Vitamin D and Bone fragility in Individuals with Osteogenesis Imperfecta: A Scoping Review

Vitamin D affects several body functions, and thus general health, due to its pleiotropic activity. It plays a key role in bone metabolism, and its deficiency impacts bone development, leading to bone fragility. In osteogenesis imperfecta (OI), a group of hereditary connective tissue disorders characterized by bone fragility, additional factors, such as vitamin D deficiency, can affect the expression of the phenotype and aggravate the disorder. The aim of this scoping review was to assess the incidence of vitamin D deficit in OI patients and the association between vitamin D status and supplementation in individuals affected by OI. We searched the PubMed Central and Embase databases and included studies published between January/2000 and October/2022 evaluating vitamin D measurement and status (normal, insufficiency, deficiency) and supplementation for OI. A total of 263 articles were identified, of which 45 were screened by title and abstract, and 10 were included after a full-text review. The review showed that low levels of vitamin D was a frequent finding in OI patients. Vitamin D supplementation was mainly indicated along with drug therapy and calcium intake. Even if widely used in clinical practice, vitamin D supplementation for OI individuals still needs a better characterization and harmonized frame for its use in the clinical setting, as well as further studies focusing on its effect on bone fragility.


Introduction
Vitamin D has a key role in different metabolic and development mechanisms, acting in particular in bone remodeling, skin differentiation, and immune system regulation [1]. Its role in calcium homeostasis and bone metabolism is well characterized; it is known that vitamin D deficiency affects different body mechanisms and systems [1][2][3][4][5][6][7][8], and recently its status has been evaluated in disorders other than osteoporosis or bone diseases, such as diabetes [9], sleep disorders [10], rheumatic diseases [11], and COVID-19 [12].
A cutoff level of 50 nmol/L (or 20 ng/mL) to define vitamin D deficiency has been established in guidelines by different societies (Endocrine Society Task Force on Vitamin D; Institute of Medicine (IOM, Washington DC, USA)) [29], and when serum/plasma 25(OH)D concentration is below 75 nmol/L (or 30 ng/mL) is considered insufficient [29,30].
Vitamin D exists in two forms: vitamin D3 or cholecalciferol, which is synthesized in the skin after exposure to sunlight or ultraviolet light, and ergocalciferol or vitamin D2 which is obtained by irradiation of plants or plant materials, or foods [30]. Vitamin D is mainly acquired by sunlight (90%) on the skin (deep layers of the epidermis) from 7-dehydrocholesterol and absorbed by the small intestine. In liver microsomes, hydroxylation occurs in 25-hydroxyvitamin D3 (25(OH) vitamin D), and then a second hydroxylation occurs in the kidneys by 1α-hydroxylase (encoded by CYP27B1) to active vitamin D Collagen type I is secreted by osteoblasts and is the most abundant constituent of the bone matrix. The mineral component of the bone is located within and between the collagen fibers. In the extracellular space, the osteoblasts produce alkaline phosphatase leading to the formation of mineral crystals in the gap regions between the collagen molecules [42].
In OI, pathogenic variants in COL1A1 or COL1A2 genes cause quantitative or qualitative abnormalities in collagen fibers. In fact, in collagen-type-I-related OI the collagen molecules are over-modified, the collagen fibers are thinner, and the bone matrix is hypermineralized, leading to bone fragility [43]. The bone in OI shows an abnormal architecture, with a lower trabecular number and connectivity, and lower trabecular thickness and volumetric bone mass. All these features contribute to bone fragility in OI [43].
The gold standard for the drug treatment of moderate to severe OI is bisphosphonates, antiresorptive drugs acting to inhibit bone resorption by osteoclasts [44], and in recent decades, several studies have focused on bisphosphonate treatment for children and adults with OI [44][45][46][47][48].
In light of the recent molecular knowledge, other treatment approaches have been evaluated [49][50][51], such as the use of denosumab (a monoclonal antibody directed against receptor activator of nuclear factor kappa B ligand (RANKL)) [52] or sclerostin antibody [53]. Furthermore, clinical trials are focusing on the use of antibodies acting on TGF-beta signaling [54] and cell or gene therapy [55].
Long before the first clinical classification of OI was established and the pathogenesis of the disorder was known, some authors had already considered including vitamin D supplementation in the management of the disease [3,56,57]. It is known that vitamin D supplementation has a beneficial effect on reducing the complications associated with vitamin D deficiency, including the low bone mineral density of the hip, spine, and arm bones [58]. The rationale for the use of vitamin D supplementation was that its deficiency had been directly linked to an increased fracture risk and severity in children [59]. Supplementation given to children classified as vitamin D deficient could have clinically useful benefits for peak bone mass [59][60][61][62].
As for children and adults in general [59,61,63], in OI patients, an adequate level of vitamin D is important for maintaining bone metabolism balance in a condition already with an increased risk of fracture. The optimal management of OI depends on the early and correct diagnosis of the disease and includes a multidisciplinary approach with pharmacological therapy, orthopedic follow-up, occupational therapy, physiotherapy, dietitians, and social workers [60,63,64]. Adequate vitamin D apport is part of the management of OI as in other conditions with bone fragility [65][66][67][68][69][70].
The aim of this scoping review was to evaluate the association between vitamin D status and supplementation in individuals affected by osteogenesis imperfecta.

