 19781681). In the Ppib-KO mouse, only 5–11% of α1(I) P986 residues were hydroxylated in osteoblasts and fibroblasts, and 1–2% in tissue (PMID: 24968150).

Collagen folding delay: "Collagen folds more slowly in the absence of CyPB," with 5–8 minute delays in triple helix formation documented in Ppib-KO osteoblasts. This implicates CyPB as the predominant collagen PPIase, though residual CsA-sensitive activity (possibly FKBPs) provides partial redundancy (PMID: 24968150).

Lysyl hydroxylation defects — a distinctive biochemical signature: Beyond the prolyl-3 hydroxylation defect, CyPB deficiency creates tissue-specific dysregulation of helical lysyl hydroxylation and glycosylation by disrupting lysyl hydroxylase (LH) chaperone complexes: - In bone: site-specific underhydroxylation at crosslinking residues α1(I) K87 (57% vs. 98% in wild-type) and α2(I) K87 (45% vs. 77%), leading to an altered collagen crosslink profile - In skin: >80% decrease in total hydroxylysine; positive regulatory LH chaperones (FKBP65/Sc65/P3H3) drop to <11% of wild-type while negative regulator HSP47 increases - "CyPB deficiency profoundly affects Lys post-translational modifications of collagen likely by modulating LH chaperone complexes" (PMID: 30562343)

Crosslink abnormalities: In Ppib-KO bone: - Nearly 3-fold increase in the helical lysine-involved crosslink hydroxylysinonorleucine (HLNL) - Lysylpyridinoline (LP) increased 4–5-fold - HP/LP (hydroxylysylpyridinoline/lysylpyridinoline) ratio decreased 4.25- to 5.6-fold - Deoxy-HHMD and LNL (non-hydroxylysine crosslinks) form exclusively in KO tissue (PMID: 24968150)

These crosslink alterations reduce bone mechanical competence independently of the prolyl-3 hydroxylation defect.

ER trafficking defect: In CyPB-deficient fibroblasts, "a significant amount of collagen remained in the ER" rather than trafficking properly to the Golgi after ascorbic acid stimulation (PMID: 19997487), indicating disrupted intracellular collagen transport.

Reduced collagen deposition: Collagen deposited into insoluble extracellular matrix by Ppib-KO osteoblasts was decreased ~80%, linked to altered fibrillogenesis and abnormal fibril structure (PMID: 24968150).

Collagen Fibril Morphology

Transmission electron microscopy of Ppib-KO mice shows collagen fibrils are abnormally enlarged — on average 1.45 times wider than controls (114.6 ± 22.4 nm vs. 78.6 ± 12.4 nm diameter) (PMID: 19997487), indicating disrupted fibril assembly.

Causal Chain Summary

Biallelic PPIB loss-of-function mutation
    ↓
Absence of cyclophilin B (CyPB) in ER
    ↓
[Branch 1] Reduced P3H1 stability → decreased Pro-986 3-hydroxylation
[Branch 2] Disrupted PPIase activity → delayed collagen triple-helix folding
[Branch 3] Disrupted LH chaperone complexes → abnormal helical lysyl hydroxylation
    ↓
Altered collagen crosslink chemistry (LP↑, HP/LP ratio↓)
Abnormal fibril morphology (enlarged fibrils)
Reduced collagen matrix deposition
    ↓
Defective bone matrix organization and mechanical incompetence
    ↓
Severe osteoporosis, bone fragility, susceptibility to fractures

Cell Types and Biological Processes Involved

Primary cell types: - Osteoblasts (CL:0000062): Primary producers of type I collagen; most severely affected by CyPB deficiency - Chondrocytes (CL:0000138): Type II collagen also lacks Pro3-hydroxylation in KO mice - Fibroblasts (CL:0000057): Show ER retention of collagen and reduced P3H1 levels - Odontoblasts (CL:0000060): Involved in dentinogenesis imperfecta

GO Biological Processes: - Collagen fibril organization (GO:0030199) - Peptidyl-proline modification (GO:0018208) - Post-translational protein modification (GO:0043687) - Collagen biosynthetic process (GO:0032964) - Bone mineralization (GO:0030282) - Protein folding (GO:0006457) - Endoplasmic reticulum to Golgi vesicle-mediated transport (GO:0006888)

