Development of Algorithm for Clinical Management of Sickle Cell Bone Disease: Evidence for a Role of Vertebral Fractures in Patient Follow-up

Bone Health Impairment in Patients with Hemoglobinopathies: From Biological Bases to New Possible Therapeutic Strategies

Hemoglobinopathies are monogenic disorders affecting hemoglobin synthesis. Thalassemia and sickle cell disease (SCD) are considered the two major hemoglobinopathies. Thalassemia is a genetic disorder and one of the major hemoglobinopathies determined by an impairment of globin chain production, which causes an alteration of erythropoiesis, an improvement in hemolysis, and an alteration of iron homoeostasis. In SCD, the mutations are on the β-globin chain of hemoglobin which results in a substitution of glutamic acid by valine with consequent formation of Hemoglobin S (HbS). Several factors are involved in bone metabolism alteration in patients with hemoglobinopathies, among them hormonal deficiency, bone marrow hyperplasia, iron overload, inflammation, and increased bone turnover. Bone metabolism is the result of balance maintenance between bone deposition and bone resorption, by osteoblasts (OBs) and osteoclasts (OCs). An impairment of this balance is responsible for the onset of bone diseases, such as osteoporosis (OP). Therefore, here we will discuss the alteration of bone metabolism in patients with hemoglobinopathies and the possible therapeutic strategies to contain and/or counteract bone health impairment in these patients, taking into consideration not only the pharmacological treatments already used in the clinical armamentarium, but also the new possible therapeutic strategies.

A bone to pick-cellular and molecular mechanisms of bone pain in sickle cell disease

The bone is one of the most commonly affected organs in sickle cell disease (SCD). Repeated ischemia, oxidative stress and inflammation within the bone is largely responsible for promoting bone pain. As more individuals with SCD survive into adulthood, they are likely to experience a synergistic impact of both aging and SCD on their bone health. As bone health deteriorates, bone pain will likely exacerbate. Recent mechanistic and observational studies emphasize an intricate relationship between bone remodeling and the peripheral nervous system. Under pathological conditions, abnormal bone remodeling plays a key role in the propagation of bone pain. In this review, we first summarize mechanisms and burden of select bone complications in SCD. We then discuss processes that contribute to pathological bone pain that have been described in both SCD as well as non-sickle cell animal models. We emphasize the role of bone-nervous system interactions and pitfalls when designing new therapies especially for the sickle cell population. Lastly, we also discuss future basic and translational research in addressing questions about the complex role of stress erythropoiesis and inflammation in the development of SCD bone complications, which may lead to promising therapies and reduce morbidity in this vulnerable population.

Prevalence of bone complications in young patients with sickle cell disease presenting low bone mineral density

PURPOSE

Bone fragility in sickle cell disease (SCD) has been previously reported even in young patients, but the clinical consequences and specific management remain unclear. The objective of this study was to assess the prevalence of bone fragility in sickle cell patients and to evaluate the potential risk factors and associated complications.

METHODS

We conducted a single-center cross-sectional study. Bone mineral densitometry (BMD) at the lumbar spine and the hip, Vertebral Fracture Assessment (VFA) and biological measurements were performed in patients aged between 20 and 40 years.

RESULTS

One hundred and thirty-eight patients with sickle cell disease were included between June 2020 and December 2021. One hundred and one patients (73.2 %) were from Sub-Saharan Africa, 13 from North Africa (9.4 %), 11 from the Caribbean (7.9 %), 6 from the Indian Ocean. A Z-score < -2 was found in 43 patients (31.2 %) at the lumbar spine, in 4 patients (3 %) at the total hip, and in 5 patients (3.7 %) at the femoral neck. 59 patients (46.8 %) had vertebral deformities. Fragility fractures were recorded in 9 patients (10.8 %). Patients with low BMD had lower BMI (21.3 (19.0, 24.0) versus 24.0 (20.7, 26.1) Kg/m2, p = 0.003), lower osteonecrosis history (7 % versus 25.3 %, p = 0.011) and lower hemoglobin levels (9.0 (8.0, 10.0) versus 10.0 (9.0, 11.0) g/dL, p < 0.01). No association was found between history of fracture and low BMD.

CONCLUSION

Young patients with SCD commonly have low BMD at the lumbar spine, but the prevalence of fragility fracture was low. Low BMD - specifically at the spine - may not be tantamount to bone fragility.

Endocrinopathies in Hemoglobinopathies: What Is the Role of Iron?

