This study is unique because it is the first to examine factors associated with bone deficits using a comprehensive characterization of bone in a group of young subjects with thal, previously identified with low bone mass, compared to a healthy control cohort from the same region. DXA was used to assess spine and whole body bone mineral density, and pQCT evaluated peripheral volumetric density and strength parameters. What was particularly novel was the evaluation of deficits after adjustment for growth, pubertal delay and lean mass deficits.
We have shown that whole body bone mineral deficits assessed by DXA are only partially attributable to reduced skeletal size and lean mass deficits in patients with Thal. Specifically, deficits in spine aBMD and whole body BMC Z-score persisted in patients with Thal after adjustments were made for gender, height Z-score, and lean mass index Z-score. In this group of subjects, despite the presence of pubertal delay, Tanner stage alone was not a strong predictive factor in the bone deficits observed. Similarly, the compartmental deficits in cortical BMC, area, and thickness assessed by pQCT in a weight bearing bone (tibia) were only partially influenced by muscle mass deficits. In contrast, the differences observed between the Thal and controls in periosteal circumference and tibial strength (section modulus) were mediated by muscle mass differences. Interestingly, though differences in bone density and other geometric parameters were observed, fracture incidence was similar between the two groups. Differences in exposure to fracture risk, such as physical activity patterns / sports participation, and age-specific fracture incidence were not accounted for in this comparison.
These findings are not consistent with observations made in several other chronic conditions in which low bone mass occurs in the context of significant growth deficits. In pediatric patients with cystic fibrosis, after correction for significant linear growth deficits, the whole body bone mineral deficits disappeared [21
]. In patients with nephrotic syndrome on high dose prednisone therapy and short stature, when height deficits were considered, bone mass assessed by DXA was no longer significantly different between patients and healthy controls [22
]. Additionally, in patients with Crohn's disease, adjustment for lean mass deficits eliminated the bone mass deficits observed [23
]. In other chronic conditions with reduced growth velocity and decreased bone turnover, significant increases in cortical vBMD have been observed [24
]. However, in the present study though reduced bone formation was observed in the context of growth failure, we did not observe an effect on cortical vBMD.
Other factors that may have contributed to decreased BMC in patients with Thal include endocrinopathies, increased hematopoietic activity and nutritional deficits. It must be noted that though many of these subjects with Thal were transfused, they were maintained on chronic chelation therapy and few had extremely high current levels of hepatic iron. Despite this treatment, roughly 15% of these young subjects had iron-related hypogonadism, hypothyroidism and/or diabetes, perhaps a reflection of historically elevated body iron. We attempted, through statistical modeling, to control for the effects of pubertal delay on bone, however due to the small sample size we could not include other endocrinopathies into our models. The presence of these endocrinopathies could explain some of the remaining skeletal deficits observed in this group of subjects. Young Thal patients without endocrinopathy have aBMD by DXA within the normal range (25) further supporting the hypothesis that endocrinopathies may contribute to the observed bone deficits.
Recently, Gurevitch et al. used a mouse model to show that chronic blood loss leads to augmentation of the hematopoietic microenvironment and subsequent osteoporosis [26
]. Therefore, increased red cell turnover, either in the non-transfused patient, or the transfused patient at the end of the transfusion cycle, may create pressure on the closely linked osteohematopoietic system by increasing the number of osteoclasts, and thereby intensifying resorption of bone tissue [25
]. The effects of hyperactive hematopoiesis and oxidative stress were not directly assessed in the present study and could play a role in the etiology of low bone mass in thalassemia.
Deficiency of vitamin D and other bone forming nutrient deficiencies may also play a role in the development of reduced bone mass in thalassemia [8
]. More subjects with Thal than controls were consistently taking calcium and vitamin D supplements for their low bone mass. Despite these supplements, more Thal had deficient levels of vitamin D, a finding which was predictive only in the model for tibial trabecular density (data not shown). Simplified assessments of physical activity and inactivity and dietary intake used in this study did not prove to be strong predictors of bone mass in this sample.
Because DXA is a 2-dimensional image and may underestimate bone density in patients with severe growth and skeletal deficits, three-dimensional volumetric measures using pQCT are particularly useful both to distinguish trabecular from cortical bone, and to reduce the influence of patient size on measurements. Others have used pQCT to assess bone in adult patients with thalassemia with somewhat similar results to ours. In 2004 and again in 2008, Ladis et al compared findings at the 4% site of the distal radius in adult patients with thalassemia major and intermedia (21 to 44 years) with scans from healthy adult controls [28
]. They observed significant reductions in the trabecular bone compartments of adult patients compared to the controls, with the Thal intermedia group being the most severely affected. The trabecular deficits observed at this non-weight-bearing site (radius) in the study by Ladis were similar to those seen in the weight bearing tibia in our study. The lower trabecular vBMD we found in adult subjects with Thal, however, was no longer significant after controlling for tibial length and vitamin D level.