Study Design
We followed the PRISMA-ScR guidelines (10) and Joanna Briggs Institute Methods manual for scoping reviews (11) as a reference to develop this study.
The review included the 5 following steps: (a) definition of the research question; (b) identification of relevant studies; (c) selection of the studies; (d) data chart; (e) data extraction and summary of the results.

Definition of the Research Question
The formulation of the search string followed the PICO system (12): -P: individuals affected by OI; -I: vitamin D status, measurement, and supplementation; -C: healthy subjects; -O: vitamin D status and supplementation.

Search and Selection of the Studies
The literature revision was performed in the PubMed Central and Embase databases from January 2000 to October 2022. The eligibility criteria were: (1) studies evaluating the use of vitamin D measurement, status, and supplementation in individuals with a genetic or clinical diagnosis of OI; (2) including randomized and non-randomized studies, observational studies, case reports, and case series; (3) studies written in English or Italian; and (4) studies on pediatric and adult populations.
Meta-analyses, book chapters, short communications, letters to the editor, and conference abstracts were excluded.
The following search string was formulated: Two independent reviewers performed the literature search and article selection, and the titles and abstracts of all the studies were reviewed to determine their eligibility. In case of disagreement on the suitability of the paper, a third author was consulted. Then, the full-text versions of the selected papers were extracted.

Data Extraction
The relevant data were extracted in a predefined form including (1) general paper information: first author and year of publication, the country where the study was conducted, the study design, aims, and duration; (2) the study population characteristics; and (3) vitamin D level, status, or measurement, and the main study results.

Results
The PRISMA flowchart is shown in Figure 1 and the PRISMA-ScR checklist is reported in Appendix A. A total of 263 potentially relevant studies were identified, and of those, 218 were excluded after the title and abstract screening. Of the 45 remaining studies, screened according to the eligibility criteria, only 10 were included. Among them, three were cross-sectional, one was a clinical trial, two were case-control studies, and four were retrospective studies. The study selection procedure is shown in the PRISMA flowchart ( Figure 1). The data extracted from eligible articles and a detailed summary are presented in Table 2.      In the Iranian series [71], vitamin D deficiency was observed in more than 40% of cases, but the mean levels of vitamin D in OI patients (even if with criteria for deficiency) were higher than in healthy controls, which might be related to supplementary consumption in patients.
Lower levels of vitamin D were described in a series of OI adolescents where the average concentration was similar for all OI types; moreover, the most severely affected patients had the lowest vitamin D levels [78]. The percentage of vitamin D deficiency reported by Edouard et al. [79] in a study that included 315 OI patients was similar to the percentage found in children and adolescents with bone fragility (about 20%) by Bowden et al. [80].
Bowden and colleagues evaluated 85 children with osteopenia or osteoporosis, 24 of them with an OI diagnosis, observing that vitamin D deficiency was prevalent in this population, regardless of the specific diagnosis of bone disease [80].
Winzenberg and colleagues obtained similar results about the prevalence of vitamin D deficiency, comparing OI patients with a control group of healthy individuals [60].
In a series of 97 OI patients from Norway, normal vitamin D levels were observed and only 10% of the patients showed osteoporotic T scores [77].
A percentage as high as 80% for vitamin D deficiency was found in 52 Brazilian OI patients by Zambrano et al. [73]. This prevalence did not differ statistically by OI type; nevertheless, a deficient or insufficient level of vitamin D was observed in 100% of OI type III patients [73].
A positive association between the BMD z-score and serum vitamin D was reported in the largest study selected that involved 315 Canadian OI patients affected by diverse severity levels of the disease (OI type I, III, or IV): serum vitamin D (25OH vitamin D) levels were associated independently with the LS-aBMD Z score [79]. In another study, even if more than half of the OI children had low lumbar bone mass [64,79], there was no association between vitamin D level and BMD parameters [64]. The same observation was made in a retrospective study of 71 patients, where no relationship between vitamin D and the indicator of bone mass was described [79].
Fracture and height are the main clinical features and outcomes of OI. Chagas and colleagues evaluated the nutritional status of OI patients and found that body composition is a relevant risk factor for fractures [76].
Interestingly, a positive correlation between vitamin D levels and height was observed, independent of the OI type [75]. In the series described by Zambrano and colleagues, this correlation was found in children who received vitamin D supplementation [73].
Wekre and colleagues observed that in adult OI patients bone turnover tended to be increased and osteoporosis and lower vitamin D levels were more prevalent in OI type III than in other OI types [77]. Similar data were found by Zambrano et al. in children and adolescents, observing that, even if the prevalence did not depend on the OI type, deficient or insufficient levels of vitamin D were observed in all cases affected by OI type III [73].

