First discover the disorder and then find the patients
Bielefeld biochemists confirm cause of initially unclear symptoms
The sequencing of the human genome has made it possible: nowadays scientists can discover potential disorders for which there are no known patients. Such a disorder is ‘MPS III-E’, originally also called ‘Dierks’s disorder’ after its discoverer. Doctors at the Hadassah-Hebrew University Medical Center in Jerusalem (Israel) found consistent symptoms of progressive blindness in three patient families that indicated the presence of a previously unknown genetic defect. Genome analyses at the University of Lausanne (Switzerland) delivered a candidate gene on which Professor Dr. Thomas Dierks from Bielefeld University had already published a research article in 2012. Studies by Dierks’ research team now revealed that these patients indeed suffer from ‘MPS III-E’. The researchers are now presenting their analysis in the journal ‘Genetics in Medicine’, published by the Nature Publishing Group.
The scientists at Bielefeld had studied the disorder in mice. In these animals, they switched off the gene responsible for producing the enzyme arylsulfatase G. This ensured that the carbohydrate heparan sulfate could no longer be fully degraded. ‘As a result, the carbohydrate accumulated in the cells,’ explains Thomas Dierks. This accumulation takes place in the lysosomes, the ‘recycling plants’ of the cells. Due to this metabolic dysfunction, the breakdown also of other substances such as lipids and proteins gradually comes to a standstill. The accumulation then increases the size of the lysosome until it destroys the cell – and that brings about the disorder.
To make sure whether the mutated gene in the patients actually does lead to the enzyme defect, Dierks’ team studied the patient gene in a cell culture. This culture produced the enzyme (arylsulfatase G). They found that ‘the enzyme actually was seriously damaged. It is unable to break down the heparan sulfate,‘ says Dierks.
Now that the cause of the symptoms is known, it is possible to think about an enzyme replacement therapy for the patients. ‘The required enzyme can be produced biotechnologically with the help of cell cultures,’ says Dierks. For the treatment, the enzyme is injected intravenously. It is then carried through the body in the blood and should break down the heparan sulfate in the tissues. ‘However, the brain is hard for the enzyme to reach. Moreover, the disorder has to be diagnosed through gene tests before the symptoms appear if treatment is to commence before damage has become irreparable.’ Now that the symptoms resulting from this hereditary disorder in humans are known, Dierks expects that the genetic defect will also be diagnosed in more people. ‘Further patients should be found particularly among the unexplained cases with Usher syndrome – this indicates a combined loss of vision and hearing.’
Alongside Bielefeld University, the Hadassah-Hebrew University Medical Center in Jerusalem (Israel), University of Lausanne (Switzerland), Kiel University, the Institute of Technology Technion in Haifa (Israel), and the University of Leicester (Great Britain) were involved in the study.
Samer Khateb, Björn Kowalewski, Nicola Bedoni, Markus Damme, Netta Pollack, Ann Saada, Alexey Obolensky, Tamar Ben-Yosef, Menachem Gross, Thomas Dierks, Eyal Banin, Carlo Rivolta, Dror Sharon: A homozygous founder missense variant in arylsulfatase G abolishes its enzymatic activity causing atypical Usher syndrome in humans. Genetics in Medicine. http://doi.org/10.1038/gim.2017.227, published on 4 January 2018. The article is available via: http://rdcu.be/D3Sw.
Researchers discover hereditary enzyme deficiency (press release from 11.06.2012): http://bit.ly/2B8plDl
Prof. Dr. Thomas Dierks, Bielefeld University
Faculty of Chemistry, Biochemistry I
Telephone: +49 521 106-2092