Aneuploidy and Skeletal Health

Abstract

The normal human chromosome complement consists of 46 chromosomes comprising 22 morphologically different pairs of autosomes and one pair of sex chromosomes. Variations in either chromosome number and/or structure frequently result in significant mental impairment, and/or a variety of other clinical problems, among them, altered bone mass and strength. Chromosomal syndromes associated with specific chromosomal abnormalities are classified as either numerical or structural and may involve more than one chromosome. Aneuploidy refers to the presence of an extra copy of a specific chromosome, or trisomy, as seen in Down’s syndrome (trisomy 21), or the absence of a single chromosome, or monosomy, as seen in Turner syndrome (a single X chromosome in females: 45, X). Aneuploidies have diverse phenotypic consequences, ranging from severe mental retardation and developmental abnormalities to increased susceptibility to various neoplasms and premature death. In fact, trisomy 21 is the prototypical aneuploidy in humans, is the most common genetic abnormality associated with longevity and is one of the most widespread genetic causes of intellectual disability. In this review, the impact of trisomy 21 on the bone mass, architecture, skeletal health and quality of life of people with Down syndrome will be discussed.

Introduction

The discovery in 1956 that the correct chromosome number in humans was 46 ushered in the modern human cytogenetics era¹. In the ensuing 50+ years, a series of major chromosomal syndromes with altered numbers of chromosomes were reported, e.g. Down syndrome (trisomy 21), Turner syndrome (45, X) and Klinefelter syndrome (47, XXY)². It is now well-established that chromosome abnormalities contribute significantly to genetic disease resulting in reproductive loss, infertility, congenital anomalies, abnormal sexual development, intellectual disability and altered pathogenesis of malignancy, as well as decreases in bone mass and strength. Specific and recurrent chromosome abnormalities have been associated with more than 60 identifiable syndromes and are present in at least 50% of spontaneous abortions, 6% of stillbirths, about 5% of couples with two or more miscarriages and approximately 0.5% of newborns³. In women aged 35 or over, chromosomal abnormalities are detected in around 2% of all pregnancies^4,5. Of particular interest to bone biologists is the decreased bone mass reported in the prototypical human aneuploidy Down syndrome^6–11. This review is focused on the mechanisms and clinical consequences of low bone mass on the function and quality of life of people with Down syndrome.

Complex phenotypes of Down syndrome

Originally described in 1866¹², Down syndrome is characterized by trisomy of human chromosome 21 (Hsa21). The hundreds of genes on Hsa21 that are at some level of dosage imbalance are presumed to affect a wide-range of developmental pathways and tissues¹³. Trisomy 21 is the most common symptomatic chromosomal abnormality compatible with survival into adulthood and the probability of fetal trisomy 21 is strongly associated with maternal age¹⁴. Although there appears to be a trend toward increased maternal age in developed countries¹⁵, the trend appears to be offset by the widespread availability of prenatal screening and evolving barriers to elective abortion, ensuring that the incidence of Down syndrome has stabilized at around one in 1000 to one in 800 live births^15,16.

Indeed, the life expectancy of Down syndrome patients born today has increased dramatically from around 9 years in the early part of the twentieth century to greater than 60 years in the second decade of the 21st century^17,18. The substantial increase in survival is due primarily to improvements in both medical and social care and indicates that the life expectancy of people with Down syndrome is approaching that of the general population¹⁹. The increased longevity is also now accompanied by a range of significant mid-life health concerns²⁰. These findings are of relevance to the US and have considerable implications in terms of the counselling information provided to families at risk of having a child with Down syndrome.

Numerous environmental and hormonal factors have been reported to contribute to low bone mineral density (BMD) in Down syndrome patients²⁰. The low BMD in these patients is associated with impairments in skeletal maturation and bone-mass accrual that predispose to fragility fractures, with the likelihood for these fractures increasing with extended life expectancy^21,22. Several factors have been suggested to contribute to these skeletal impairments and susceptibility to fragility fractures, including muscle hypotonia, low rates of physical activity, endocrine (hypothyroidism, hyperparathyroidism, hypogonadism), and autoimmune disorders (celiac disease) which can lead to inadequate nutrition²⁰. Low activity levels, low sunlight exposure and anticonvulsant therapy have also been associated with decreased bone mass but these are not consistent risk factors in Down syndrome, leaving the underlying pathophysiology unclear⁹. In any case, all Down syndrome patients should be encouraged to maintain adequate calcium levels, sufficient sunlight exposure (Vitamin D) and healthy dietary and exercise habits as can be tolerated (Table 1). It is important that Down syndrome patient and their caregivers and physicians recognize that conditions such as celiac disease or the use of anti-seizure medications adversely affect vitamin D levels, so diligence is required to ensure patients remain vitamin D sufficient.

