SED (spondyloepiphyseal dysplasia) is a group of genetic disorders affecting the vertebrae and the ends of the long bones. There are several types of SED, in different severity levels – some are very severe and some are relatively mild.
The main characteristics of all SED disorders are:
- Flattened vertebrae / vertebrae abnormalities
- Delay in the long bone development (in severe cases breathing problems are caused by abnormal chest development)
- Vision impairments occasionally accompanied by a cleft palate
What are the symptoms of SED?
People with SED have short stature from birth, but their limbs are usually of average length. Adult maximal height ranges from 90 centimeters to 140 centimeters. In some cases, bone growth delay appears at around two years of age, certain patients suffer from breathing problems resulting from this delay.
In addition, there may be deformities of the vertebrae in the neck. The disorder may also result in skeletal abnormalities that can cause arthritis and decreased joint mobility in early stages of life. A quarter of people with SED suffer from mild to moderate hearing loss and in many cases show distinct facial features.
What causes SED?
Skeletal disorders under the SED spectrum are caused by a mutation in a gene called COL2A1. This gene codes the instructions for production of the protein collagen type 2, that is essential to bone and cartilage development. In addition, other genes take part in collagen type 2 production, which makes the genetic test that diagnoses this disorder very complex. For this reason, there is currently no regular test that can detect the different disorders in the different genes.
However, the disorder can be diagnosed relatively easily by testing the levels of collagen type 2 via amniocentesis or chorionic villus sampling (CVS).
How is the disorder inherited?
SED disorder can be inherited through autosomal recessive inheritance or autosomal dominant inheritance, but it is usually passed down through autosomal dominant inheritance. This means that in most cases, passing on a mutated gene from one parent is enough for the child to suffer from SED.