Genetics of Diffuse Idiopathic Skeletal Hyperostosis and Ossification of the Spinal Ligaments

PURPOSE OF REVIEW

The study aims to provide updated information on the genetic factors associated with the diagnoses 'Diffuse Idiopathic Skeletal Hyperostosis' (DISH), 'Ossification of the Posterior Longitudinal Ligament' (OPLL), and in patients with spinal ligament ossification.

RECENT FINDINGS

Recent studies have advanced our knowledge of genetic factors associated with DISH, OPLL, and other spinal ossification (ossification of the anterior longitudinal ligament [OALL] and the yellow ligament [OYL]). Several case studies of individuals afflicted with monogenic disorders, such as X-linked hypophosphatemia (XLH), demonstrate the strong association of fibroblast growth factor 23-related hypophosphatemia with OPLL, suggesting that pathogenic variants in PHEX, ENPP1, and DMP1 are associated with FGF23-phosphate wasting phenotype and strong genetic factors placing patients at risk for OPLL. Moreover, emerging evidence demonstrates that heterozygous and compound heterozygous ENPP1 pathogenic variants inducing 'Autosomal Recessive Hypophosphatemic Rickets Type 2' (ARHR2) also place patients at risk for DISH and OPLL, possibly due to the loss of inhibitory plasma pyrophosphate (PPi) which suppresses ectopic calcification and enthesis mineralization. Our findings emphasize the importance of genetic and plasma biomarker screening in the clinical evaluation of DISH and OPLL patients, with plasma PPi constituting an important new biomarker for the identification of DISH and OPLL patients whose disease course may be responsive to ENPP1 enzyme therapy, now in clinical trials for rare calcification disorders.

A Multicenter Allelic Analysis of Diffuse Idiopathic Skeletal Hyperostosis: Nature Versus Nurture?

BACKGROUND

Diffuse idiopathic skeletal hyperostosis (DISH) is an incompletely defined disease process with no known unifying pathophysiological mechanism.

OBJECTIVE

To our knowledge, no genetic studies have been performed in a North American population. To summarize genetic findings from previous studies and to comprehensively test for these associations in a novel and diverse, multi-institutional population.

METHODS

Cross-sectional, single nucleotide polymorphism (SNP) analysis was performed in 55 of 121 enrolled patients with DISH. Baseline demographic data were available on 100 patients. Based on allele selection from previous studies and related disease conditions, sequencing was performed on COL11A2, COL6A6, fibroblast growth factor 2 gene, LEMD3, TGFB1, and TLR1 genes and compared with global haplotype rates.

RESULTS

Consistent with previous studies, older age (mean 71 years), male sex predominance (80%), a high frequency of type 2 diabetes (54%), and renal disease (17%) were observed. Unique findings included high rates of tobacco use (11% currently smoking, 55% former smoker), a higher predominance of cervical DISH (70%) relative to other locations (30%), and an especially high rate of type 2 diabetes in patients with DISH and ossification of the posterior longitudinal ligament (100%) relative to DISH alone (100% vs 47%, P < .001). Compared with global allele rates, we found higher rates of SNPs in 5 of 9 tested genes ( P < .05).

CONCLUSION

We identified 5 SNPs in patients with DISH that occurred more frequently than a global reference. We also identified novel environmental associations. We hypothesize that DISH represents a heterogeneous condition with both multiple genetic and environmental influences.

Functional characterization of a unique mutant of ALK2, p.K400E, that is associated with a skeletal disorder, diffuse idiopathic skeletal hyperostosis

Bone morphogenetic protein (BMP) signaling regulates the physiological and pathological development of skeletal tissues. Activin receptor-like kinase 2 (ALK2) is a BMP type I transmembrane serine/threonine kinase receptor. Recently, a p.K400E mutation was found in ALK2 in a patient with diffuse idiopathic skeletal hyperostosis (DISH), which is a disorder characterized by calcification and ossification of spinal ligaments and entheses. We report here the functional characterization of ALK2 p.K400E in vitro. Cells overexpressing ALK2 p.K400E activated BMP signaling in response to osteogenic BMP ligands. However, ALK2 p.K400E was not activated by a nonosteogenic ligand, Activin A. BMP signaling through ALK2 p.K400E was further enhanced by the coexpression of a BMP type II receptor. The type II receptor increased the phosphorylation level of ALK2 p.K400E, suggesting that ALK2 p.K400E is a hypersensitive mutant to the BMP type II receptor kinases. Our findings suggest that pathological calcification and ossification in DISH are caused by overactivated BMP signaling through ALK2 p.K400E enhanced by type II receptors in response to osteogenic BMPs rather than Activin A.

