The Genetic Basis of Recessive Pediatric Brain Disorders
Unlike adult disorders, which are frequently due to dominant genetic or multifactorial causes, many pediatric disorders, especially severe or life-threatening types, are due to recessive genetic mutations. In such families, two healthy parents have one or more children with severe disease. Having such a child frequently leads the parents to a genetic counselor, to ask how their genetic makeup could have led to their child's condition, and whether the condition could be seen again in future children.
Many pediatric brain diseases have no known causes. Most human genes are expressed in the developing brain but have no known function. These observations led us to the hypothesis that permeates our work: many of these connections between diseases and genes remain to be established. If we can elucidate these connections and understand how these mutations give rise to a specific disease, we hope to understand more about the assembly of the map and circuits of the human brain. Synergy between clinical and basic science allows for advances in human health, for fundamental insight into mechanisms of disease, and for discovery of potential treatments.
Focus on the Middle East, North Africa, and Central Asia
To understand the function of recessive disease genes, we study populations where both copies of a gene are missing. In most cases of recessive disease in the United States, physicians assume that two different mutations in the same gene were inherited. However, in practice, to identify the causes of such diseases, it is most useful to have a situation in which we can assume that the child inherited the same mutation from both parents. Such inheritance is quite common in regions of the world with high rates of consanguinity in which parents are related to one another. Because the rates of consanguinity are 100-times higher in many parts of the Middle East, North Africa, and Central Asia, we focus recruitment efforts in these areas.
For this reason, we have established collaborations with physician-scientists in major cities in many countries with high rates of consanguinity rates. Physicians from our group make regular visits to counterparts in Egypt, Lebanon, Libya, Kuwait, Jordon, Saudi Arabia, Pakistan, Turkey, Morocco, Qatar, Oman, and neighboring countries to evaluate patients, ascertain for research participation, and subsequently to link disease to gene. These discoveries are then translated in a "bench-to-bedside" approach to develop new diagnostic tests, consider potential points of treatment, and help clarify basic mechanisms of brain development. Having enrolled over 5,000 families and studied over 5,000 exomes, we leverage expertise in bioinformatics and stem cell modeling to make new discoveries.
Molecular Mechanisms of Cerebellar Development
One focus of our research is the development of the cerebellum, the main brain region that controls balance. Surprisingly, we now know that the cerebellum is responsible for much more than just balance: it establishes much of our basic bodily rhythms and a lot of higher cognitive processing. In Joubert syndrome, the most common inherited cerebellar malformation, the midline, or vermis, is underdeveloped. We have contributed to the identification of more than six different genetic causes of this disease, and there appear to be many more causes yet to identify. We found that the gene products appear to localize to the region of the cell known as the primary cilium, an organelle that, until recently, was thought to be vestigial. It is now established, however, that the primary cilium plays critical roles in most regions of the body. We are exploring how dysfunction of the more than 30 genes can lead to cerebellar development disorders and whether defective signaling at the primary cilium is a major contributor. We are focused on understanding the structural components and signaling mechanisms of the primary cilium using a combination of ultrastructural analysis and patient-derived induced pluripotent stem cell modeling of human cerebellar tissue.
Potentially Treatable Diseases
The entries for most conditions in pediatric neurology textbooks nearly always end with the sad truth that most of these conditions have no treatment other than supportive care. As our list of candidate genes grows through expanded patient recruitment, we prioritize study of mutations that predict treatment might be possible. We recently uncovered several different conditions, previously considered untreatable, for which potential treatments emerged. We found mutations in SRD5A3 link with a defect in protein N-linked glycosylation due to a biochemical block in the conversion of polyprenol to dolichol, essential for synthesis of the precursor glycan. We found that mutations in BCKDK lead to a depletion of branched chain amino acids, and result in autism. We found that mutations in AMPD2 lead to a depletion of GTP, a key cellular energy source, resulting in neurodegeneration. In each condition, nutritional supplementation with the depleted cellular metabolite rescued at least some aspects of disease. We are researching whether such novel forms of treatment are feasible in patients, and interested to apply this approach to other poorly understood, untreatable conditions.
Focal Disorders of Cortical Development
Epilepsy is present in about 4% of the general population, and a substantial portion of children with epilepsy do not respond to anticonvulsant treatment. In such cases, a focal disorder of cortical development is often identified on brain MRI, representing a part of the brain that was generated incorrectly. Our hypothesis is that such patches are constituted by cells carrying a genetic mutation that predisposes them to disrupted cortical organization. Over the past 10 years, the trend in the neuroscience field has been to surgically resect these lesions as a way to cure the patient of epilepsy. Such surgery can have a positive impact on overall health and cognition, in addition to resolving the epileptic seizures.
In order to advance this field we have initiated a Focal Cortical Dysplasia Registry to organize neurologists and neurosurgeons to assist with enrollment into our research protocol for molecular study. Our hypothesis is that, like the field of cancer, there are many mutations that can disrupt cellular signaling and result in cellular dysfunction. However, in the brain, these mutations do not lead to cancer because neural cells do not have the capacity to divide and accumulate mutations. We have a large and growing library of resected tissue samples from which we are performing specially designed sequencing approaches to molecularly characterize these patches. Our goal is to uncover new mechanisms of disease and develop new treatments.
Our preliminary data support this model of de novo mosaic mutations in brain as a cause for focal patches. In 2012 we reported the first mutations in AKT3, PIK3CA, and MTOR in brain samples from patients with the focal dysplasia known as Hemimegalencephaly. Since then, the results have been replicated in dozens of labs, and extended to the more focal lesions observed commonly in patients. The mutations we uncovered were striking for several reasons: 1) They occurred only in resected brain and were not present in other tissues of the body. 2) They were present in just a small fraction of the cells (8-10% of cells resected at surgery). 3) The mutations were distributed across the entire cortical diseased patch. 4) The identified mutations were in three of the key genes in the MTOR signaling pathway known to regulate cell size and cell signaling. 5) The mutations were the same ones observed in solid malignancies, already known to activate the pathway.
We recently explored how mutations in such a small percent of cells could give rise to disruption of the entire patch of cortex. We expected that such a small percent of cells with a mutation should not interfere with the behavior of surrounding cells, but the neuropathological features of resected brains suggested that there might be an effect of mutation-carrying cells on nearly non-mutated cells. By profiling mutation-carrying neural cells, we uncovered a pathway in which these cells are stimulated to release a secreted factor known as Reelin, which can instruct the nearby cells to abort development. We think that this is one of the mechanisms by which mutation in a small subset of cells can give rise to severe cortical developmental disease and epilepsy.
The Gleeson lab also receives funding from the NIH, the Simons Foundation, and the California Institute for Regenerative Medicine.
As of February 29, 2016