Recent milestones in achondroplasia research

Neurological Manifestations of Achondroplasia

PURPOSE OF REVIEW

This review is to delineate the neurological complications seen in patients with achondroplasia.

RECENT FINDINGS

As the understanding of the genetics of this disorder has advanced, the possibility of targets for intervention which might modify the development and management of the neurological complications of this disease may be identified. Achondroplasia is a hereditary short-limbed dwarfism which has been known for millennia. The genetic defect is a gain of function sequence variation in the fibroblast growth factor receptor 3 (FGFR3). This gene normally regulates (inhibits) bone growth thus the gain of function results in abnormal or excessive inhibition of growth. The resulting bone is subject to distortion and the result is that bone impinges on nervous tissue, most commonly at the foramen magnum, spinal canal, and nerve root outlet foramen. Awareness of the range of these complications will, hopefully, allow early and more effective intervention so as to ameliorate the nature and severity of the long-term effects of the neurological complications in patients with achondroplasia.

Whole-body and segmental analysis of body composition in adult males with achondroplasia using dual X-ray absorptiometry

Achondroplasia is a condition characterized by a genetic mutation affecting long bone endplate development. Current data suggests that the bone mineral content (BMC) and bone mineral density (BMD) of achondroplasic populations are below age matched individuals of average stature (controls). Due to the disproportionate limb-to-torso length compared to controls however, the lower BMC and BMD may be nullified when appropriately presented. The aim of this study was to measure whole-body and segmental body composition in adult males with achondroplasia (N = 10, 22 ±3 yrs), present data relative to whole-body and whole-limb values and compare all values to age matched controls (N = 17, 22 ±2 yrs). Dual X-ray absorptiometry (DEXA) was used to measure the in vivo mass of the whole-body and 15 segments, from which BMD, BMC, fat free mass (FFM) and body fat mass were measured. BMC of lumbar vertebrae (L1-4) was also measured and presented as a volumetric BMD (BMDVOL). The achondroplasic group had less BMC, BMD and FFM, and more body fat mass than controls as a whole-body measure. The lower achondroplasic BMC and BMD was somewhat nullified when presented relative to whole-body and whole-limb values respectively. There was no difference in lumbar BMDVOL between groups. Whole-body BMD measures presented the achondroplasic group as 'osteopenic'. When relative to whole-limb measures however, achondroplasic BMD descriptions were normal. Further work is needed to create a body composition database for achondroplasic population's, or for clinicians to present achondroplasic body composition values relative to the whole-limb.

Intraoperative computed tomography for cervicomedullary decompression of foramen magnum stenosis in achondroplasia: two case reports

The authors report two cases of cervicomedullary decompression of foramen magnum (FM) stenosis in children with achondroplasia using intraoperative computed tomography (iCT). A 14-month-old girl with myelopathy and retarded motor development, and a 10-year-old girl who had already undergone incomplete FM decompression was presented with myelopathy. Both patients underwent decompressive sub-occipitalcraniectomy and C1 laminectomy without duraplasty using iCT. It clearly showed the extent of FM decompression during surgery, which finally enabled sufficient decompression. After the operation, their myelopathy improved. We think that iCT can provide useful information and guidance for sufficient decompression for FM stenosis in children with achondroplasia.

Advances in research on and diagnosis and treatment of achondroplasia in China

Achondroplasia is a rare autosomal dominant genetic disease. Research on achondroplasia in China, however, has received little emphasis. Around 80-90% of cases of neonatal achondroplasia result from mutations in fibroblast growth factor receptor 3 (FGFR3) according to polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP). Recently, genetic research on achondroplasia in China made a major breakthrough by revealing two novel mutations located on the FGFR3 gene, thus helping to complete the pathological molecular map of achondroplasia. There are still, however, unknown aspects of the diagnosis and treatment of achondroplasia. This review will summarize advances in research on and the clinical diagnosis and treatment of achondroplasia in China.

Lower limb lengthening in patients with disproportionate short stature with achondroplasia: a systematic review of the last 20 years

Parents of children suffering from disproportionate short stature due to achondroplasia may wish to have surgical leg lengthening carried out for the child. The aim is not to increase height, but rather to achieve physiological proportions in the body. In a systematic review of the literature on the topic dating from the last 20 years, the surgical approaches used for this purpose were analyzed in accordance with the Preferred reporting items for systematic reviews and meta-analyses (PRISMA) criteria. Twelve studies show that to date, involvement of the child in decision-making at the start of treatment has been expected and that it is recommended from the age of 12. In highly heterogeneous patient groups, with varying factors involved and different techniques being used, lengthening (often by more than 10 cm) is described. High complication rates are reported, with many setbacks often requiring repeat surgery. Using PALEY'S multiplier method, the expected standing height, sitting height, and leg length can be predicted and an individualized treatment approach can be planned and operative procedures could be started in early childhood as PERETTI suggests. As the patients are unable to be involved in decision-making as young children, these data may provide a basis for offering differentiated advice to parents, who usually consult a pediatric orthopedist at a very early stage in the child's life.