Hemoglobinopathies, including β-thalassemia and sickle cell disease (SCD), are common genetic blood disorders. Endocrine disorders are frequent manifestations of organ damage observed mainly in patients with β-thalassemia and rarely in SCD. Iron overload, oxidative stress-induced cellular damage, chronic anemia, and HCV infection contribute to the development of endocrinopathies in β-thalassemia. The above factors, combined with vaso-occlusive events and microcirculation defects, are crucial for endocrine dysfunction in SCD patients. These endocrinopathies include diabetes mellitus, hypothyroidism, parathyroid dysfunction, gonadal and growth failure, osteoporosis, and adrenal insufficiency, affecting the quality of life of these patients. Thus, we aim to provide current knowledge and data about the epidemiology, pathogenesis, diagnosis, and management of endocrine disorders in β-thalassemia and SCD. We conducted a comprehensive review of the literature and examined the available data, mostly using the PubMed and Medline search engines for original articles. In the era of precision medicine, more studies investigating the potential role of genetic modifiers in the development of endocrinopathies in hemoglobinopathies are essential.

Bone loss is ameliorated by fecal microbiota transplantation through SCFA/GPR41/ IGF1 pathway in sickle cell disease mice

Bone loss is common in sickle cell disease (SCD), but the molecular mechanisms is unclear. Serum insulin-like growth factor 1 (IGF1) was low in SCD subjects and SCD mice. To determine if decreased IGF1 associated with low bone mass in SCD is due to reduced SCFA production by gut microbiota, we performed reciprocal fecal microbiota transplantation (FMT) between healthy control (Ctrl) and SCD mice. uCT and histomorphometry analysis of femur showed decreased bone volume/total volume (BV/TV), trabecular number (Tb.N), osteoblast surface/bone surface (Ob.S/BS), mineralizing surface/ bone surface (MS/BS), inter-label thickness (Ir.L.Th) in SCD mice were significantly improved after receiving Ctrl feces. Bone formation genes Alp, Col1, Runx2, and Dmp1 from SCD mice were significantly decreased and were rescued after FMT from Ctrl feces. Transplantation of Ctrl feces increased the butyrate, valerate, and propionate levels in cecal content of SCD mice. Decreased G-coupled protein receptors 41 and 43 (GPR41 and GPR43) mRNA in tibia and lower IGF1 in bone and serum of SCD mice were partially restored after FMT from Ctrl feces. These data indicate that the healthy gut microbiota of Ctrl mice is protective for SCD bone loss through regulating IGF1 in response to impaired bacterial metabolites SCFAs.

Sickle cell bone disease and response to intravenous bisphosphonates in children

Children with sickle cell disease (SCD) have the potential for extensive and early-onset bone morbidity. This study reports on the diversity of bone morbidity seen in children with SCD followed at three tertiary centers. IV bisphosphonates were effective for bone pain analgesia and did not trigger sickle cell complications.

INTRODUCTION

To evaluate bone morbidity and the response to intravenous (IV) bisphosphonate therapy in children with SCD.

METHODS

We conducted a retrospective review of patient records from 2003 to 2019 at three Canadian pediatric tertiary care centers. Radiographs, magnetic resonance images, and computed tomography scans were reviewed for the presence of avascular necrosis (AVN), bone infarcts, and myositis. IV bisphosphonates were offered for bone pain management. Bone mineral density was assessed by dual-energy X-ray absorptiometry (DXA).

RESULTS

Forty-six children (20 girls, 43%) had bone morbidity at a mean age of 11.8 years (SD 3.9) including AVN of the femoral (17/46, 37%) and humeral (8/46, 17%) heads, H-shaped vertebral body deformities due to endplate infarcts (35/46, 76%), and non-vertebral body skeletal infarcts (15/46, 32%). Five children (5/26, 19%) had myositis overlying areas of AVN or bone infarcts visualized on magnetic resonance imaging. Twenty-three children (8/23 girls) received IV bisphosphonate therapy. They all reported significant or complete resolution of bone pain. There were no reports of sickle cell hemolytic crises, pain crises, or stroke attributed to IV bisphosphonate therapy.

CONCLUSION

Children with SCD have the potential for extensive and early-onset bone morbidity. In this series, IV bisphosphonates were effective for bone pain analgesia and did not trigger sickle cell complications.

Sickle cell disease promotes sex-dependent pathological bone loss through enhanced cathepsin proteolytic activity

Age- and sex-dependent bone loss occurs in a Townes mouse model of SCD, with female mice more prone to trabecular bone loss.

Reduced cathepsin activity leads to increased thickness and density of cortical and trabecular bone in the Townes mouse model of SCD.

