A research article in the August 2015 issue of the JCI identifies de novo missense mutations in two genes that encode protein phosphatase 2A (PP2A) subunits, PPP2R5D and PPP2R1A, in individuals with syndromic intellectual disability (ID). Mechanistically, the authors demonstrate that patient-derived mutations in these genes disrupted PP2A function by inhibiting dephosphorylation regulated by the PP2A subunit encoded by PPP2R5D. Their research provides the first evidence that PP2A dysfunction is causative of pathogenesis in a subset of ID patients. We asked lead author Gunnar Houge about his work.
What was your path to becoming a physician-scientist?
I started as a physician in 1984 at the age of 24. During the first 10 years, I did basic research while working part-time as a physician. Because of my PhD (1992) on cyclic AMP-dependent protein kinase and apoptosis, I worked in the anatomy and cell biology department at that time. But I missed having clinical contact, so I decided in 1996 to change fields completely to medical genetics. That leap of fields was accompanied by a change in my research interests. Now I run a lab while seeing patients in clinic. I am happy that I made the change because in medical genetics you have the advantage that research questions manifest in a number of patients that walk through the door. You have all these very challenging and interesting patients that you see, and now you have the tools to really find out a lot about them. I feel quite privileged to work in medical genetics these days because we can do so much more than before. My background in basic research on cAMP-regulated protein phosphorylation helped me to generate a hypothesis for a mechanism of PP2A dysregulation in ID. Even though kinases and protein phosphatases are two faces of the same coin, only one side (the kinases) has received much scientific attention so far.
What considerations did you make in switching careers?
It was a resetting of my whole career. After 10 years in basic research and having a permanent position at the university as an associate professor, I quit for several reasons. The main reason was lack of clinical contact; another minor issue was the need to write several grant applications annually. At that time in Norway you rarely got big grants, so I had to write many applications, which took me a couple of weeks each year. Although we were doing okay in this regard, I moved into a clinical field in which research is more or less what I do in the hospital on a daily basis. Thus, I can do my research on data that have already been collected through diagnostic testing done by the hospital. The patient in the JCI article, for instance, was picked up in one of my outpatient clinics (this time in middle Norway) and followed up by routine genetic testing. Thus, in this work, grant money was mainly needed to fund the biochemistry performed by our collaborators in Belgium — and of course to fund the Deciphering Developmental Disorders (DDD) project, of which this work is only a small part. However, switching careers also meant I had to accept that I would not have that many publications during the first 5–6 years because I had to build up a research program at the hospital. Another inherent advantage of this discipline of research is that in my department, we have six researchers already paid by the hospital to do the bioinformatics work to explore interesting cases.
How did you get involved with such a large and international group of researchers?
The story started some years ago when I saw the first patient in Norway. I saw a young patient and found her condition to be very peculiar. We didn’t know what she had. Then the technique of trio exome sequencing came along. Approximately two years ago, we found the first mutation, de novo E198K in PPP2R5D. Partly through my connections in my role as the Secretary-General of the European Society of Human Genetics, we found other patients with identical or functionally similar mutations. Most of these patients had been found through the DDD project or by two Dutch genetic departments, primarily in Nijmegen and later also in Utrecht. This project has gone well because everybody wanted to collaborate, and when there are so many patients from so many different places, nobody is really capable of doing it all.
Is there a cut-off age at which you would think that a potential therapy against would no longer be efficacious?
Many cases of ID are due to structural aberrations in the brain for which not much can be done. For instance, in the PPP2R1A mutations, there is agenesis of the corpus callosum, which obviously cannot be reversed. But when it comes to the PPP2R5D mutations, the brain looks quite normal in size and shape, but there is a bit of extra fluid around, and sometimes inside, the brain that can raise suspicion for pseudohydrocephalus. The main problem appears to be brain function, not structure. So if this phosphatase is important for synaptic signaling, then it’s possible that some cases of ID might be improved, not necessarily fully reversed. So I don’t think you necessarily need to have a young patient to have an effective treatment, but this is of course pure speculation.
With whole-exome sequencing, you are unraveling various genotypic changes that may be associated with different phenotypes than previously reported. How will this affect the type of genetic testing that’s ordered by the referring physicians?
