Mutation analysis in the causative genes for two X-linked mental retardation disorders: Simpson-Golabi-Behmel Syndrome (SGBS) and Coffin-Lowry Syndrome (CLS) (B1014)

Aim/Principal research question

The aim of the research was to find out more about the, at that time, recently identified causative genes for two developmental genetic disorders: Simpson-Golabi-Behmel Syndrome (SGBS) and Coffin-Lowry Syndrome (CLS). Both disorders involve growth and brain function abnormalities. Less was known about the CLS gene and so the initial aim of this part of the project was to find out about its structure and organisation. During the project this information became available from another group. With the aid of an extension to the grant, we were then able to address the same research questions for both genes: to identify the range of mutations and their position in the gene and to try to find out whether this related in any way to the severity or nature of the disorders in individual patients. A subsidiary aim of the project was to compare the efficiency and ease of use of two methods of mutation screening .

Factors of interest

1) Type of mutation found in each gene: the GPC3 gene underlying Simpson-Golabi-Behmel Syndrome and the RSK2 gene underlying Coffin Lowry Syndrome.

Methodology

Two methods were used for mutation detection. The first was a standard method based on running DNA fragments through a gel matrix which separates them on the basis of both size and shape. A proportion of mutations cause a change in shape in the DNA fragment when compared to fragments taken from unaffected individuals without the mutation. The method is called single-stranded conformation polymorphism (SSCP). The second method is a sequencing based method again based on comparing the gel running pattern from affected and unaffected individuals, this time of a set of subfragments produced for each piece of the gene. This method is called bi-directional dideoxy fingerprinting (bi-DDF). Two advantages were claimed for this method. Firstly that it detected 100% of mutations and secondly that it pinpointed the position of the mutation within the fragment. With both methods the final characterisation of the mutation was by DNA sequencing of the altered fragments.

Sample groups

DNA samples were prepared from blood samples taken with consent from 61 patients who had been diagnosed as having Coffin Lowry Syndrome and from 61 patients who had been diagnosed as having Simpson-Golabi-Behmel Syndrome. Control DNA samples were also obtainied from unaffected individuals. The patients were usually referred by a clinical genetics specialist.

Outcome measures

Detection of altered DNA fragment in either the SSCP or bi-DDF screen and then full characterisation of the mutation by sequencing. Identification of a mutation in a patient is a significant step in that it confirms the diagnosis, enables genetic information to be provided to the patient's family and a specific mutation screening test to be offered where appropriate.

Findings

We found mutations in 16 of the 61 DNAs from patients with Simpson-Golabi-Behmel syndrome and 28 mutations in the 61 DNAs from patients with Coffin-Lowry syndrome. In these samples SSCP and bi-DDF were equally effective at detecting mutations but SSCP was a considerably easier procedure to carry out. In both genes mutations were distributed without an obvious pattern and there was no clear relationship between the mutation type or position and the severity or pattern of the patient's disease. Many of the mutations in the Coffin-Lowry syndrome patients were missense mutations and for this type of mutation it is more difficult to be sure that they are the causative. The genes underlying both disorders, however, lie on the X chromosome and usually it is only males that are affected. It is possible, therefore, to use the presence of a missense mutaiton as a marker for the affected gene even when the direct relationship between the mutation and the disorder has not been established. This is important for carrying out further genetic tests in family members.

Implications for future research

The regulatory regions for each gene also need to be screened for mutations as these may account for some of the patients where no mutation has been discovered to date. It may also be that further genes are involved in each disrder and that these will need to be identified and characterised. The mechanism of disease causation still needs to be established for both disorders.

Citation

Lindsay S (2003). Mutation analysis in the causative genes for two X-linked mental retardation disorders: Simpson-Golabi-Behmel Syndrome (SGBS) and Coffin-Lowry Syndrome (CLS) (B1014). The Research Findings Register. Summary number 1137. Retrieved 24 December 2005, from http://www.ReFeR.nhs.uk/ViewRecord.asp?ID=1137