Hand and foot abnormalities associated with genetic diseases

INTRODUCTION

The small bones and soft tissues of the hands and feet can be affected by systemic disorders, and frequently, the findings are quite unique and virtually diagnostic for some genetic or metabolic disorders.

MATERIALS AND METHODS

Photographs and imaging studies for the hands and feet are available in a digitized system, which has been approved by our hospital institutional review board. Examination of these and their description can establish a relationship with some degree of certainty to a series of highly variable and uncommon clinical disorders.

RESULTS

Description of the clinical, physiologic and genetic characteristics, and illustrations of hand and foot abnormalities are provided for an array of diseases, including Ellis-van Creveld syndrome, fibrodysplasia ossificans progressiva, achondroplasia, Kniest dysplasia, pseudo- and pseudo-pseudohypoparathyroidism, acromegaly, nail-patella syndrome, Marfan's disease, cartilage-hair hypoplasia, and several forms of mucopolysaccharidosis.

CONCLUSIONS

The findings support the concept that many genetic disorders can often be diagnosed by clinical and imaging examination of the patient's hands and feet.

Achondroplasia: manifestations and treatment

Achondroplasia, the most common skeletal dysplasia, is caused by a mutation of fibroblast growth factor receptor-3. This disorder is characterized by frontal bossing, midface hypoplasia, otolaryngeal system dysfunction, and rhizomelic short stature. Orthopaedic manifestations are exhibited in the spine and the extremities. In the infant with achondroplasia, foramen magnum stenosis may result in brainstem compression with apnea and sudden death. Thoracolumbar kyphosis is seen in most infants, but typically it resolves when the child begins to walk. Anatomic anomalies of the vertebral column place the patient at risk for spinal stenosis as early as the first decade and especially during adulthood. Radial head dislocation is one manifestation in the upper extremity. Lower extremity alignment often is characterized by genu varum, which may require correction osteotomy. Medical and surgical options are available to increase patient height, but indications are controversial, and treatment often consumes a large portion of the child's life.

Acute neurological deficit after minor trauma in an infant with achondroplasia and cervicomedullary compression. Case report and review of the literature

Cervicomedullary compression at the foramen magnum in patients with achondroplasia can be associated with apnea, neurological deficits, and sudden death. Decompressive operations are often performed in symptomatic patients. In asymptomatic patients, the indications for prophylactic decompression are controversial. The authors present the case of a previously neurologically intact 4-month-old girl with achondroplasia who presented with severe hemiparesis after a low-velocity motor vehicle accident. Imaging studies demonstrated osseous compression of the medulla and upper cervical spinal cord with associated parenchymal signal changes. To the authors' knowledge this is the first reported case of a new neurological deficit after a minor trauma in this patient population. The authors review the relevant literature, focusing on the indications for cervicomedullary decompression in infants with achondroplasia. They propose that asymptomatic patients with achondroplasia and osseous compression at the foramen magnum should be offered prophy-lactic surgery if T2-weighted magnetic resonance imaging signal changes in the spinal cord are observed. Prophylactic surgery can be considered an option in patients whose imaging studies do not show signal changes in the spinal cord but demonstrate significant osseous compression and absence of visible subarachnoid spaces.

Snail1 is a transcriptional effector of FGFR3 signaling during chondrogenesis and achondroplasias

Achondroplasias are the most common genetic forms of dwarfism in humans. They are associated with activating mutations in FGFR3, which signal through the Stat and MAPK pathways in a ligand-independent manner to impair chondrocyte proliferation and differentiation. Snail1 has been implicated in chondrocyte differentiation as it represses Collagen II and aggrecan transcription in vitro. Here we demonstrate that Snail1 overexpression in the developing bone leads to achondroplasia in mice. Snail1 acts downstream of FGFR3 signaling in chondrocytes, regulating both Stat and MAPK pathways. Moreover, FGFR3 requires Snail1 during bone development and disease as the inhibition of Snail1 abolishes its signaling even through achondroplastic- and thanatophoric-activating FGFR3 forms. Significantly, Snail1 is aberrantly upregulated in thanatophoric versus normal cartilages from stillborns. Thus, Snail activity may likely be considered a target for achondroplasia therapies.