Discussion
The impact of vitamin D on bone metabolism and calcium-phosphate homeostasis is well documented in the medical literature [1,32,33]. Cases of rickets due to vitamin D deficiency have been reported since the 17th century [81]. More recently, the role of vitamin D in multiple body systems and several diseases has been revised; nevertheless, only a few studies have evaluated vitamin D status and collected specific data related to the effect of vitamin D supplementation in individuals with OI. In addition, most of these studies could not confirm the correlation between vitamin D status and the increase in the clinical severity of the disease. Even if the number of studies on this topic is limited, most of them highlighted insufficient or deficient levels of vitamin D in OI patients as a frequent finding and observed few correlations between vitamin D status and other factors.
Insufficient or deficient vitamin D levels were reported in a variable percentage (from 20% to 80%) in different studies [73,[78][79][80], confirming that vitamin D deficiency is a prevalent condition in OI, as in the general population. In particular, the same prevalence of vitamin D deficiency/insufficiency as in children or adolescent patients with bone fragility was observed [60,80].
Vitamin D insufficiency or deficiency was found in OI patients regardless of the degree of disease severity [73,75], even if all type III patients showed insufficient or deficient levels in a series [73] or the most severely affected individuals in an adolescent cohort had lower levels of vitamin D [78] in another study.
These findings about the prevalence of insufficient or deficient levels of vitamin D in OI are in accordance with data in the healthy population, and the fact that vitamin D deficiency is a diffuse health concern. [13][14][15][16][17]. Some authors have suggested that patients with more severe diseases also have restricted mobility and so less exposure to the sun. This hypothesis could also explain the fact that no effect of seasonality on vitamin D insufficiency was found in some studies [73,78,80]. The season of assessment was not related to the vitamin D concentration [73], although not all studies reported this information. Height is one of the main clinical features that varies with disease severity in OI. Unlike what has been found for the severity of the disease, a positive correlation between vitamin D levels and height, not depending on OI type, was observed. [75]. The same correlation was found in another series with patients who received vitamin D supplementation [73].
These described series included pediatric populations, while available data about vitamin D status in adult OI patients are insufficient. In a series of 97 OI patients from Norway, normal vitamin D levels were observed [77] and only 10% of the patients showed osteoporotic T scores.
Moreover, in an adult OI patient series, bone turnover tended to be increased and osteoporosis and lower vitamin D levels were more prevalent in OI type III than in other OI types [77].
Correlations between vitamin D and other health factors, such as bone mass index, parathormone (PTH) levels, or body composition, were not observed in any of the studies.
A positive association between the BMD z-score and serum vitamin D was reported in two of the selected studies [71,79]; however, this result was not confirmed in other articles [63,78].
A negative correlation between serum vitamin D and PTH levels has been observed in OI [73] as well as a positive correlation with alkaline phosphatase according to previous reports [75,76,80,82]. However, this association was not confirmed in all the studies, i.e., in the Brazilian study with 52 OI children, no correlation with bone markers was found [73]. This inverse relationship between vitamin D levels and PTH has been reported previously in healthy populations [1,[81][82][83][84][85][86][87][88].
