Achondroplasia is a rare genetic autosomal disease. In about 80 percent of people with achondroplasia, it is not inherited and only about 20 percent of people with achondroplasia receive the affected FGFR3 gene from their parent who has had achondroplasia. Most people who suffer from achondroplasia have normal parents and achondroplasia is a result of some types of new gene alterations or mutations. These are spontaneous mutations that occur in the fibroblast growth factor receptor 3 (FGFR3) gene.
Fibroblast growth factor receptor 3 (FGFR3) is a receptor tyrosine kinase and the function of the FGFR3 gene is to provide instructions for the production of FGFR3 protein which helps in the bone growth, development and maintenance. It also helps in the maintenance of brain tissue, kidney, development of intestines, etc.
It is observed that more than 99% of achondroplasia patients suffer from one of the two mutations at the exon 10 of the FGFR3 gene. Of this, in 98% cases, the mutation is c.1138G>A and in about 1% of individuals, the mutation is found to be c.1138G>C. These both result in a p.Gly380Arg substitution in the FGFR3 protein. In other words, mutations in the FGFR3 gene convert glycine (Gly) into arginine (Arg) on the 380th amino acid which leads to the processing of dysfunctional proteins (i.e. overactive proteins or under production of proteins). This affects the chondrocyte proliferation and calcification and also inhibits the growth and development of cartilage and long bones which ultimately results in an external phenotype of achondroplasia.
Understanding genetic testing for achondroplasia
Genetic testing helps in the identification of manipulation in chromosomes, genes, or proteins which can help a doctor to rule out or detect a suspected genetic condition. Genetic testing is also helpful in detecting the chances of developing or passing on a genetic disorder to the children from parents. This can be done during pregnancy with the help of amniocentesis and chorionic villus sampling and after the birth through specific blood tests.
Molecular tests or gene tests are used to study single genes or short lengths of DNA. Chromosomal genetic tests help in the analysis of whole chromosomes or long lengths and biochemical genetic tests help to study the amount or activity levels of proteins.
For achondroplasia, molecular genetic testing is used for identification of a heterozygous pathogenic variant in FGFR3. Molecular genetic testing can be done in two ways:
Targeted analysis for achondroplasia
Targeted analysis test is a molecular test which is used to confirm the diagnosis and identification of the disease occurring due to mutations. Targeted analysis is not offered as a panel test and for achondroplasia. It is done for c.1138G>A (p.Gly380Arg) and c.1138G>C (p.Gly380Arg).
Targeted analysis involves sequence analysis of selected exons through Bi-directional Sanger Sequence Analysis. During targeted analysis, bidirectional sequencing of the target involves coding of exon 10 of the FGFR3 gene in achondroplasia and mutations in this area is detected through this test.
Multigene panel approach
Multigene panel testing is used for parallel sequencing of multiple pre-specified genes that help in the identification of pathologic DNA variation. For a multigene panel to test achondroplasia, FGFR3 and other genes of interest are taken into account.
With sequence analysis through a multi panel approach, more than 99% of cases are detectable. These tests help in the detection of small deletions and duplications which take place in a single exon of the FGFR3 gene. If the deletions and duplications are very small within a targeted large exon, there are chances that they might not be detected in some cases.
Before these tests, the patients should consider meeting a genetic counselor and understand all possible outcomes of the test.