GO Cellular Components: - Endoplasmic reticulum lumen (GO:0005788) - Collagen-containing extracellular matrix (GO:0062023) - Extracellular matrix (GO:0031012)

7. Anatomical Structures Affected

Skeletal System (Primary)

• Long bones (UBERON:0002492): Femur, tibia, humerus, radius, ulna — cortical thinning, bowing, fractures
• Spine (UBERON:0001617): Platyspondyly, kyphoscoliosis
• Skull (UBERON:0003129): Decreased calvarial ossification, wormian bones (sutural ossification)
• Ribs (UBERON:0002228): Fractures, narrow thorax
• Teeth (UBERON:0007759): Dentinogenesis imperfecta

Secondary Organ Involvement

• Lung / respiratory system (UBERON:0001004): Restrictive lung disease secondary to narrow chest; recurrent pneumonias in severe cases
• Ear (UBERON:0001690): Sensorineural hearing loss (cochlea); possibly also conductive component
• Eye / sclera (UBERON:0001827): Blue sclerae from abnormal scleral collagen
• Skin (UBERON:0002097): Documented loose/lax skin; abnormal collagen crosslinking

Tissue and Cell Level

• Bone tissue (UBERON:0002481): Primary affected tissue — osteoporotic cortical and trabecular bone
• Connective tissue (UBERON:0002384): Broadly affected given ubiquitous type I collagen
• Cartilage (UBERON:0002418): Type II collagen also affected in KO model

Subcellular Level

• Endoplasmic reticulum (GO:0005783): Site of CyPB action; collagen retained here in CyPB deficiency
• Golgi apparatus (GO:0005794): Impaired collagen trafficking
• Extracellular matrix (GO:0031012): Reduced collagen deposition, abnormal fibril assembly

8. Temporal Development

Onset

• Perinatal lethal form: Fractures present in utero (detectable by ultrasound at 2nd trimester); bowed long bones at birth; respiratory failure common
• Severe deforming (non-lethal) form: Fractures at birth or early infancy; wheelchair dependence by age 7 documented (PMID: 19781681)
• Moderate form: Onset with fractures in early childhood; slower progression

Progression

• Disease course: Chronic, progressive
• Fracture accumulation: Multiple fractures through childhood and adolescence, often with minimal trauma
• Deformity progression: Progressive kyphoscoliosis and long-bone deformity from repeated fracture-healing cycles
• Lifespan: Variable — perinatal lethality in severe cases; the Ppib-KO mouse has a lifespan of 40–50 weeks with progressive kyphosis (PMID: 19997487)

Critical Periods

• Prenatal period: Fracture accumulation in utero in severe cases
• Early childhood: Most critical for fracture management and mobility preservation; "The age at onset of long bone fractures is a critical predictor of future ambulatory ability"
• Puberty: BMD typically improves relative to childhood during growth but deformity may worsen

9. Inheritance and Population

Epidemiology

• Overall OI prevalence: ~1 in 15,000–20,000 births across all types
• OI type IX specifically: Extremely rare; estimated incidence in Chinese population ~1/1,000,000 (PMID: 34659339); worldwide frequency not established due to rarity
• Fraction of all OI: OI types caused by CRTAP, LEPRE1, and PPIB mutations collectively account for a small minority of OI cases (~5% of total OI); PPIB variants are the least common of the three

Inheritance Pattern

• Autosomal recessive (AR)
• Penetrance: Complete — biallelic loss-of-function results in disease
• Expressivity: Variable (perinatal lethal to moderate)
• Consanguinity: Multiple reported families involve parental consanguinity, consistent with AR inheritance of rare alleles

Founder Effect

• East Asian (Chinese) populations have a specific founder variant c.509G>A/p.Gly170Asp with estimated mutation age 65,160 years (~3,258 generations), implying spread from a single common ancestral haplotype. This provides the basis for targeted carrier screening in Chinese populations (PMID: 34659339).

Carrier Frequency

• Based on gnomAD data for the Chinese founder allele (AF ~0.000924), carrier frequency is approximately 1/540 in East Asian populations for this single variant
• No reliable carrier frequency data for global PPIB pathogenic variants

Sex Ratio

No sex predilection — autosomal recessive, affects males and females equally.