These findings corroborate the higher bone remodeling markers in metabolic disorders and response to low vitamin D levels, regardless of the OI type [83].
The change in PTH level occurs as a physiologic response to low levels of vitamin D and leads to high bone turnover, bone resorption, and osteoporosis [31,32]. The inverse correlation between vitamin D and PTH levels may be the expression of the effect of vitamin D deficiency on bone health, as an additional factor contributing to low bone mass and so to worsening of the disease [32,33]. Moreover, monitoring and supplementation of vitamin D should be advised for managing pediatric patients with osteopenia or osteoporosis, and also for the management of OI [80].
Overweight or body composition are also related to vitamin D levels: in particular in the study by Wilsfold and colleagues, overweight was another risk factor for lower levels of vitamin D in OI [75]. Some studies also revealed an association between vitamin D levels and fat/overweight in both children and adults not affected by OI [82][83][84][85][86][87][88][89][90][91].
Nutritional status in OI patients, and in particular body composition, is a relevant risk factor for fractures [76]. Increased body fat had a negative effect on bones, according to the inverse association between total body fat and bone mineral content in children previously described [91][92][93][94][95][96][97][98]. This correlation was also found in a small OI series: type III OI patients showed a decreased lean body mass (LBM) compared to controls and presented a higher percentage of body fat. In addition, compared with patients with type I OI, those with type III OI presented with lower body mass, height, length, and lean body mass (LBM), and higher BMI and number of fractures. This is in accordance with the fact that weight was proven to be related to lower bone mass in children [97]. Other studies will be needed to evaluate if the overweight and vitamin D levels correlation is independent of other factors (severity of the disease, physical activity, or sun exposure), in particular in OI patients. Indeed only a few of the studies collected detailed information about nutritional status or body composition in OI.
A number of studies on OI have highlighted optimizing lifestyle factors and nutrition (including calcium and vitamin D) along with physical exercise, as part of the management of the disease and also for bone health in general [64][65][66][67][68][69][70][99][100][101][102][103]. The estimated effects on BMD of vitamin D supplementation are probably relatively modest; moreover, appropriate levels of vitamin D can optimize the benefits of bisphosphonate treatment in adults [75].
No conclusive data are available about supplementation, nor about the optimal dose of supplementation, in the case of normal levels of vitamin D. Plante and colleagues compared two treatment groups (2000IU and 400IU). The supplementation with vitamin D at 2000IU increased serum 25OHD concentrations in children with OI more than supplementation with 400IU, but no significant differences in LS-aBMD z-score changes were detected and only about 20% of the cases had baseline vitamin D levels < 50 nmol/L [74].
The main limitations of the studies are that they were conducted in small series, showed different severity in clinical expression in few cases evaluated, and different molecular bases/mutations in collagen type I underlying the disease. Furthermore, the outcomes were different between the studies, and some data were subjective as they were patient self-reported, which can impact the reliability of the results. In most of the series, concomitant treatment or evaluation during clinical trials with drugs for the treatment of the disease represent confounding factors in evaluating vitamin D level effects per se in the disease. The majority of the studies had no molecular characterization of the cases, and only two studies reported COL1A1 or COL1A2 genetic testing, but not other genes correlated to OI.