I think you are absolutely right. In my lab, for example, we are changing now from doing gene-by-gene diagnostics to gene panels. We have different panels depending on the clinical situation. I am a dysmorphologist in that I look at patients and try to find out which genetic mutation they have based on their dysmorphic clinical features. But I must admit, we medical geneticists are sometimes really bad at guessing and can often be surprised when the disease-causing mutation eventually is found. The phenotype is always on a spectrum of mild to severe manifestations of the mutation/disease, and only in the middle range the phenotype may be easily recognizable (i.e., “classical”). Perhaps the same story will apply to these protein phosphatases, because we have not really seen the mild end of the phenotypic spectrum. It is entirely possible that there is a mild form that does not present with ID, for instance. It may turn out that soon we will perform genetic testing first and look at the patients afterwards, which would be a new way of doing things. Importantly, in such a diagnostic scenario we should only query for genetic information that we are able to interpret in a clinical context.
What advice do you have for physician-scientist trainees in regards to how to choose a career niche?
You should follow your heart and interests, because then everything will naturally follow. You should aim for a PhD and then do a postdoctoral fellowship. If you want to practice translational medicine today, you have to have an understanding of the science. Otherwise, you will be overrun by all kinds of suggestions from various sources that you could not resist with good counter-arguments because you are not well versed in the literature and the scientific method. I highly recommend pursuing international connections, meaning that you should have at least one period of scientific training in another country. I was in Paris for two years when I did cancer research back in the 1990s. I now go on short sabbaticals every five years somewhere that will result in long-term collaborations.
With the publication of your work in the JCI and the DDD project already ongoing, what do you foresee as the next big leap in your field of research?
I think the next big leap will be to understand why patients have this intellectual disability, because it is quite a brain-specific phenotype. What is really the brain malfunction? Even though the phosphatase is present in all cells of the body, the regulatory B56δ subunit [encoded by the PPP2R5D gene] is mostly expressed in the brain. This is probably why it is so specific. It is entirely possible that mutations in other regulatory subunits would also result in particular phenotypes. First we will need to better understand what’s happening at the synapse level, which substrates are hyperphosphorylated, and if the dysphosphorylation dynamics may be corrected by chemical substances. With more understanding, we will be able to think about therapeutic interventions. And obviously with genetic testing, the benefit is that the sample is very readily available from the patient’s blood. The genetic diagnosis should be straightforward because these mutations appear to be clustered in small regions of these genes. Functional neuroimaging would of course be a helpful adjunct to understanding what’s going on in these brains.
Gunnar Houge, MD, PhD, received his MD from the University of Bergen in 1983 and his PhD from the same institution in 1993. His research has included work at the Hôpital Saint-Louis in Paris, Radboud University Nijmegen Medical Center in Nijmegen, and the University of Queensland in Brisbane. He currently has appointments at the Center for Medical Genetics and Molecular Medicine at Haukeland University Hospital and the Department of Clinical Science at the University of Bergen. He is also the Deputy Secretary-General of the European Human Genetics Association.
Freddy T. Nguyen is an MD/PhD candidate at the University of Illinois at Urbana-Champaign. He is the founder of the American Physician Scientists Association and served on the Associate Member Council of the American Association for Cancer Research. His research interests currently lie at the intersection of biomedical optics and cancer research. He received his BS in chemistry and BA in mathematics from Rice University.
Chirag Patel earned his MD and PhD from the University of Texas Medical School at Houston. He completed a preliminary year of residency training in internal medicine at East Tennessee State University and is currently a resident physician in neurology at the University of California at Los Angeles David Geffen School of Medicine. Dr. Patel received his B.S. and M.S.E. in biomedical engineering from Johns Hopkins University.
Here we report inherited dysregulation of protein phosphatase activity as a cause of intellectual disability (ID). De novo missense mutations in 2 subunits of serine/threonine (Ser/Thr) protein phosphatase 2A (PP2A) were identified in 16 individuals with mild to severe ID, long-lasting hypotonia, epileptic susceptibility, frontal bossing, mild hypertelorism, and downslanting palpebral fissures. PP2A comprises catalytic (C), scaffolding (A), and regulatory (B) subunits that determine subcellular anchoring, substrate specificity, and physiological function. Ten patients had mutations within a highly conserved acidic loop of the
Gunnar Houge, Dorien Haesen, Lisenka E.L.M. Vissers, Sarju Mehta, Michael J. Parker, Michael Wright, Julie Vogt, Shane McKee, John L. Tolmie, Nuno Cordeiro, Tjitske Kleefstra, Marjolein H. Willemsen, Margot R.F. Reijnders, Siren Berland, Eli Hayman, Eli Lahat, Eva H. Brilstra, Koen L.I. van Gassen, Evelien Zonneveld-Huijssoon, Charlotte I. de Bie, Alexander Hoischen, Evan E. Eichler, Rita Holdhus, Vidar M. Steen, Stein Ove Døskeland, Matthew E. Hurles, David R. FitzPatrick, the Deciphering Developmental Disorders (DDD) study, Veerle Janssens