Geographic Distribution

OI type IX cases have been reported in multiple ethnic and geographic contexts: - North America (consanguineous families; one Native American family) - China/Taiwan (multiple families with founder variant) - Middle Eastern consanguineous families (not specifically OI IX, but recessive OI broadly)

10. Diagnostics

Clinical Criteria

No OI-type-IX-specific diagnostic criteria exist. Diagnosis follows the general OI diagnostic approach combined with molecular confirmation:

1. Clinical recognition of bone fragility with multiple fractures, ± blue sclerae, ± DI, ± hearing loss
2. Exclusion of non-accidental injury
3. Radiographic assessment (wormian bones, osteopenia, long-bone bowing, platyspondyly)
4. DXA bone mineral density (reduced Z-score for age/height)
5. Molecular genetic confirmation (PPIB sequencing)

Laboratory Tests

• Routine labs are typically normal in OI (including calcium, phosphate, alkaline phosphatase); ALP may be mildly elevated
• Collagen biochemical analysis: Electrophoretic analysis of type I collagen from fibroblast culture can show slightly delayed migration (from altered PTMs); may reveal reduced P3H1 complex function
• Pro-986 hydroxylation assay: Mass spectrometric analysis can demonstrate reduced Pro-986 3-hydroxylation in collagen from patient fibroblasts (33% vs. controls 93–100%) (PMID: 19781681)

Imaging

• Radiographs (X-ray): Primary modality — reveals osteopenia, long-bone bowing, fractures, platyspondyly, wormian bones
• Prenatal ultrasound: Short/bowed long bones in severe cases detectable in 2nd trimester
• DXA: Quantitative BMD measurement; height-adjusted Z-scores used in children; reveals severely reduced BMD
• CT: For assessment of scoliosis, basilar invagination

Genetic Testing

• Preferred approach: Next-generation sequencing (NGS) gene panel for OI-related genes (includes COL1A1, COL1A2, PPIB, CRTAP, LEPRE1/P3H1, and others)
• Whole exome sequencing (WES): Highly effective; identified novel PPIB variants in multiple families (PMID: 34659339: average WES coverage 242×)
• Sanger sequencing: Targeted confirmation of variants and parental carrier testing
• ACMG/AMP classification: All reported PPIB variants are pathogenic or likely pathogenic
• Carrier testing: Targeted Sanger sequencing of familial variant for parental/sibling testing
• Prenatal diagnosis: Chorionic villus sampling or amniocentesis with targeted PPIB analysis is feasible once familial variant is known

Available genetic tests: NIH GTR lists 34 clinical tests for OI type IX conditions, including sequence analysis (31 tests), deletion/duplication analysis (18), and targeted variant analysis (7).

Differential Diagnosis

11. Outcome / Prognosis

Survival and Mortality

• Perinatal lethal form (Type II-equivalent): Death in utero or within days of birth from respiratory failure secondary to rib fractures and pulmonary hypoplasia
• Severe deforming form (Type III-equivalent): Variable survival; one reported patient died at 16 months from pneumonia (PMID: 27625864); survival into adulthood possible with intensive support
• Life expectancy: Not established for OI IX specifically; severe OI overall has significantly reduced life expectancy, primarily from pulmonary and neurological complications (basilar invagination)

Morbidity

• Mobility: Progressive loss of ambulatory ability in severe cases; wheelchair dependence may occur by mid-childhood
• Respiratory: Restrictive lung disease is a major cause of morbidity and death in severe cases
• Hearing: Early-onset bilateral sensorineural hearing loss documented; requires audiological follow-up
• Chronic pain: From recurrent fractures and skeletal deformity

Prognostic Factors

• Severity of skeletal phenotype at birth — most important prognostic indicator
• Achievement of motor milestones — children achieving independent sitting/standing by age 12 have better ambulatory prognosis
• Spinal deformity — severe kyphoscoliosis worsens pulmonary and cardiovascular prognosis
• Response to bisphosphonate treatment — improves BMD and may reduce fracture rate

12. Treatment

Bisphosphonate Therapy (Standard of Care)

Bisphosphonates are the most established pharmacological treatment across all OI types, including recessive forms:

• Intravenous pamidronate: Most widely used in children; typical regimen 1–3 mg/kg every 3–4 months; reduces fracture rate and improves lumbar spine BMD (~25% annual increase in young children); PMID: 25054949
• Intravenous zoledronic acid: 0.05 mg/kg/dose; comparable efficacy to pamidronate with fewer infusions; both increase lumbar spine BMD (51.8% vs. 67.6% respectively)
• Oral alendronate: Alternative for milder cases
• Duration: Typically continued through childhood growth; drug holiday considered after reaching final height

No PPIB-specific pharmacological trials exist; bisphosphonate data for OI IX is extrapolated from broader OI studies.

