Bone health in children with Angelman syndrome at the ENCORE Expertise Center

Angelman syndrome (AS) is a rare genetic disorder characterized by a severe developmental delay, epilepsy, and movement disorders [1, 2]. The estimated prevalence of AS is 1 in 24,000 [3]. AS is caused by the loss of function of the maternally inherited ubiquitin protein ligase E3A (UBE3A) gene [4]. AS is an imprinting disorder; the paternal gene for UBE3A is silenced in neurons. The loss of the maternal gene can occur due to a microdeletion of the 15q11.2-q13 region, a pathological variant of the UBE3A gene, uniparental paternal disomy (UPD), or an imprinting center defect (ICD) [5]. Children with a deletion are known with a more severe phenotype [1, 6, 7].

In 2010, the multidisciplinary ENCORE Expertise Center for AS was established at the Erasmus MC Sophia Children’s Hospital in Rotterdam, The Netherlands [1], and recognized by the National Federation of Universities and the European Reference Network ITHACA. We noticed the occurrence of (multiple) fractures — after minor trauma — in some children with AS. Previous studies showed that children with neurological disabilities might have reduced bone health, so-called secondary osteoporosis [8]. Known associated factors for reduced bone health are genetic predisposition, malnourishment, immobilization, late onset of puberty, use of anti-seizure medication (ASM) and/or corticosteroids, low vitamin D level, and endocrine disorders [8, 9]. Low bone health and fractures in people with AS have not been frequently reported. A case report described a girl with recurrent fractures [10]. Coppola et al. described a cohort of 18 AS patients, all walking, in whom 44% had a low bone mineral density (BMD) on dual-energy X-ray absorptiometry (DEXA). BMD was significantly lower in the group using ASM for a longer period of time and in patients with older age [11]. There was no association with sex, BMI, onset of puberty, or vitamin D level. The study did not report on the occurrence of fractures. An abnormal BMD was also found in 38% of 18 children with idiopathic epilepsy without AS, but decrease in BMD was less pronounced. A possible primary factor of the syndrome itself was suggested [10]. A study in a larger population including a longitudinal perspective on bone health in children with AS was never performed.

The aim of this study was to gain more insight in bone health, its longitudinal pattern, and risk factors for reduced bone health in children with AS that may be amenable to preventative measures to reduce fracture risk.

Bone health was assessed at least once in 91 of 120 eligible children with AS of 4 years of age or older; 40 children had two or more BHI measurements. Reasons for parents for not having an X-ray were time, behavioral issues of the child, or were unknown. In 5 children, it was not possible to calculate the BHI-SDS at most recent visit due to technical failure of the software program.

In a large cohort of 91 children, including longitudinal follow-up, we show that children with AS have a lower bone health than the population reference [12]. Moreover, BHI-SDS decreased significantly with age. Mean BHI-SDS in children with a deletion was below the normal range and significantly lower than in those with a non-deletion. Children with a deletion were younger than non-deletion children at the most recent assessment, which could make this latter finding even more worrying, as BHI-SDS also showed to decrease with age in this study and normally bone mass increases during puberty [21]. The negative effect of deletion subtype persisted when controlled for age and puberty and was confirmed in the longitudinal analysis.

In addition to the deletion genotype, older age, not walking independently, and late onset of puberty were most strongly associated with lower BHI-SDS. We also showed that AS children that walk with support had a significantly higher BHI-SDS than non-ambulatory children. In general, in children with neurological disorders, lack of physical and weight bearing activity is associated with less bone deposition and bone loss and higher risk of fractures [8, 22]. Appropriate physical activities and exercise for children with AS seem important. Since walking with support has a positive effect on bone health, we advise to stimulate the use of walking frames and other walking aids and to keep using them since we know that some children lose the ability to walk independently in puberty [1]. Vertical pressure stimulates bone remodeling [8], so regular use of a standing frame can positively affect bone health in non-ambulatory children. In the general population, the association of reduced bone health with late onset of puberty is established as a physiological phenomenon [23, 24]. When a child with AS shows late onset of puberty, parents may be reluctant to start puberty induction. The negative effect on bone health can be taken into account in shared decision-making.

In literature, use of ASM (in particular use of enzyme-inducing ASM such as phenytoin, carbamazepine, and also the non-enzyme-inducing valproic acid) is indicated as a strong risk factor for low bone health [8, 23, 25, 26]. ASM can activate the CYP450 pathway leading to a lower 1,25(OH)₂ vitamin D level (carbamazepine, phenytoin, phenobarbital, valproic acid), inhibits osteoblast and stimulates osteoclast activity (carbamazepine, phenytoin, phenobarbital and valproic acid), and/or creates a weak acidosis with decreased bone mineralization (topiramate, zonisamid). A longer period of ASM use might have a stronger negative effect on bone health [8, 25, 26]. Use of three or more ASM induces a stronger risk of osteoporosis than use of one ASM [25]. We found a similar association of BHI-SDS with ASM use in general, with valproic acid specifically, and with the number of ASM. However, these associations disappeared when corrected for age, genotype, mobility, BMI-SDS, and onset of puberty. As mean age in our cohort was 11 years, we could not analyze the effect of long-term ASM use.