Sickle cell disease (SCD) is the most common hereditary blood disorder in the United States. SCD is frequently associated with osteonecrosis, osteoporosis, osteopenia, and other bone-related complications such as vaso-occlusive pain, ischemic damage, osteomyelitis, and bone marrow hyperplasia known as sickle bone disease (SBD). Previous SBD models have failed to distinguish the age- and sex-specific characteristics of bone morphometry. In this study, we use the Townes mouse model of SCD to assess the pathophysiological complications of SBD in both SCD and sickle cell trait. Changes in bone microarchitecture and bone development were assessed by using high-resolution quantitative micro–computed tomography and the three-dimensional reconstruction of femurs from male and female mice. Our results indicate that SCD causes bone loss and sex-dependent anatomical changes in bone. SCD female mice in particular are prone to trabecular bone loss, whereas cortical bone degradation occurs in both sexes. We also describe the impact of genetic knockdown of cathepsin K– and E-64–mediated cathepsin inhibition on SBD.

Dietary ω-3 Fatty Acid Supplementation Improves Murine Sickle Cell Bone Disease and Reprograms Adipogenesis

Sickle cell disease (SCD) is a genetic disorder of hemoglobin, leading to chronic hemolytic anemia and multiple organ damage. Among chronic organ complications, sickle cell bone disease (SBD) has a very high prevalence, resulting in long-term disability, chronic pain and fractures. Here, we evaluated the effects of ω-3 (fish oil-based, FD)-enriched diet vs. ω-6 (soybean oil-based, SD)- supplementation on murine SBD. We exposed SCD mice to recurrent hypoxia/reoxygenation (rec H/R), a consolidated model for SBD. In rec H/R SS mice, FD improves osteoblastogenesis/osteogenic activity by downregulating osteoclast activity via miR205 down-modulation and reduces both systemic and local inflammation. We also evaluated adipogenesis in both AA and SS mice fed with either SD or FD and exposed to rec H/R. FD reduced and reprogramed adipogenesis from white to brown adipocyte tissue (BAT) in bone compartments. This was supported by increased expression of uncoupling protein 1(UCP1), a BAT marker, and up-regulation of miR455, which promotes browning of white adipose tissue. Our findings provide new insights on the mechanism of action of ω-3 fatty acid supplementation on the pathogenesis of SBD and strengthen the rationale for ω-3 fatty acid dietary supplementation in SCD as a complementary therapeutic intervention.

Mechanisms of Bone Impairment in Sickle Bone Disease

Sickle bone disease (SBD) is a chronic and invalidating complication of Sickle cell disease (SCD), a multisystem autosomal recessive genetic disorder affecting millions of people worldwide. Mechanisms involved in SBD are not completely known, especially in pediatric age. Among the hypothesized pathogenetic mechanisms underlying SBD are bone marrow compensatory hyperplasia and bone ischemic damage, both secondary to vaso-occlusive crisis (VOC), which leads to cell sickling, thus worsening local hypoxia with a negative impact on osteoblast recruitment. Furthermore, the hypoxia is a strong activator of erythropoietin, which in turn stimulates osteoclast precursors and induces bone loss. Hemolysis and iron overload due to a chronic transfusion regimen could also contribute to the onset of bone complications. Vitamin D deficiency, which is frequently seen in SCD subjects, may worsen SBD by increasing the resorptive state that is responsible for low bone mineral density, acute/chronic bone pain, and high fracture risk. An imbalance between osteoblasts and osteoclasts, with a relative decrease of osteoblast recruitment and activity, is a further possible mechanism responsible for the impairment of bone health in SCD. Moreover, delayed pubertal growth spurt and low peak bone mass may explain the high incidence of fracture in SCD adolescents. The aim of this review was to focus on the pathogenesis of SBD, updating the studies on biochemical, instrumental, and biological markers of bone metabolism. We also evaluated the growth development and endocrine complications in subjects affected with SCD.

Zebrafish: A Suitable Tool for the Study of Cell Signaling in Bone

In recent decades, many studies using the zebrafish model organism have been performed. Zebrafish, providing genetic mutants and reporter transgenic lines, enable a great number of studies aiming at the investigation of signaling pathways involved in the osteoarticular system and at the identification of therapeutic tools for bone diseases. In this review, we will discuss studies which demonstrate that many signaling pathways are highly conserved between mammals and teleost and that genes involved in mammalian bone differentiation have orthologs in zebrafish. We will also discuss as human diseases, such as osteogenesis imperfecta, osteoarthritis, osteoporosis and Gaucher disease can be investigated in the zebrafish model.