Introduction 

Acrodysostosis describes a group of rare genetic disorders that influence the bone development of affected individuals.¹ Mutations in the type 1 regulatory subunit of cAMP-dependent protein kinase alpha (PRKAR1A) or the cAMP-specific phosphodiesterase 4D (PDE4D) may lead to the development of acrodysostosis type 1 and acrodysostosis type 2, respectively.² PRKAR1A and PDE4D are essential components of the GPCR-Gsα-cAMP-protein kinase A (PKA) signalling pathway.³ Activation of the PKA signalling pathway leads to cAMP production, which has important roles in memory, gene regulation and immune function.⁴ As such, a dysregulation in cAMP levels could be the underlying biochemical basis of acrodysostosis.⁵ The inheritance of acrodysostosis may occur in an autosomal manner, but most cases of this rare disease are sporadic, and differing phenotypic features may be observed in patients possessing identical mutations.² This article aims to educate the readers on acrodysostosis, with emphasis on the pathophysiology and skeletal abnormalities. Through a better understanding of acrodysostosis, this article intends to support healthcare professionals in diagnosing and managing this rare condition and foster further research into effective treatments. 

Pathophysiology

Acrodysostosis arises from a mutation in the PRKAR1A gene or the PDE4D gene. A majority of the gene mutations leading to acrodysostosis are sporadic or de novo mutations,¹⁰ but documentation of autosomal dominant inheritance is observed in some cases of acrodysostosis type 2.² The PRKAR1A gene is responsible for coding one of the regulatory subunits of PKA (type 1- alpha), and the protein phosphorylates specific enzymes.¹² PDE4D hydrolyses phosphodiester bonds, such as the phosphodiester bond in cAMP.¹³ 

Mutations in the PRKAR1A and PDE4D genes may lead to disruptions in the cAMP/PKA pathway, which regulates bone growth and development. The binding of cAMP to PKA leads to the activation of PKA, which goes on to phosphorylate other proteins and modulate their activity. Some mutations in the PRKAR1A gene alter the functionality of the regulatory subunits or inhibit the attachment of cAMP.⁶ Therefore, this results in changes to PKA activation; the dysregulation in PKA activity has severe consequences for chondrocyte differentiation and endochondral bone formation. This is observed in acrodysostosis patients, many of which possess the skeletal manifestation of abnormal bone formation and growth retardation.¹⁴ Defects in the PDE4D gene disrupt the ability of the phosphodiesterase to cleave cAMP, thus elevating cAMP levels and leading to prolonged activation of PKA. The dysregulation of the cAMP/PKA pathway has harsh implications on other components of the signalling pathways, such as the parathyroid hormone (PTH) and PTH-related protein (PTHrp).² PTH is an example of a GPCR signalling hormone that regulates calcium and phosphate homeostasis by indirectly elevating cAMP levels.¹⁵ PTH also has a role in directly stimulating bone development via osteoblasts,¹¹ and attenuation in PTHrp signalling may lead to problems in endochondral ossification.²   

Overview of skeletal system involvement in acrodysostosis

As mentioned previously, the two types of acrodysostosis arise from differences in the targeted gene. Type 1 acrodysostosis is frequently associated with resistance to multiple G-protein coupled receptor signalling hormones, whereas type 2 acrodysostosis affected individuals typically do not exhibit hormone resistance and endocrine issues.⁵ On the contrary, both acrodysostosis subtypes display similar skeletal abnormalities:

• Short stature
• Peripheral dysostosis
  • Enlarged big toe³  
  • Reduced length of metacarpals (bones of the hand)  and metatarsals (bones of the feet)
  • Brachydactyly⁶
• Scoliosis⁷
  • Abnormal curvature of the spine sideways
• Kyphosis and spinal stenosis⁷
  • Kyphosis is the extreme rounding of the upper back⁸ 
  • Spinal stenosis is the tightening of the spinal cord in the lower back⁹ 
• Macrocephaly
• Misaligned teeth¹⁰ 
• Advanced bone age²
• Facial dysostosis
  • Broad face 
  • Orbital hypertelorism
  • Maxillonasal hypoplasia (underdevelopment of the upper jaw and the nasal bone)³ 

Peripheral dysostosis and short stature are often congenital signs of acrodysostosis, but the severity of each skeletal manifestation varies on an individual basis. Since mutations are mostly inherited sporadically, this makes it difficult to suspect genetic origins early on and may potentially delay the diagnostic process.¹⁰ However, on the rare occasion of multigenerational familial inheritance of acrodysostosis, clinical manifestations may also differ between family members affected by this disease. There were noteworthy phenotypic differences within one family possessing identical mutations, such as the absence of short stature in one individual, whereas the other family member was affected.² Therefore, significant phenotypic variability should be considered in mind for clinical diagnosis, administration of medical treatment and future research into the subtypes of acrodysostosis.  