Achondroplasia

Achondroplasia is the most common form of short limb dwarfism in human beings, affecting more than 250,000 individuals worldwide. More than 95% of patients have the same point mutation in the gene for fibroblast growth factor receptor 3 (FGFR3) and more than 80% of these are new mutations. The mutation, which causes gain of FGFR3 function, affects many tissues, most strikingly the cartilaginous growth plate in the growing skeleton, leading to a variety of manifestations and complications. The biology of FGFR3 and the molecular and cellular consequences of the achondroplasia mutation are being elucidated, providing a more complete understanding of the disorder and a basis for future treatments targeted directly at relevant pathogenetic pathways. Furthermore, the natural history of the condition, which has been well delineated in childhood and adolescence, is being defined more fully in adults with achondroplasia; most of the serious complications can be modified favourably or prevented by anticipation and early treatment. Possible future treatments include chemical inhibition of receptor signalling, antibody blockade of receptor activation, and alteration of pathways that modulate the downstream propagation of FGFR3 signals.

Clinical hypochondroplasia in a family caused by a heterozygous double mutation in FGFR3 encoding GLY380LYS

In classical achondroplasia (Ach), a glycine residue is replaced by an arginine at codon 380 in exon 10 of the fibroblast growth factor receptor 3 gene (FGFR3). Here we report on a mother and daughter with hypochondroplasia (Hch) caused by a new heterozygous double mutation (1138_1139GG > AA) at the same codon 380, but encoding a lysine instead of the usual arginine. Previous functional assays of these codon 380 amino acid substitutions demonstrated a lesser activation of receptor signaling by lysine compared to arginine [Webster and Donoghue, 1996; EMBO J 15:520-527]. This could explain the milder phenotype observed in our patients. Several other rare double mutations were previously described in both FGFR2 and FGFR3 and interpreted as resulting from positive selection of spermatogonial cells owing to gain-of-function in the encoded protein [Goriely et al., 2005; Proc Natl Acad Sci USA 102:6051-6056]. The present case contributes additional support for this hypothesis.

Advances in understanding etiology of achondroplasia and review of management

PURPOSE OF REVIEW

A summary of management and current research in achondroplasia (OMIM 100800). The most common nonlethal skeletal dysplasia, achondroplasia presents a distinct clinical picture evident at birth. Substantial information is available concerning the natural history of this dwarfing disorder. Diagnosis is made by clinical findings and radiographic features. Characteristic features include short limbs, a relatively large head with frontal bossing and midface hypoplasia, trident hands, muscular hypotonia, and thoracolumbar kyphosis. Children commonly have recurrent ear infections, delayed motor milestones, and eventually develop bowed legs and lumbar lordosis. People with achondroplasia are generally of normal intelligence.

RECENT FINDINGS

The genetic cause of achondroplasia was discovered in 1994. Subsequent research efforts are designed to better characterize the underlying possible biochemical mechanisms responsible for the clinical findings of achondroplasia as well as to develop possible new therapies and/or improve intervention.

SUMMARY

Establishing a diagnosis of achondroplasia allows families and clinicians to provide anticipatory care for affected children. Although the primary features of achondroplasia affect the skeleton, a multidisciplinary approach to care for children with achondroplasia helps families and clinicians understand the clinical findings and the natural history of achondroplasia in order to improve the outcome for each patient.

Bone biology and the pathogenesis of osteoporosis

Much is now known about skeletal biology and the changes that take place during diseases. Skeletal development is programmed by the sequential activation of specific genetic pathways that culminate in the production of the adult skeleton, which is light but strong. Systemic hormones including parathyroid hormone, vitamin D metabolites, and calcitonin regulate blood calcium levels and contribute to the overall calcium economy of the body. Many other hormones have subtle but important effects on skeletal behaviour and its modelling and remodelling activity. At a local level, the integration of cellular differentiation and function within the microenvironment of bone is under the influence of a large number of cytokines and growth factors. Osteoporosis is a very common disorder and is a result of perturbation in these regulatory mechanisms. Much has been learnt in recent years about the many pathogenic processes that contribute to bone loss and fragility. Several drug treatments are now available to prevent bone loss and reduce the incidence of fractures, and there are prospects for the development of further novel pharmacological interventions that may modify some of the pathogenic processes themselves. Among the newer pathways for pharmacological intervention, the calcium-sensing receptor and the receptor activator of nuclear factor kappa B ligand/receptor activator of nuclear factor kappa B/osteoprotegerin system involved in osteoblast-osteoclast interactions offer exciting opportunities.