Whole exome sequencing of patients with diffuse idiopathic skeletal hyperostosis and calcium pyrophosphate crystal chondrocalcinosis

OBJECTIVES

DISH/CC is a poorly understood phenotype characterised by peripheral and axial enthesopathic calcifications, frequently fulfilling the radiological criteria for Diffuse Idiopathic Skeletal Hyperostosis (DISH, MIM 106400), and in some cases associated with Calcium Pyrophosphate Dihydrate (CPPD) Chondrocalcinosis (CC). The concurrence of DISH and CC suggests a shared pathogenic mechanism. In order to identify genetic variants for susceptibility we performed whole exome sequencing in four patients showing this phenotype.

MATERIALS AND METHODS

Exome data were filtered in order to find a variant or a group of variants that could be associated with the DISH/CC phenotype. Variants of interest were subsequently confirmed by Sanger sequencing. Selected variants were screened in a cohort of 65 DISH/CC patients vs 118 controls from Azores. The statistical analysis was performed using PLINK V1.07.

RESULTS

We identified 21 genetic variants in 17 genes that were directly or indirectly related to mineralization, several are predicted to have a strong effect at a protein level. Phylogenetic analysis of altered amino acids indicates that these are either highly conserved in vertebrates or conserved in mammals. In case-control analyses, variant rs34473884 in PPP2R2D was significantly associated with the DISH/CC phenotype (p=0.028; OR=1.789, 95% CI= 1.060 - 3.021)).

CONCLUSION

The results of the present and preceding studies with the DISH/CC families suggests that the phenotype has a polygenic basis. The PPP2R2D gene could be involved in this phenotype in an as yet unknown way.

Ectopic calcification among families in the Azores: clinical and radiologic manifestations in families with diffuse idiopathic skeletal hyperostosis and chondrocalcinosis

OBJECTIVE

Twelve families that were multiply affected with diffuse idiopathic skeletal hyperostosis (DISH) and/or chondrocalcinosis, were identified on the island of Terceira, The Azores, potentially supporting the hypothesis that the 2 disorders share common etiopathogenic factors. The present study was undertaken to investigate this hypothesis.

METHODS

One hundred three individuals from 12 unrelated families were assessed. Probands were identified from patients attending the Rheumatic Diseases Clinic, Hospital de Santo Espírito, in The Azores. Family members were assessed by rheumatologists and radiologists. Radiographs of all family members were obtained, including radiographs of the dorsolumbar spine, pelvis, knees, elbows, and wrists, and all cases were screened for known features of chondrocalcinosis.

RESULTS

Ectopic calcifications were identified in 70 patients. The most frequent symptoms or findings were as follows: axial pain, elbow, knee and metacarpophalangeal (MCP) joint pain, swelling, and/or deformity, and radiographic enthesopathic changes. Elbow and MCP joint periarticular calcifications were observed in 35 and 5 patients, respectively, and chondrocalcinosis was identified in 12 patients. Fifteen patients had sacroiliac disease (ankylosis or sclerosis) on computed tomography scans. Fifty-two patients could be classified as having definite (17%), probable (26%), or possible (31%) DISH. Concomitant DISH and chondrocalcinosis was diagnosed in 12 patients. Pyrophosphate crystals were identified from knee effusions in 13 patients. The pattern of disease transmission was compatible with an autosomal-dominant monogenic disease. The mean age at which symptoms developed was 38 years.

CONCLUSION

These families may represent a familial type of pyrophosphate arthropathy with a phenotype that includes peripheral and axial enthesopathic calcifications. The concurrence of DISH and chondrocalcinosis suggests a shared pathogenic mechanism in the 2 conditions.

COL6A1, the candidate gene for ossification of the posterior longitudinal ligament, is associated with diffuse idiopathic skeletal hyperostosis in Japanese

STUDY DESIGN

Genetic screening of collagen 6A1 gene (COL6A1) in patients with diffuse idiopathic skeletal hyperostosis (DISH) recruited in Japan and the Czech Republic.

OBJECTIVE

To investigate allelic associations between DISH and nucleotide variants of COL6A1.

SUMMARY OF BACKGROUND DATA

DISH is a skeletal hyperostotic disease characterized by ligamentous ossification of the anterolateral side of the spine. Ossification of the posterior longitudinal ligament (OPLL) is a related disorder with DISH, and COL6A1 was identified as a susceptibility gene to OPLL. COL6A1 was examined for susceptibility in DISH patients from Japan and the Czech Republic.

