Overview

Acromesomelic dysplasia (AMD) is a rare genetic condition that impacts bone development, with an incidence rate reportedly less than 1 in a million in some literature. Unlike other forms of dwarfism, which affect the whole body, AMD results in shorter middle and/or distal sections of the arms and legs. As a result, individuals with AMD commonly have disproportionately short forearms, lower legs, hands, and feet, while their torsos and heads develop more typically.

The condition stems from mutations in certain genes crucial for normal bone development. These genetic alterations disrupt the signalling pathways responsible for bone elongation during early growth. Children with AMD generally exhibit signs of shortened limbs early in childhood. However, despite this physical condition, their overall health is generally unaffected.

Causes

AMD is a rare genetic condition caused by mutations in certain genes crucial for the development of a healthy skeletal system. These mutations interrupt signalling pathways that control how bones grow, especially in the limbs, leading to the characteristic shortening of the arms and legs.

There are five different types of AMD, each associated with mutations in different genes:

1. AMD-type Maroteaux is caused by mutations in the (Natriuretic Peptide Receptor 2) NPR2 gene, which produces a receptor that helps regulate bone growth. In healthy individuals, when the correct protein binds to the NPR-B receptor coded for by this gene, intracellular signalling is increased, which in turn increases chondrocyte proliferation. Chondrocytes give rise to cartilage, which can eventually ossify into bone. As such, NPR2 controls an increase in chondrocyte number, the maintenance of cartilage, and the differentiation of cartilage into bone. In AMD-type Maroteaux, this receptor doesn’t function properly or at all.. Therefore, chondrocyte activity isn’t encouraged in the growth plate of bones, and as a result, less cartilage is laid down, so there is less available to become bone, resulting in shorter limbs
2. Grebe-type AMD is linked to mutations in the GDF5 (Growth Differentiation Factor 5) gene. This gene acts on chondrocytes in the growth plates of the long bones (e.g., in the arms or legs), encouraging them to multiply and differentiate into cartilage. Once the cartilage template is formed, it also plays a role in maintaining the extracellular matrix (the space surrounding the cartilage), as well as supporting its transition to bone (ossification)
3. Hunter-Thompson type AMD is also associated with the GDF5 gene, but involves different mutations than the Grebe type. Consequently, it results in a different, less severe physical presentation of AMD due to the various roles GDF5 plays in the formation, maintenance, and further differentiation of cartilage
4. Dominantly inherited acromesomelic dysplasia (AMD-H) type Strudwick is connected to mutations in the NOG (Noggin) gene, which has multiple downstream effects. In healthy individuals, noggin binds to members of the bone morphogenetic protein (BMP) family of signalling molecules. BMP-2, 5, and 7 are involved in chondrocyte differentiation into cartilage and bone-forming cells (osteoblasts). Noggin plays an important role in controlling when cartilage and bone form, as well as ensuring that gaps in the limbs remain bone-free so joints can form to ensure a good range of motion
5. AMD with genital anomalies is related to mutations in the BMPR1 B (Bone Morphogenetic Protein Receptor Type 1B) gene, another gene that influences bone formation and development. This is a key receptor in the BMP superfamily, which is responsible for the development of bone and cartilage as well as sexual development, among others. Consequently, mutations in this gene lead to genital abnormalities

These genetic mutations interfere with signalling pathways that guide bone growth. In a healthy individual, these pathways send signals to cells in the developing limbs, instructing them on how to grow and differentiate into specific types of bone tissue. However, in individuals with AMD, the mutated genes lead to disruptions in these signals. As a result, the bones, particularly in the middle and distal parts of the limbs, do not grow as they should, leading to the shortening of the arms and legs.

AMD is an inherited condition, meaning it is passed down from parents to their children through genes. It is typically inherited in an autosomal recessive manner, meaning that a child must inherit two copies of the mutated gene, one from each parent, to develop the disorder. If both parents are carriers of the mutated gene, there is a 25% chance with each pregnancy that the child will have AMD. Only acromesomelic dysplasia (AMD-H) type Strudwick is inherited dominantly, meaning that even a single copy of the faulty gene is inherited from either parent, and then the AMD will exhibit symptoms. This dominance means that there is a 50% likelihood of any child inheriting this mutation.

The condition is not more common in either sex, and affects those of various ethnic backgrounds. The genetic mutations that cause AMD can occur in any population, and there is no evidence to suggest that any particular ethnic group is more prone to the disorder.

The effects of AMD begin to influence limb development early in pregnancy, during the critical stages when the limbs are forming.

• Limb Bud Development (Around 4 Weeks): Limb development starts around the 4th week of pregnancy with the appearance of limb buds. Although the genetic mutations responsible for AMD are present from conception, their physical impact on limb formation begins at this stage as disrupted signalling pathways start affecting normal bone growth
• Formation of Mid-Limbs and Hands/Feet (5 to 8 Weeks): From 5 to 8 weeks of pregnancy, the limb buds start to elongate and differentiate into the upper and lower parts of the limbs, including the hands and feet. During this period, the genetic mutations causing AMD begin to affect bone growth, resulting in the shortened limbs characteristic of the condition

The shortened limbs associated with AMD can typically be detected during prenatal ultrasounds in the second trimester, around 18 to 20 weeks, when detailed imaging can reveal limb length abnormalities.

All of the bones that are typically long and tubular appear shortened in this case of AMD-type Maroteaux. However, no joints are fused.

Symptoms and treatments

Limb-related abnormalities

• Shortened Limbs: Significant shortening of the limbs, especially in the middle (mesomelic) and end (acromelic) segments, is typical of AMD
• Bone Deformities: The bones in these shortened limbs may have abnormal shapes or may be underdeveloped, leading to visible skeletal deformities

Reduced Joint Flexibility: Stiff or restricted movement in joints can result, particularly in areas like the elbows, knees, wrists, and ankles.

Growth and physical development:

• Short Stature: Due to short limbs, individuals often have a notably short stature, with a body length that is disproportionately shorter compared to the torso
• Delayed Motor Development: Limb deformities and associated joint issues may lead to delays in physical milestones, such as learning to walk

Other skeletal characteristics

• Spinal Issues: Certain types of AMD might be associated with mild spinal curvature (scoliosis) or other vertebral anomalies
• Facial Appearance: Some forms of AMD might include distinct facial features, although this varies depending on the specific type

Additional Symptoms

• Genital Differences: In some forms of AMD, particularly those with associated genital anomalies, individuals may have underdeveloped or atypically formed genitalia
• Hand and Foot Abnormalities: In more severe cases, individuals may have extremely short or even missing fingers and toes, and sometimes fused digits (syndactyly)

These symptoms typically emerge early in life, often becoming evident during infancy or early childhood, and their severity can vary depending on the specific type of AMD.

Treatments include orthopaedic surgery to realign bones, lengthen bones or stabilise joints. Braces and physiotherapy can also help to improve range of motion and improve strength. Non-steroidal anti-inflammatory drugs (NSAIDS) like ibuprofen can also be taken to help manage joint pain.

Summary

AMD is a rare genetic condition that affects bone development, particularly shortening the middle and distal parts of the limbs while the torso and head develop normally. The condition is caused by mutations in specific genes, leading to disrupted bone growth signalling pathways. AMD can result in significantly shortened limbs, skeletal deformities, reduced joint mobility, and, in some types, genital abnormalities. The disorder is usually inherited in an autosomal recessive manner, although one type, AMD-H Strudwick, is inherited dominantly. Symptoms typically appear early in life, and prenatal ultrasounds may detect limb abnormalities during the second trimester.