Conclusions
This scoping review of the current evidence related to the incidence of vitamin D deficit in OI patients and the association between vitamin D status and supplementation in individuals affected by OI suggests that low levels of vitamin D (deficiency and insufficiency) is a frequent finding in OI individuals with different OI types. Only a limited number of studies focused on vitamin D status and on the benefit of its supplementation in the OI population; vitamin D supplementation was mainly indicated along with drug therapy and calcium intake. In light of the evidence, vitamin D may be considered in the follow-up and management of OI patients, as part of a multidisciplinary approach [64][65][66][67][68][69][70][100][101][102][103][104].
Nevertheless, it is important to consider that the evidence presented on vitamin D supplementation for OI does not change the risk of fracture, the primary outcome of OI.
Even if widely used in clinical practice, vitamin D supplementation for OI individuals still needs a better characterization and harmonized frame for its use in clinical practice, as well as further studies focusing on its effect on bone fragility and homeostasis.

Structured summary 2
Provide a structured summary that includes (as applicable): background, objectives, eligibility criteria, sources of evidence, charting methods, results, and conclusions that relate to the review questions and objectives.

Rationale 3
Describe the rationale for the review in the context of what is already known. Explain why the review questions/objectives lend themselves to a scoping review approach.
1 Objectives 4 Provide an explicit statement of the questions and objectives being addressed with reference to their key elements (e.g., population or participants, concepts, and context) or other relevant key elements used to conceptualize the review questions and/or objectives.

Protocol and registration 5
Indicate whether a review protocol exists; state if and where it can be accessed (e.g., a Web address); and if available, provide registration information, including the registration number.

Eligibility criteria 6
Specify characteristics of the sources of evidence used as eligibility criteria (e.g., years considered, language, and publication status), and provide a rationale.

4-5
Information sources * 7 Describe all information sources in the search (e.g., databases with dates of coverage and contact with authors to identify additional sources), as well as the date the most recent search was executed.

4-5
Search 8 Present the full electronic search strategy for at least 1 database, including any limits used, such that it could be repeated. 5 Selection of sources of evidence † 9 State the process for selecting sources of evidence (i.e., screening and eligibility) included in the scoping review. 5 Data charting process ‡ 10 Describe the methods of charting data from the included sources of evidence (e.g., calibrated forms or forms that have been tested by the team before their use, and whether data charting was done independently or in duplicate) and any processes for obtaining and confirming data from investigators.

6
Data items 11 List and define all variables for which data were sought and any assumptions and simplifications made. 6 Critical appraisal of individual sources of evidence §

12
If done, provide a rationale for conducting a critical appraisal of included sources of evidence; describe the methods used and how this information was used in any data synthesis (if appropriate).
- Describe the methods of handling and summarizing the data that were charted. 5-12

Selection of sources of evidence 14
Give numbers of sources of evidence screened, assessed for eligibility, and included in the review, with reasons for exclusions at each stage, ideally using a flow diagram.

5, 6
Characteristics of sources of evidence 15 For each source of evidence, present characteristics for which data were charted and provide the citations. 6 Critical appraisal within sources of evidence 16 If done, present data on critical appraisal of included sources of evidence (see item 12). -

Results of individual sources of evidence 17
For each included source of evidence, present the relevant data that were charted that relate to the review questions and objectives. 7-12 Synthesis of results 18 Summarize and/or present the charting results as they relate to the review questions and objectives. 7-13

Summary of evidence 19
Summarize the main results (including an overview of concepts, themes, and types of evidence available), link to the review questions and objectives, and consider the relevance to key groups.

13,14
Limitations 20 Discuss the limitations of the scoping review process. 15

Conclusions 21
Provide a general interpretation of the results with respect to the review questions and objectives, as well as potential implications and/or next steps.

Funding 22
Describe sources of funding for the included sources of evidence, as well as sources of funding for the scoping review. Describe the role of the funders of the scoping review. JBI = Joanna Briggs Institute; PRISMA-ScR = Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews. * Where sources of evidence (see second footnote) are compiled from, such as bibliographic databases, social media platforms, and Web sites. † A more inclusive/heterogeneous term used to account for the different types of evidence or data sources (e.g., quantitative and/or qualitative research, expert opinion, and policy documents) that may be eligible in a scoping review as opposed to only studies. This is not to be confused with information sources (see first footnote). ‡ The frameworks by Arksey and O'Malley (6) and Levac and colleagues (7) and the JBI guidance (4,5) refer to the process of data extraction in a scoping review as data charting. § The process of systematically examining research evidence to assess its validity, results, and relevance before using it to inform a decision. This term is used for items 12 and 19 instead of "risk of bias" (which is more applicable to systematic reviews of interventions) to include and acknowledge the various sources of evidence that may be used in a scoping review (e.g., quantitative and/or qualitative research, expert opinion, and policy document). From: Tricco et al. [105].