Overview of achondrogenesis

Achondrogenesis is a group of rare genetic skeletal disorders that severely affect bone and cartilage development in unborn babies. It is characterised by dwarfism, shortened limbs in relation to the trunk, abnormal rib development, and anomalies in the vertebrae and other bones. Babies with achondrogenesis also have a higher chance of life-threatening health issues, which can often result in stillbirth or death shortly after birth. 

Achondrogenesis type IA and type IB are autosomal recessive conditions which can only be inherited when both parents carry a specific gene. In contrast, achondrogenesis type II is an autosomal dominant genetic condition which can be inherited from a single parent carrying a specific gene. Doctors can often detect these serious bone development problems during ultrasound examination performed between the 14th and 17th weeks of pregnancy.¹

Achondrogenesis classification (type I and type II)

Achondrogenesis types I and II are rare genetic disorders that affect bone development and cause conditions like short limbs and a small chest. Two main types of achondrogenesis include:

Achondrogenesis type I 

This type of achondrogenesis is often detected before birth through ultrasounds, and is characterised by severe skeletal abnormalities. Babies born with this form of achondrogenesis usually have a very short trunk and limbs, a small chest with short ribs, and a prominent forehead. Type I achondrogenesis can lead to significant skeletal deformities due to poorly ossified (formed) bones in the spine and limbs. Sadly, infants with this type of achondrogenesis are usually stillborn or die shortly after birth. 

Type I achondrogenesis is subdivided into type IA and type IB.

Achondrogenesis type II

This type of achondrogenesis leads to skeletal abnormalities that are significant, but less severe than those associated with Type 1 achondrogenesis. Babies with type II achondrogenesis have underdeveloped bones and short ribs; this can affect the spine and the limbs. They also usually have small chests and short limbs, but they do not possess the distinctive facial features seen in type I. Unlike type I, infants with type II may survive for a short period after birth. 

Both type I and type II achondrogenesis are inherited in an autosomal recessive manner, meaning that parents must carry a specific gene to pass the disorder to their child. These specific genes are involved in the formation of cartilage and bone during foetal development. 

Genetic basis of achondrogenesis

Autosomal recessive inheritance 

Everyone has 23 pairs of chromosomes (equalling 46 total), with 22 pairs being autosomal (i.e. that determine traits other than sex) and the 23rd being the sex chromosomes (X and Y). Recessive inheritance typically refers to genes on the first 44 chromosomes. Autosomal recessive conditions only manifest when an individual has two copies of a disease-causing gene. For this to happen, they must inherit one ‘faulty’ or disease-causing gene copy from each of their parents. As such, if both parents carry gene variants associated with achondrogenesis, but don’t show symptoms of disease themselves, any offspring they have will have a 1 in 4 (25%) chance of being born with the disease.²

In contrast, just one copy of a disease-causing gene variant is needed for an individual to manifest disease. In these cases, only one parent needs to pass on a disease-causing gene to their child. 

Gene mutations

Achondrogenesis type I and type II are skeletal disorders which are caused by mutations in different genes.

Achondrogenesis type IA is caused by mutations in the GMAP-210 gene. Normally, GMAP-210 encodes proteins that control the function of the Golgi apparatus (organelles that process and package proteins before they are used in the body). Mutations in this gene cause faulty or dysfunctional GMAP-210 proteins to be produced. In turn, the Golgi apparatus becomes dysfunctional, resulting in cartilage defects and abnormal bone development.⁵

Achondrogenesis type IB (also known as Fraccaro-type achondrogenesis) is caused by mutations in the SLC26A2 gene. These mutations disrupt cartilage development, resulting in improper bone formation. Babies with this type of achondrogenesis generally have short fingers, short trunk and limbs, oddly shaped toes, and a large abdomen.¹ 

Achondrogenesis type 2, the most severe form of the disorder, is caused by mutations in the COL2A1 gene, which is involved in collagen production. Collagen is the most abundant protein in our bodies, and makes up a large part of our bones, muscles, and connective tissue. This type of achondrogenesis leads to significant skeletal abnormalities that often lead to death or significant impairments and damage. Achondrogenesis type II can be detected on an X-ray through the presence of short long bones, improper development of the pelvis and spine, short ribs, and poor mineralisation of certain bones like the iliac and skull bones.¹ 

Embryological development

Normal cartilage development (chondrogenesis)

Chondrogenesis is the process through which cartilage forms in the body. It begins when mesenchymal stem cells converge on sites where cartilage is developing, following chemical and molecular signals. These stem cells then turn into chondrocytes (cartilage-producing cells), which create a special material called extracellular matrix (ECM). This matrix, which is composed mainly of collagen (type II), proteoglycans, and glycoproteins, gives cartilage its structure and helps it grow. As foetal development progresses, cartilage in certain areas may be replaced by bone through a process called endochondral ossification. The balance between the production and degradation of the ECM is critical for healthy cartilage and bone production. Growth factors, cytokines, and mechanical signals all play a role in regulating this balance. As seen in achondrogenesis, problems in this process can lead to various disorders affecting cartilage and other parts of the skeleton.³

Role of chondrocytes and extracellular matrix (ECM) in skeletal development

Cartilage is composed of cells known as chondrocytes. By maintaining the extracellular matrix (ECM), chondrocytes produce a cartilage matrix.⁴ Chondrocytes arrange themselves to form growth plates at the ends of bones during bone development. Depending on the stage of bone development, different groups of chondrocytes within the growth plates produce various ECM components and signalling molecules. The extracellular matrix (ECM) fills the space between cells in the body, provides lubrication, absorbs shock and handles stress during joint movement.