In neurotypical children, the prevalence of fractures increases with age with a peak at 11–12 years in girls and 13–14 years in boys. At the end of adolescence, 25 to 40% of the girls and 30 to 50% of the boys had one fracture [27]. The 22% in our cohort is lower, but relatively high if you bear in mind that the mean age of this cohort is 11 years, substantial less children walk, and walk at an older age, putting them at a lower risk to break a bone. Hypotonia, balance problems, and crouch gait, as known to exist in children with AS [1, 2], will make it easier to fall and hamper the ability to break a fall properly, once they walk. In 11 of the 18 children though, the fracture was not related to a recalled trauma or related to minor trauma, suggesting a more fragile bone health.

Neurotypical children show an increase in BMD with a peak bone mass around 25 years for women and 30 for men [21]. As BHI-SDS is already lower in childhood and even decreases with age, we hypothesize that young adults with AS will have a lower peak bone mass than their neurotypical peers. This hypothesis should be studied in future research, by analyzing bone health and fracture prevalence in our patients after the age of 18 years. There is no publication on fracture prevalence in adults with AS beside a general remark on lower bone health in 20% of 53 adults of the cohort study of Prasad et al. [28]. From studies in adults with other neurogenetic conditions with ID, it is known that low bone health and increased fracture risk is prevalent. In a study with 30.522 individuals between 40 and 64 years with ID, low-trauma fractures were seen three times more often than in a non-ID population, with higher age as one of the risk factors [29]. Berkvens et al. followed 136 patients aged 18 to 79 years with ID and epilepsy and showed that 50% had osteopenia and 26.5% osteoporosis assessed with DEXA. During seven years of follow-up, 59% had at least one fracture, of whom 35% had one or more major osteoporotic fractures [30]. Considering our finding of low BHI-SDS in children with AS, we propose the monitoring bone health in adults with AS, performing bone health assessment at least when presenting with a fracture, and starting treatment when osteoporosis is diagnosed.

DEXA is considered the gold standard for bone health assessment [23]. A recent systematic review on measures of bone quality concluded that DEXA and DXR showed the strongest significant correlation [31]. DXR provides information on cortical thickness and metacarpal length and width, representing volumetric BMD, while DEXA is unable to measure bone depth, representing areal BMD. Although future studies are warranted, DXR can be a promising tool to better predict fracture risk [31]. DEXA does not provide information about bone age. DXR is a rapid and easy and therefore more suitable for children with ID and a low-cost measure of bone health.

It is unclear whether the lower BHI-SDS in children with AS has an AS specific origin. Many factors already known to be associated with low bone health are present in AS, such as immobility and ASM use, suggesting that secondary factors likely play a role. But that does not explain why children with a deletion are more severely affected, even after adjustment for mobility and ASM use. Furthermore, it is remarkable that BHI-SDS is already lower than normal at a young age in all children with ASA role for UBE3A gene dysfunction with effect on nuclear hormone receptor function and possible effect on vitamin D and hormone function related to bone turnover has been suggested [10]. A recent mouse model study confirmed that UBE3A protein is involved in the nuclear hormone receptor function and cholesterol synthesis [32]. Another mouse model study reports on NIPA2 positively regulating osteogenic capacity of osteoblasts [33]. NIPA2 is one of the other genes besides UBE3A deleted in AS of the deletion type [5, 34]. UBE3A dysfunction in bone is not likely to be a causative factor, as the UBE3A gene is only imprinted in neurons. Also, we did not see a difference in bone health between children with AS due to UPD/ICD and a pathological variant of the UBE3A gene, and normal expression levels are expected in bone in UPD/ICD children compared to 50% expression in the pathological variant group. Further research is needed to unravel a possible primary AS-specific mechanism for low bone health.

The results of this study combined with previous findings of osteoporosis in adults with ID and epilepsy, which confirm the need for proper guidance and treatment in children and adults with AS. As we are the first to study bone health in a large cohort of children with AS, our data can be used to protocol assessment of bone health in follow-up. We advise to measure bone health at the age of 7, 13–14, and 17 years. At the age of 7, most children showed whether they can walk and/or have manifested epilepsy (and use of ASM). At the age of 13 (girls) and 14 (boys), pubertal development and bone health can be taken into account in the decision to induce puberty, if appropriate. At the age of 17, puberty has completed and assessment provides information for follow-up in adulthood. Assessment of bone health should be considered at any time when a child develops a fracture, especially after non-significant trauma.

Current advice for people at risk for osteoporosis are sufficient sun exposure, suppletion of vitamin D, and optimal nutritional status [8, 23, 35, 36]. Choice of ASM can be considered in the light of their possible negative effects on bone health. Sufficient exercise; maintenance of walking; if needed with support, use of standing table in non-ambulant children; and treatment of late puberty were already discussed. Lastly, when children with AS show unexplained discomfort, even without clear trauma, always consider a fracture. When osteoporosis is officially diagnosed [37], bisphosphonate treatment may be considered [37, 38].

We showed that children with AS display low bone health, significantly decreasing with age. Deletion genotype, immobility, and late onset of puberty were significant risk factors. Future studies should focus on follow-up of bone health and fracture prevalence in adulthood, intervention measurements to stimulate bone mass deposition and prevent loss of bone mass with age, and better understanding of underlying molecular-genetic mechanisms. Monitoring and guidance of bone health should become a regular part of clinical follow-up in AS.