MAXO term: MAXO:0000647 (chemotherapy) — not applicable; pharmacotherapy with bisphosphonates: NCIT:C15986 (Pharmacotherapy); specific agents: CHEBI:60753 (pamidronate), CHEBI:74699 (zoledronic acid)

Emerging Pharmacological Treatments

• Setrusumab (UX143): Anti-sclerostin monoclonal antibody promoting bone formation; Phase 2b ASTEROID trial showed BMD improvement in adult OI; Phase 3 ORBIT and COSMIC trials missed primary endpoint (fracture rate reduction) but achieved secondary endpoints (BMD increase); FDA Breakthrough Therapy designation granted. No specific data for OI IX.
• Anti-TGF-β antibodies (fresolimumab): Preclinical studies showing promise in OI mouse models
• 4-Phenylbutyrate (4-PBA): Chemical chaperone shown to rescue ER stress in recessive OI zebrafish models; targets the cellular stress from impaired prolyl hydroxylation complex
• Denosumab: Anti-RANKL antibody; small studies in severe pediatric OI; not standard

MAXO term: MAXO:0000950 (supportive care) for symptom management

Surgical Management

• Intramedullary rodding (telescoping rods — Fassier-Duval, Bailey-Dubow): Prophylactic stabilization of femora/tibiae prone to bowing and fracture; telescoping rods accommodate growth; telescopic nailing significantly reduces fracture rates (33.3% vs. 75%) and deformity rates (23.3% vs. 50%) vs. traditional nails
• Osteotomy with internal fixation: For correction of severe long-bone deformity
• Spinal surgery: For severe progressive scoliosis (>40–50°); high complication risk given osteoporotic bone

MAXO term: MAXO:0000004 (surgical procedure)

Supportive Care

• Physiotherapy: Aquatic therapy, low-impact exercise; improves muscle strength and mobility; MAXO:0000011 (physical therapy)
• Occupational therapy: Adaptive equipment, fall prevention
• Calcium and Vitamin D supplementation: Standard nutritional support
• Hearing aids / audiological management: For sensorineural hearing loss; recommended for all PPIB-OI patients (PMID: 27625864)
• Respiratory support: Supplemental oxygen, ventilatory assistance in severe cases

Gene Therapy (Preclinical)

• AAV-based gene editing for collagen mutations is in preclinical development; no specific PPIB-targeted gene therapy trials
• CRISPR/Cas9 and gene addition approaches are in animal model testing
• Stem cell therapy (mesenchymal stem cell transplantation — BOOSTB4 trial) is under investigation for severe OI, potentially applicable to recessive forms

13. Prevention

Primary Prevention

• Genetic counseling for families with a known PPIB mutation — essential before further pregnancies (MAXO:0000079 genetic counseling)
• Cascade screening of siblings and relatives of affected individuals and known carriers
• Preimplantation genetic diagnosis (PGD): Available for couples where both are identified carriers

Secondary Prevention (Early Detection)

• Prenatal diagnosis: Molecular analysis of CVS or amniocentesis for known familial PPIB variants; prenatal ultrasound for skeletal dysplasia signs in at-risk pregnancies
• Newborn screening: Not currently part of national newborn screening programs; no metabolite-based test exists; diagnosis requires clinical suspicion + genetic testing
• Founder variant carrier screening: In East Asian (Chinese) populations, screening for the c.509G>A/p.Gly170Asp founder allele may be considered in preconception or prenatal settings given carrier frequency ~1/540

Tertiary Prevention (Preventing Complications)

• Fracture prevention: Bisphosphonate therapy, intramedullary rodding, avoidance of contact sports
• Hearing loss monitoring: Audiological testing at diagnosis and regular follow-up
• Respiratory monitoring: Pulmonary function tests; prompt treatment of respiratory infections
• Scoliosis surveillance: Regular spinal imaging; early intervention when curvature progresses
• Nutritional optimization: Calcium, Vitamin D, protein intake

14. Other Species / Natural Disease

Animal Models

OI type IX is not documented as a naturally occurring disease in veterinary species. The PPIB orthologue is highly conserved across mammals.