Table 1.

Options for the maintenance of bone health in Down syndrome

DIETARY

Adequate calcium (Indicate normal range)

Age<50 (1,000 mg/day) total dietary and supplemental calcium

Age>50 (1,200 mg/day) total dietary and supplemental calcium

Vitamin D

Age<50 (400–800 international units (IU) daily*)

Age>50 (800–1,000 international units (IU) daily*)

Healthy food choices and a well-balanced diet

Dairy products, fish, fruits and vegetables

EXERCISE

Weight-bearing exercise as indicated and tolerated

Support and encourage appropriate activity levels

High-impact weight-bearing exercise e.g.: Hiking, stair-climbing

Low-impact weight-bearing exercise e.g.: Fast walking, low-impact aerobics

Non-Impact exercise as indicated and tolerated

Increase muscle strength to decrease the risk of falls and fracture

Improve balance, posture and daily function

Muscle-strengthening exercise e.g.: Functional movements (standing and rising up on your toes), Using elastic exercise bands.

ENDOCRINE

Endocrine management decisions

Routine Bone Density measurements (annual)

Follow Thyroid hormone levels, (T3, T4, TSH)

Identify Celiac disease

^*According to the Institute of Medicine (IOM), the safe upper limit of vitamin D is 4,000 IU per day for most adults

It is interesting to speculate that the low BMD and low bone turnover may be the result of the presumed low activity levels of Down syndrome children. Although not a consistent risk factor in these individuals, mechanistically an altered set-point for the “mechanostat”²³ would be a plausible explanation for the lower accrual of bone mass. The subsequent low turnover state of Down syndrome adults would maintain such a low bone mass phenotype. The resolution of this intriguing question will require longitudinal studies of activity and/or nutritional levels in children with trisomy Hsa21.

Down syndrome pathologies

Although the underlying genetic cause of Down syndrome, trisomy Hsa21, is the same in all individuals with the disorder, penetrance of the resulting pathologies varies¹³. The majority of Down syndrome patients have learning difficulties, craniofacial alterations and muscle hypotonia. However, the minority have congenital heart malformations, leukemia or gut morphologic abnormalities. The severity of the specific defects is variable and the extent of cognitive impairment varies widely between individuals with trisomy 21, although a majority of pediatric Down syndrome patients will require treatment for intellectual or growth retardation.

Compared with the normal population, individuals with Down syndrome have an 18-fold increased risk of developing leukemia²⁴. Specifically, Down syndrome is associated with a 500-fold increased risk of acute megakaryoblastic leukemia. In contrast to blood tumors, individuals with Down syndrome are at a significantly lower risk for developing virtually all types of malignant solid tumors demonstrating that trisomy 21 protects against tumor growth²⁵.

Growth retardation is a cardinal feature of Down syndrome that has been suggested to be attributable to growth hormone (GH) deficiency, secondary to hypothalamic dysfunction²⁶. Thyroid dysfunction is also more common in individuals with Down syndrome than in the general population, and hypothyroidism that is present at birth or develops during childhood or adolescence is the most commonly reported thyroid abnormality in individuals with Down syndrome²⁷. Hypothyroidism can be either congenital or acquired at any age after birth, with the primary risk factors being female and old age²⁷.

Patients with Down syndrome also have a high frequency of abnormalities in sexual development such that infertility, cryptorchidism, small testes and delayed puberty have been reported²⁸. Interestingly, Hsiang et al. evaluated gonadal function in noninstitutionalized patients with Down syndrome (53 boys and men and 47 girls and women)²⁹. When Down syndrome patients with thyroid dysfunction were excluded, the mean ages of onset and completion of puberty were normal in both sexes, dispelling another common mistaken belief regarding Down syndrome. In addition, hormone studies of the 23 sexually mature men indicated that serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels were significantly elevated beyond the normal range, but that mean testosterone levels were within the normal range, confirming a diagnosis of partial, primary gonadal dysfunction²⁹. These observations have been suggested to predispose individuals with Down syndrome to high bone turnover and bone loss^6,7,30–34. However, this proposed mechanism regarding the mechanism of low bone mass is one that we and others have recently dispelled in both mice and humans with Down syndrome^8,9,35.