Management and treatment

Treatment of acrodysostosis is managed by a range of multidisciplinary specialists, so as to target specific clinical features observed on a case-by-case basis. Although there are no standardised treatment protocols for this rare disease, below are some suggested management and treatment options for affected individuals:

• Corrective surgeries
  • Acrodysostosis patients with severe skeletal abnormalities may undergo surgical procedures.
  • Surgeries may be performed to correct maxillo-nasal hypoplasia in some cases¹⁰      
• Bracing
  • The improvement of specialised braces to treat spine deformities, such as kyphosis, provides a non-surgical option for patients with acrodysostosis. However, individuals with severe kyphosis (possessing a spinal curve greater than 70 degrees) are recommended for surgery¹⁶   
• Dental braces
  • Misaligned teeth can be corrected with dental braces.
• Physical therapy
  • Patients experiencing weakened mobility and muscle strength may benefit from partaking in tailored physical therapy programs¹⁰  
• Hormone therapy
  • Hormone replacement therapy may be administered to patients who display signs of hormone resistance and help manage conditions, such as hyperparathyroidism and pseudohypoparathyroidism¹⁷     
• Medications
  • Patients who experience pain in their lower back, due to spinal deformities, and legs¹⁰ may be prescribed pain relief medication to ease their discomfort
• Bone density monitoring
  • Since advanced bone age is a common clinical feature of acrodysostosis-affected individuals, periodic monitoring of bone health may give insight into the effectiveness of treatments

There is ongoing research and potential future therapies that aim to increase the quality of life for affected individuals. Since the root of the disease lies with defective PRKAR1A or PDE4D genes, it would be wise to develop therapies that correct specific mutations. Gene therapy techniques are currently unavailable towards the treatment of acrodysostosis in humans but remain a promising approach; gene therapy conducted on a mouse model of acrodysostosis resulted in correction of the mutant PRKAR1A gene, as well as the observation of enhanced skeletal growth.¹⁸

FAQs

What diagnostic tests are used to confirm acrodysostosis?

Initial diagnosis of acrodysostosis may be suspected by physical examination through the identification of classic clinical features, such as brachydactyly. Although physical examination alone may not be sufficient for diagnosis, as other genetic disorders display similar phenotypic features, such as Albright Hereditary Osteodystrophy (AHO). ³ Medical professionals may also seek the health history of additional family members to check for hereditary patterns, although acrodysostosis most often occurs sporadically. Radiographic imaging, such as X-rays and skull radiographs, is used to identify specific skeletal abnormalities. Although again, these clinical features may overlap with other phenotypically related diseases. Molecular genetic testing seeks out mutations in PRKAR1A or PDE4D genes, which are known to cause acrodysostosis types 1 and 2, respectively. However, genetic testing is only made available at specialised laboratories.¹⁰ 

Summary 

Acrodysostosis is a rare genetic disorder characterised by distinct skeletal abnormalities. It is primarily caused by mutations in the PRKAR1A and PDE4D genes, which disrupt the cAMP/PKA signalling pathway essential for bone growth and development. Patients typically present with short stature, brachydactyly, facial dysmorphisms, and occasionally, spinal deformities like kyphosis and scoliosis. Although these skeletal abnormalities are not limited to acrodysostosis.

Therefore, diagnosis involves a thorough clinical evaluation, radiographic imaging, and genetic testing to confirm specific mutations. The management and treatment of acrodysostosis requires a combination of medical, therapeutic, and supportive interventions, which aim to alleviate symptoms, improve function, and support the overall well-being of individuals with acrodysostosis. Although not yet available, advancements in gene therapy hold potential for future treatment options targeting the underlying genetic causes of acrodysostosis, specifically the PRKAR1A and PDE4D genes.