NCBI Taxon: Mus musculus (10090) — primary model

15. Model Organisms

Ppib Knockout Mouse Model

The Ppib-null (cyclophilin B-deficient) mouse is the principal model organism for OI type IX and was characterized by Choi et al. (PMID: 19997487):

Phenotype: - Severe osteopenia with dramatically reduced trabecular bone volume - Increased trabecular separation, reduced trabecular number - Progressive kyphosis, evident by 8 weeks, progressively worsening - Reduced body size and weight - Lifespan 40–50 weeks - Loose, lax skin (Ehlers-Danlos-like properties) - Normal femur-to-tibia ratios (no rhizomelia) - Normal fertility and birth appearance

Collagen findings: - Abnormally enlarged collagen fibrils (114.6 ± 22.4 nm vs. 78.6 ± 12.4 nm in controls) — "on average 1.45 times wider than similar samples from littermate control mice" (PMID: 19997487) - Near-total loss of Pro-986 3-hydroxylation (1–5% vs. near-100% in controls) - Severely disrupted helical lysyl hydroxylation and crosslink pattern (PMID: 24968150) - 80% reduction in collagen matrix deposition by osteoblasts - Reduced total skin collagen content; reduced skin tensile strength

Molecular findings: - P3H1 levels substantially reduced; CRTAP levels unaffected — demonstrating CyPB-dependent P3H1 stabilization - Collagen retained in ER rather than trafficking to Golgi - Residual collagen PPIase activity attributable to cyclosporine A-sensitive enzymes (suggesting FKBP redundancy)

Model limitations (Human-Model Mismatch considerations): - Mice do not show rhizomelia observed in some human cases - Mouse lifespan (40–50 weeks) and the degree of phenotypic severity may not fully recapitulate the human perinatal lethal form - Mice show pronounced skin laxity; this is less well-documented in human patients - Mouse bone biology differs from human (faster remodeling, different cortical organization)

Genetic model type: Constitutive knockout (whole-body Ppib deletion)

Available resources: - MGI: Available through IMPC and KOMP2 programs - JAX: Ppib-null alleles available for research

Summary of Key Evidence Citations

Suggested MONDO ID: MONDO:0012978 (Osteogenesis imperfecta type IX)

Key HPO terms: - HP:0002757 (Recurrent fractures) - HP:0000939 (Osteoporosis) - HP:0002645 (Wormian bones) - HP:0002808 (Kyphosis) - HP:0002650 (Scoliosis) - HP:0000592 (Blue sclerae) - HP:0000703 (Dentinogenesis imperfecta) - HP:0000407 (Sensorineural hearing impairment) - HP:0004322 (Short stature) - HP:0001382 (Joint hypermobility) - HP:0000926 (Platyspondyly)

Key GO terms (Biological Process): - GO:0030199 (Collagen fibril organization) - GO:0018208 (Peptidyl-proline modification) - GO:0032964 (Collagen biosynthetic process) - GO:0006457 (Protein folding)

Key CL terms: - CL:0000062 (Osteoblast) - CL:0000057 (Fibroblast) - CL:0000138 (Chondrocyte)

Sources: - PPIB Mutations Cause Severe Osteogenesis Imperfecta (PMC2756556) - Severe Osteogenesis Imperfecta in Cyclophilin B–Deficient Mice (PMC2777385) - Abnormal Type I Collagen PTM and Crosslinking in Cyclophilin B KO Mouse (PMC4072593) - Osteogenesis imperfecta caused by PPIB mutation with congenital hearing loss (PMC5020983) - A Founder Pathogenic Variant of PPIB Unique to Chinese Population (PMC8511635) - Cyclophilin B control of lysine PTMs of skin type I collagen (PMC6602281) - Osteogenesis Imperfecta - StatPearls (NBK536957) - OMIM #259440 — Osteogenesis Imperfecta, Type IX - NIH GTR — Osteogenesis imperfecta type 9 - Setrusumab for OI — Phase 2b ASTEROID Study (PMC11371902) - Mutations in PPIB delay procollagen chain association (HMG, PMID:21282188) - Bisphosphonate therapy for osteogenesis imperfecta (PMC6611487)