METHODS

Seven single nucleotide polymorphisms of COL6A1 were genotyped by direct sequencing. The allele frequencies were compared between 97 Japanese DISH patients and 298 Japanese controls, and between 96 Czech DISH patients and 96 Czech controls by chi2 test.

RESULTS

The intron 32 (-29) single nucleotide polymorphisms of COL6A1 was significantly associated with the Japanese DISH patients (chi2 = 9.33; P = 0.0022), but not with the Czech DISH patients.

CONCLUSIONS

Because COL6A1 could be a susceptibility to the occurrence of DISH and OPLL in the Japanese population, we consider that COL6A1 could be responsible for the hyperostotic state, leading to ectopic bone formation in the spinal ligament.

Hyperleptinemia in female patients with ossification of spinal ligaments

In order to examine the involvement of leptin in the ossification of spinal ligaments (OSL), the present study examined (i) serum levels of leptin and insulin in OSL patients and controls, (ii) serum leptin levels in children of OSL females with severe obesity, (iii) the expression of leptin receptor mRNA in human spinal ligaments, and (iv) effects of leptin on cultured human ligament cells. In the OSL females, serum leptin levels were significantly higher than those of the control females, and the levels were positively correlated to the serum insulin levels, while in the control females, there was a tendency of inverse correlation. The daughters of OSL females with severe obesity also had high serum leptin levels, although they had not developed OSL. The expression of leptin receptor mRNA was confirmed in the ligaments, but leptin did not influence the alkaline phosphatase activity nor procollagen type I carboxyl-terminal peptide content of the ligament cells. These findings suggest that leptin is involved genetically and indirectly with the pathogenesis of OSL in female patients.

Calcitonin simultaneously regulates both periosteal hyperostosis and trabecular osteopenia in the spinal hyperostotic mouse (twy/twy) in vivo

The twy (tiptoe-walking-Yoshimura) mouse, established in Japan in 1978 by brother-sister mating of ICR strain mice, is a valuable mutant as a model of ossification of the posterior longitudinal ligament (OPLL). OPLL causes severe myelopathy and has been thought to be very similar to ankylosing spinal hyperostosis (ASH) and diffuse idiopathic skeletal hyperostosis (DISH). In the twy mouse, both an increase in vertebral cortical membranous bone formation and a decrease in trabecular bone mass due to accelerated bone resorption occur simultaneously. This process is attributed to an inherited autosomal recessive single gene (twy). Calcitonin's suppression of bone resorption has been well established in the past, whereas the effects of this hormone on bone formation remain to be defined. Of particular interest is the simultaneous action of calcitonin on the abnormally accelerated bone formation and resorption. Thirty twy mice and 14 ICR mice were divided into seven groups, and changes induced by calcitonin on vertebral cortical appositional rate and on trabecular bone mass were investigated histomorphometrically. Results were (1) osteoclastic activity on trabecular surface was clearly suppressed by chicken calcitonin injected subcutaneously for 4 weeks; (2) no significant difference between the lumbar vertebral periosteal bone formation of calcitonin (CA) and vehicle-administrated twy mice groups. However, on the periosteal surface of the cervical vertebrae of the 6-week-old twy mice, the abnormally accelerated bone formation was suppressed by CA administration. This was also true for the elderly twy mice, although the effect was less pronounced. In conclusion, CA suppressed the abnormally hyperactivated periosteal bone formation. Results also suggested a possible therapeutic value of CA for OPLL.

Ossification of the posterior longitudinal ligament, diffuse, idiopathic skeletal hyperostosis, abnormal retinol and retinol binding protein: a familial observation

We describe a 52-year-old man who presented with diffuse idiopathic skeletal hyperostosis, ossification of the posterior longitudinal ligament, and abnormal levels of retinol and retinol binding protein (RBP). The molar retinol/retinol binding protein ratio was high, suggesting congenital functional RBP deficiency. His two sons, aged 23 and 27 years, shared the same biological abnormality without clinical symptoms. To our knowledge, this is the first case report of such a familial association.