Pathological features of achondrogenesis

Defective endochondral ossification 

Endochondral bone ossification and differentiation are affected in achondrogenesis type IA (ACG1A), a rare and lethal skeletal dysplasia. Type IA is also known as the Houston-Harris subtype. It's characterised by defective endochondral and intramembranous ossification, which results in growth restriction, short limbs and a narrow thorax.⁶

Abnormal bone and cartilage structure

The structural defects associated with achondrogenesis are the result of mutations affecting genes crucial for cartilage and bone formation. Cartilage is improperly formed, which is essential for normal bone development, resulting in severely misshapen and underdeveloped bones. It can also lead to short stature, disproportionate limbs and increased susceptibility to bone fractures.⁷ 

Abnormal skeletal growth

Achondrogenesis severely impacts skeletal growth and can cause issues such as short limbs, a small chest, and a relatively large abdomen. This is caused by mutations in genes involved in cartilage development, which is crucial for bone development. Infants typically exhibit severe growth restriction and often face a high risk of stillbirth or early death due to respiratory issues or other complications. 

Diagnosis of achondrogenesis

Achondrogenesis is a severe congenital disorder that can be diagnosed by a doctor using a combination of tests and examinations:

Physical features

Newborns with achondrogenesis show markedly shortened limbs and other skeletal abnormalities, which are generally noticeable at birth or can be observed in early prenatal ultrasound imaging.⁸ 

X-ray imaging

X-ray findings are critical for identifying abnormal bone development characteristics of achondrogenesis, which reveals skeletal dysplasia, including shortened long bones and underdeveloped vertebrae. 

Histological examination

Abnormalities in cartilage formation and bone development can be revealed by microscopic examination of tissue samples, which is often used to confirm the diagnosis after other methods suggest achondrogenesis.⁸ 

Prenatal ultrasound

Detailed ultrasounds conducted around weeks 12-14 of pregnancy can identify early skeletal anomalies and growth deficiencies like short limbs, skeletal underdevelopment and other signs of achondrogenesis. 

Genetic tests

Molecular genetic tests can be used to identify mutations in the SLC26A2 gene and confirm a diagnosis of achondrogenesis type IB. Prenatal diagnosis may also be possible by amniocentesis (performed at 15–18 weeks) or chorionic villus sampling (performed at 10–12 weeks) if a family member has been identified with specific gene mutations.⁸ 

Management and treatment 

Achondrogenesis is a serious genetic disorder that causes major skeletal issues, which has no cure. Treatment focuses on managing symptoms and providing support to the family.⁹

Supportive care

• Due to their small chest, infants may need assistance with breathing 
• Infants with achondrogenesis face feeding difficulties, so special care may be needed to ensure they receive proper nutrition

Orthopedic care

To address bone problems and improve limb function, braces or surgery might be used 

Pain management

To manage pain caused by skeletal issues, pain-alleviating medicines may be given 

Genetic counseling

Genetic counseling helps families understand the condition to discuss family planning options, and evaluate risks for future pregnancies. 

Multidisciplinary care

Specialists from various areas - such as geneticists and orthopaedic surgeons - will work together to provide the best care for children with achondrogenesis. 

Prenatal diagnosis

During pregnancy, ultrasound and genetic tests can help doctors plan the baby's care. 

Emotional support

• Counseling provides emotional support to families dealing with the challenges of this severe condition
• As the condition is very severe and usually has a limited prognosis, the overall goal is typically to improve the child’s quality of life and provide comfort to the parents

Summary

Achondrogenesis is a severe genetic skeletal disorder characterised by significant abnormalities in bone and cartilage development. It can lead to disproportionately short limbs, a small chest, and other skeletal deformities. The disorder is classified into two main types: type I, which often results in stillbirth or early death due to severe skeletal abnormalities, and type II, which, though less severe, still presents with substantial skeletal issues and a shorter survival period. Both types are inherited in an autosomal recessive manner, involving mutations in genes critical for cartilage and bone formation. Diagnosis is typically made through physical examination, imaging studies, and genetic testing. Treatment focuses on managing symptoms and providing supportive care, as there is currently no cure. Multidisciplinary care and genetic counselling play crucial roles in managing the condition and supporting affected families.