Musculoskeletal

Patients with Down syndrome experience a number of different musculoskeletal abnormalities and frequently endure osteoarthritic degeneration of the spine as well as low bone mass with resultant fractures of the long bones or vertebral bodies³⁶. In addition, people with Down syndrome are hypotonic and often have nutritional and hormonal deficiencies at critical times of bone-mass accretion, namely in infancy and adolescence³⁷. These nutritional inadequacies have been suggested to play a major role in the impairment of peak bone-mass accrual and to even correlate with the occurrence of osteoporosis³⁸. Several cross-sectional and case–control studies, that included small numbers of patients, have found an increased prevalence of low bone mass and osteoporosis in men and women with mental retardation in general, and in those with Down syndrome in particular^6,7,30,39. The majority of these studies were making inappropriate comparisons between institutionalized patients with schizophrenia and ambulatory community dwelling individuals with Down syndrome.

As a result, many of these studies missed the low bone turnover phenotype of Down syndrome. Many investigators have hypothesized that hypogonadism and endocrine dysfunction drove the increased bone resorption, leading to the low bone mass observed in Down syndrome patients. More recent studies have used larger Down syndrome patient cohorts, in comparison with non-institutionalized normal controls, rather than institutionalized individuals with mental disorders. These Down syndrome patients with normalized thyroid hormone, Vitamin D levels, adequate exercise and nutrition⁹ as well as murine Down’s syndrome models^8,35,39 have unequivocally demonstrated that low bone mass is indeed common in males and females with Down syndrome. However, the mechanism is not one of increased bone resorption, but of low bone turnover that is present despite profound hypogonadism^8,35.

In addition to the well-recognized low bone mass phenotype and increased risk of fracture characteristic of Down syndrome, trisomy Hsa21 is also associated with a number of other musculoskeletal abnormalities. Spinal conditions include upper cervical spinal instability, in particular at the atlanto-axial or occipital-cervical joints with a reported incidence of around 10–15% and scoliosis estimated to occur in around 7% of individuals^40–42. In select patients, spinal fusions are sometimes necessary to improve mobility and alleviate these problems. Extremity abnormalities including patellofemoral instability, a variety of foot conditions and several hip disorders are also common^40,43. Patellofemoral instability is one of the more common musculoskeletal issues for these patients with an estimated incidence of 10–20%^40,43. Treatment usually consists of non-operative modalities but can require soft tissue and bony procedures in persistently symptomatic or recurrent cases⁴³. Foot pathology includes pes planus or flatfoot, which is reported at rates close to 2–6% and first ray disorders including metatarsus primus varus and hallux valgus (bunions)^40,44. The management of these problems almost always begins with non-operative management, such as orthotics or shoe ware modification and only progresses to surgical options when necessary.^44,45.

The hip disorders affecting Down syndrome patients include slipped capital femoral epiphysis (SCFE), Perthes disease and hip instability¹⁶. SCFE appears to occur in approximately 1.3% of individuals⁴⁶. While the management of this entity, i.e. in situ pinning, does not differ from non Down syndrome children, there is an increased complication rate with osteonecrosis being particularly worrisome⁴⁷. The etiology of this is interesting and has been suggested to be the result of the higher incidence of endocrine abnormalities present in Down syndrome⁴⁵. Perthe’s disease occurs with an incidence similar to SCFE, around 2%⁴⁸ while hip instability occurs in around 2–5% of Down syndrome patients. The etiology appears to be multi-factorial with both bony differences and more importantly soft tissue laxity and hypermobility contributing to the development of this problem¹⁶. Interestingly, the instability and dysplasia in trisomy Hsa21 differs drastically from typical childhood developmental hip dysplasia in that they are generally asymptomatic in early childhood (<2 yo) with subsequent subluxation/dislocations and dysplasia developing later^44,46,49. In particular, Bennett and colleagues⁴⁹ detailed the natural history of hip dislocations in Down syndrome patients and suggested four specific phases; initial, dislocation, subluxation and fixed phases. These investigators’ noted that dysplasia, including posterior acetabular deficiency, occurs during the subluxation phase and that it is the repeated subluxation/dislocations that result in damage to the femoral head⁴⁹. The treatment of this issue usually begins with immobilization in the subluxing/dislocation patient without bony changes and progresses to bony procedures including femoral and acetabular osteotomies to address the bony pathology with capsular placations or tightening as needed^45,46,49,50. With the increased life expectancy, orthopaedic surgeons may expect to see an ever increasing number of Down syndrome patients with end-stage arthritis of the hip⁵¹. However, complications with orthopaedic surgery in the Down syndrome population are more common, thus early intervention to avoid surgery is preferred⁵⁰. By providing physicians and surgeons with an awareness of the unique aspects of musculoskeletal disease in the Down syndrome patient population, thoughtful patient care decisions, involvement of family and caregivers, as well as good pre-operative planning, and the appropriate implant choice for total hip arthroplasty will provide reliable pain relief and significantly improved function¹⁶.