Ladis et al. also reported cortical compartment deficits at the 4% site of the radius. However, at this site the cortical shell is extremely thin, potentially reducing the accuracy of cortical geometrical measurements and strength parameters at this site. We chose to study the tibia in the present study because the risk of partial volume effects is reduced in this peripheral bone (30). Cortical parameters were assessed at the 38% site only. Additionally, the studies by Ladis et al were limited to older patients, and did not compare the deficits observed by pQCT to traditional measures by DXA or to biochemical markers of bone turnover.
pQCT and other volumetric methods provide separate measurements of trabecular and cortical bone compartments. This may allow earlier detection of changes in bone mass within differing compartments in response to disease or therapy. However, both pQCT and high resolution-pQCT (HRpQCT) are more sensitive to movement and require the subject to be still for nearly five minutes for optimal scans; therefore it is difficult to obtain optimal scans in very young children. pQCT is limited to assessment of peripheral sites only, and QCT, HR-QCT and pQCT have limited published pediatric specific reference data. Volumetric assessments using pQCT or QCT may be used more readily in future studies due to recent publications touting the predictive value of vBMD for fracture risk (31,32). However, DXA remains the most widely used and clinically available tool to assess low bone mass, particularly in pediatric populations. Additionally, given we observed similar deficits in both DXA and pQCT in our sample, it seems reasonable to conclude that DXA continue to be used to assess low bone mass in this population of patients, particularly if the interpretation of DXA scans considers adjustment for body size deficits.
Alterations in bone turnover in adult patients with thalassemia have been explored by other investigators, but few have studied bone markers in pediatric populations with adequate controls [33
] and all failed to control for pubertal changes or skeletal mass deficits. Most have reported increased bone resorption (serum CTx, urinary NTx, deoxypyridinoline) [1
], with and without decreases in bone formation markers (bone specific alkaline phosphatase, osteocalcin) [1
]. In 2003, Domrongkitchaiporn et al. analyzed iliac crest bone biopsies and markers of bone turnover in 18 adult patients with thalassemia. Interesting, they observed reductions in trabecular bone volume but no evidence of reduced bone formation or increased bone resorption [37
]. We have shown that particularly in young patients with thalassemia, bone formation is reduced, even after correcting for skeletal size and pubertal delay. By contrast, though others have reported elevated bone resorption in older patients with Thal, after controlling for age, puberty and whole body bone content, we no longer observed differences. This suggests that at least for our marker of resorption, urinary NTx, group differences were mediated by skeletal size. The combination of reduced bone formation, particularly in younger patients during periods of rapid bone mineralization leads to reduced whole body bone mass as well as reduced cortical mass in the peripheral bones.
In order to tailor clinical therapies for patients with Thal, there needs to be a comprehensive understanding of the etiology of bone disease. Our results indicate that deficits in both bone mass and geometry are significant even in relatively young patients. Therapies to stimulate bone anabolism such as physical activity, growth hormone and/or nutritional interventions may be warranted in the younger thalassemia patient. For older patients with significant endocrinopathies, the primary defect may be an increase in bone resorption; for these individuals treatment with anti-catabolic agents such as bisphosphonates may be appropriate.
We recognize some limitations of this study. The cohort was relatively small which precluded analysis of multiple clinical variables which could have contributed to skeletal status. The subjects were limited to patients previously identified to have low bone mass by DXA (BMD Z-score <−1.0). For this reason, we could not examine pQCT parameters of less severely affected subjects with this disorder. The decision to include only Thal subjects with low BMD Z-scores excluded only 5 of 62 (8%) screened. Additionally, the Thal group assessed at Oakland represented approximately half of the transfused patients who regularly attend the thalassemia clinic at the Children's Hospital & Research Center, Oakland. The total number of patients with all types of thalassemia syndromes in North America is estimated to be less than 2,000 and roughly half are transfusion dependent [38
]. Therefore, we believe the observations from these 25 subjects may be generalizable to other transfusion dependent patients in the U.S. The strengths of this study are a complete characterization of the skeletal manifestations of Thal using DXA, pQCT and markers of bone turnover in both children and young adults. Bone parameters were compared to large healthy contemporary U.S. cohort to derive both DXA and pQCT Z-scores. Ultimately, we have found evidence of skeletal deficits that cannot be dismissed as an artifact of small bone size or delayed maturity alone.