Elevation of alkaline phosphatase activity induced by parathyroid hormone in osteoblast-like cells from the spinal hyperostotic mouse TWY (twy/twy)

We have examined the alkaline phosphatase (AP) activity of primary calvaria-derived osteoblast-like cells from the twy (tip-toe walking Yoshimura) and normal ICR control mouse. The twy mouse displays elevated osseous formation particularly in the spine, and the pathophysiological features resemble that of human ankylosing spinal hyperostosis. In the proliferative stage of cultured bone cells, parathyroid hormone (PTH) stimulation induced the elevation of AP activity of both twy and ICR mouse-derived cells. When they reached confluence, the AP activity of ICR mouse-derived cells ceased to increase with PTH stimulation. The twy mouse-derived cells, however, continued to respond to PTH, with the enzyme activity increasing even in the confluent, stationary stage. PTH stimulation also increased the intracellular cAMP content of twy mouse-derived cells but it did not influence that of ICR mouse-derived cells in the stationary stage. Moreover, stimulation with dibutyryl cAMP, but not with phorbol myristate acetate, increased the AP activity of both twy and ICR-derived bone cells irrespective of culture conditions, either in the proliferative or in the confluent stage. These data suggest that the protein kinase A-mediated pathway plays a pivotal role in bone cells with PTH stimulation, and that the uninhibited AP activity observed in twy mouse-derived bone cells might be due to some deviating process between the PTH ligand/receptor interaction and cAMP generation.

Coexistence of Paget's bone disease and diffuse idiopathic skeletal hyperostosis in males

The coexistence of Paget's bone disease (PBD) and diffuse idiopathic skeletal hyperostosis (DISH) of the spine is controversial and could have implications for the pathogenic mechanisms involved in these disorders. In order to assess the prevalence of DISH in patients with Paget's disease, a prospective controlled study was carried out in which roentgenograms were obtained from a group of 50 consecutive ambulatory patients previously diagnosed as having PBD (25 males and 25 females; mean age: 66.2 +/- 8.9 years) and these were compared with 50 age- and sex-matched subjects selected randomly from various categories of medical disorders excluding PBD. DISH was found in 12 of 50 Paget's patients, this corresponding to an incidence of 24%; 10 (83%) of these 12 subjects were males. In the control population the incidence of DISH was 6%-2 males and 1 female. This difference between the two groups was statistically significant (chi 2 test; p = 0.02). Apart from the male gender, which was clearly associated with the presence of DISH in the Paget's group (p = 0.019), no other biological variable or characteristic of PBD was linked to the presence of DISH. In conclusion, our data revealed that PBD patients have a significantly greater prevalence of DISH than non-PBD probands. It would seem that the genetic mechanism which is responsible for the susceptibility of PBD to osteoclast viral infection could be related to the pathogenia of DISH in a sex-linked manner.

Diffuse idiopathic skeletal hyperostosis (DISH) of the elbow: a cause of elbow pain? A controlled study

Elbow pain is a common complaint and elbow hyperostosis a frequent radiological condition. However, little is known about the association between the clinical and radiological findings. To evaluate the relationship between spinal and extraspinal hyperostotic features and the clinical relevance of elbow hyperostosis we have performed the first controlled, double-blinded study of 85 hospitalized probands, 33 with and 52 without thoracospinal hyperostosis on lateral chest X-ray. Elbow and shoulder hyperostosis were graded on bilateral standard radiographs. Elbow pain was assessed by an interviewer using a standardized questionnaire and extraskeletal causes of elbow pain were recorded. The prevalence of elbow hyperostosis was increased in cases with thoracospinal hyperostosis compared to controls (82% versus 58%, chi 2 = 5.32, P less than 0.025, n = 85, olds ratio (OR) 3.30 (95% CI 1.16-9.35)). Similarly, the prevalence of elbow hyperostosis was increased in cases with shoulder hyperostosis compared to controls (83% versus 60%, chi 2 = 4.51, P less than 0.05, n = 84, OR = 3.20 (95% CI 1.06-9.66)), emphasizing the multifocal nature of hyperostotic features. Elbow pain was only slightly more prevalent in cases with elbow hyperostosis compared to controls (21% versus 13%, chi 2 = 0.75, NS, OR = 1.84 (95% CI 0.46-7.44)). We conclude that elbow hyperostosis is a radiological finding of doubtful clinical relevance.

[Familial incidence of diffuse idiopathic skeletal hyperostosis]

The authors describe the incidence of diffuse idiopathic skeletal hyperostosis in three members of a family--father and two daughters. Some diagnostic problems of the disease are also discussed.

[Familial occurrence of multiple basalomas associated with hyperostotic spondylosis]

We report on brother and sister suffering from hyperostotic spondylosis and multiple basal cell carcinomas. Localisation and multiplicity of the tumors are remarkable. On account of the family tree, we assume recessive inheritance.