Cleidocranial Dysplasia is an autosomal recessive trait based genetic disorder caused by mutation of RUNX2 gene present on chromosome 6. It is characterized by a malformation of the skull bones and absent or underdeveloped clavicles or collarbones.
This mutation is usually inherited by the patient from one or both the parents; however it may also result from spontaneous mutation. There is no way to avoid spontaneous mutations. If inherited genetically, the patient with a mild form of the disorder often has only one parent with the mutated gene. In case of spontaneous mutation, the parents do not have the condition but the child develops it as a new mutation. In one-third of the diagnosed cases, there is no genetic mutation in the RUNX2 gene and the cause of the disorder is unknown. There have been only about a thousand recorded instances of people suffering from this disorder. The severity of symptoms and visible signs are also variable.
Pre-Birth Diagnosis of Cleidocranial Dysplasia
The disorder is usually diagnosed at birth, as the baby’s skeletal structure shows clear abnormalities. However, it is possible to diagnose the disorder even before birth using an ultrasound examination that may show abnormalities in the fetus.
However, an ultrasound is not the best way to determine a genetic mutation. Amniocentesis or Chorionic Villus Sampling is usually suggested to test fetus for this and any other genetic mutations if one of the parents suffer from Cleidocranial Dysplasia. These tests are usually performed in parents who have a higher than average risk of presence of genetic mutations in the fetus. Amniocentesis is conducted after the 15th week of pregnancy. Amniotic fluid is removed from the uterus sac within which the fetus is contained using a needle.
This procedure is invasive and could be risky. The fluid withdrawn will normally have the cells shed off by the fetus which are then checked by a pathologist for genetic mutations. Chorionic Villus Sampling is usually performed between the 11th and 14th week of the pregnancy. The DNA is collected from the cells of the placenta via the cervix, and there is a high risk of triggering termination of pregnancy when this test is performed. It is only recommended in high risk pregnancies, where the fetus is likely to have major genetic abnormalities.
Once the DNA sample has been obtained, the tests check for the presence of mutation in the RUNX2 gene. If a single parent has the genetic mutation, there is a high chance that the baby may not inheriting the disorder. However, if both parents are suffering from the condition, the baby will inherit the disorder.
As there is no known cure for the condition, some parents may opt for a medical termination of pregnancy should the fetus be diagnosed with Cleidocranial Dysplasia before birth. Others may choose to take the risk of having the child, hoping that the condition is mild. In such cases, genetic counselling for the parents is recommended.
Post-Birth dDiagnosis of Cleidocranial Dysplasia
The diagnosis of cleidocranial dysplasia begins with a clinical examination of the baby post birth. The most obvious symptoms include shoulders touching together in the front of the body and the presence of a wide pelvic bone and loose joints. The skeletal abnormalities are a direct result of the RUNX2 gene not regulating the production of a protein which directs the development of teeth, bones, and cartilage.
X-rays of the baby will show either partially formed or missing clavicles, under-developed shoulder blades, open area in front of the pelvic bone, and larger than normal spaces between the skull bones. All these skeletal abnormalities point to the presence of cleidocranial dysplasia. Based on the x-ray results, additional genetic tests may be required to establish the exact medical condition.
After preliminary detection of the condition, further genetic testing may be undertaken to confirm cleidocranial dysplasia. These tests determine the presence of mutation in the RUNX2 gene on the sixth chromosome. It is interesting to note that 30% patients with this condition do not have the genetic mutation, and the cause of cleidocranial dysplasia in those cases is still unknown.
Reviewed by P Surat, PhD References