Discussion

Our measurement of bone density and bone biochemical markers in community dwelling Down syndrome patients compared to the normal population⁹ was the first to challenge the idea that low bone mass in Down syndrome was not due to increased bone cell activity and lifestyle differences^13,31,33. In fact, the low bone mass phenotype was observed in a murine Down syndrome model (Ts65Dn)^8,35, and was treatable with intermittent parathyroid hormone⁸. The decreased bone turnover in Down syndrome is an entirely unexpected result, given the hypogonadism and infertility apparent in both animals and humans with the disorder^52,53. Although hypogonadism is a consistent finding in men with Down syndrome,^10,28,54 the pathophysiology of the infertility in men with Down syndrome remains unclear.

However, what is clear is that males with Down syndrome are generally infertile and have significant disruption of one or more levels of the hypothalamic-pituitary-gonadal axis resulting in elevated FSH and LH levels and inconsistent testosterone levels, that are frequently in the low-normal range^28,29,52. Whatever the final cause of the hypogonadism and infertility in males, our current appreciation of the endocrine regulation of bone would favor significantly increased, not decreased bone turnover. It is critical to note additional clinical evidence in support of this surprising observation. In the only pathologic post-mortem examination of bone from a Down syndrome patient⁵⁵, bone histology directly confirmed low bone mass and the mechanism of decreased bone turnover, although it was incorrectly diagnosed as osteoporosis⁵⁵. In that study evaluating vertebral sections from a 49-year old Down syndrome female, a complete lack of active osteoclasts and decreased osteoblast number along the bone trabeculae was noted⁵⁵.

Given the skeletal consequences of low bone mass and bone strength in Down syndrome, namely increased fracture risk and frequency^21,22, clinical intervention becomes important. In the face of low bone turnover, current anti-catabolic agents (bisphosphonates; Prolia™) are contraindicated. At the moment, no anabolic therapy is approved in this patient population. In fact, the only FDA-approved anabolic agent (intermittent parathyroid hormone; Forteo™) is approved only for the treatment of osteoporosis in men and post-menopausal women, who are at high risk for a fracture. PTH is not approved in the Down syndrome patient population, and pharmacologic intervention in people with trisomy Hsa21 is complicated by social and consenting issues and a reluctance of pharmaceutical companies to treat this underserved population. As such there is an urgent and unmet need for alternative treatments to increase bone mass and strength in Down syndrome.

A number of alternative treatment options that can increase bone mass, so called anabolic agents, exist and/or are currently under development⁵⁶. At this time, the primary bone anabolic pharmaceutical target is the inhibition of the sclerostin pathway. Sclerostin is the product of the SOST gene produced mainly by osteocytes, and is a potent negative regulator of bone formation via inhibition of the Wnt signaling pathway⁵⁷. In fact, a human anti-sclersotin antibody is in clinical development, but its use in the Down syndrome populations is likely not a priority. Although a promising therapeutic opportunity that is associated with increased bone mineral density and bone formation with decreased bone resorption in postmenopausal women with low bone mass⁵⁸, the efficacy of anti-sclerostin therapy to increase bone mass in patient populations such as Down syndrome is unknown.

As a result, the exploration of other bone anabolic approaches has led us and others to consider nutritional supplements as a unique way to increase bone mass and strength^59,60. In the Down syndrome population, the utility of a nutritional supplement in the form of an oral additive is a particularly attractive and viable therapeutic option. To provide a direct and potentially efficacious alternative bone anabolic approach, we have been pursuing the utility of a proven blueberry nutritional intervention. Numerous studies have shown robust skeletal anabolism with blueberry diets in preclinical models^61–63, as well as with other nutritional supplements^59,60,64,65. As such, these approaches provide an attractive, non-pharmaceutical option in the clinical setting of Down syndrome.

In the Down syndrome population, routine screening for thyroid hormone deficiency and celiac disease are warranted, as is monitoring vitamin D and calcium levels²⁰. The musculoskeletal consequences of poorly managed calcium and vitamin D as well as minimal physical activity, is well-documented in the general population⁶⁶. However, these deleterious consequences in Down syndrome, in the face of inherently low bone turnover, maybe exacerbated. As such, increased diligence by physicians, patients and care-givers is required.

Conclusions

In summary, this discussion provides clarification of the mechanisms that contribute to the low bone mass in Down syndrome and more importantly, provide the basis for new directions for the treatment of the osteopenia that impacts this population. In light of the elevated fracture risk and fracture rate in adult Down syndrome patients, studies that carefully assess bone mass accrual during childhood are required, as are evidence-based recommendations for bone density screening and appropriate treatment options. Indeed, we strongly support the need to consider bone anabolic therapies in appropriate healthy adult Down syndrome patients with low BMD.