2020 APBD Biomarkers Workshop
By Robert Dorsky, D.M.D., APBD Research Foundation Pharmaceutical Development Committee
On June 22, the Adult Polyglucosan Body Disease Research Foundation hosted its first Biomarker Workshop to facilitate a vigorous exchange among researchers and patient advocates regarding potential biomarkers for drug discovery and development targeting APBD.
The Workshop — hosted on a virtual platform — was organized by the Foundation’s Pharmaceutical Development Committee and brought in well-known researchers within the APBD community as well as experts from other fields to open a dialogue on potential biomarkers.
Dr. Orhan Akman Discusses Glycogen Branching Enzyme Activity in APBD
Dr. Orhan Akman, Assistant Professor of Neurology, Columbia University Medical Center, described his work in developing a biomarker, or method to measure progress in clinical trials for APBD therapeutics. The biomarker will measure glycogen production in samples taken from APBD patients. Dr. Akman began his talk with a review of two enzymes, or proteins that are essential for normal glycogen production. The first enzyme, known as glycogen synthase, transforms glucose, a sugar that enters the body directly through diet or from ingestion of carbohydrates, into a glycogen molecule which is the form in which glucose is stored. The formation of glycogen molecules is highly amplified by another enzyme, the glycogen branching enzyme or GBE. In APBD patients the GBE1 gene which produces the GBE enzyme is frequently mutated. The result is an ineffective GBE enzyme which results in the creation of a non-soluble starch-like molecule known as a polyglucosan body instead of normal, healthy glycogen.
In order to measure effective GBE1 activity Dr. Akman utilizes an ‘indirect assay’. This is a highly economical and sensitive test applied by Dr. Akman to measure glycogen production. In order to readily obtain patient samples and achieve accurate results, Dr. Akman looked for tissues (samples) which could easily be obtained from patients and would provide a reliable measure of GBE1 activity with only a minimal chance of false readings. His search led him to a type of white blood cell, or immune cell, known as a lymphocyte. Unlike other human tissues such as liver or muscle, Dr. Akman found that lymphocytes proved easily obtainable and the naturally low glycogen levels in lymphocytes made them an ideal choice for measuring glycogen levels.
Dr. Akman has learned through his assay studies that all tissues in the human body have the same GBE1 activity, none more and none less. Also he noted, this activity is constant throughout life regardless of the clinical severity of the disease in any particular patient. To summarize, therefore, in patients with a GBE1 gene mutation, which will produce the ineffective GBE enzyme, the accumulation of polyglucosans is a steady process throughout life with symptoms appearing generally after 40 or 50 years.
In a brief review of current investigations into therapies for APBD, Dr. Akman focused on gene therapy. A technique to place healthy GBE1 genes into patients shows promise for improved production of healthy glycogen molecules. Another therapy under investigation is a process to correct the defective enzyme produced by the mutated gene in APBD patients. Known as antisense oligonucleotides, these are short strands of genetic material introduced into patients. This genetic material may also be used to alter or restore the undesirable enzyme product of a mutated GBE1 gene. Dr. Akman Is currently researching a form of gene mutation in mice, known as the intronic mouse model. It is for these types of APBD mutation that Dr. Akman hopes to apply the antisense oligonucleotide.
Dr. Angela Genge Discusses Biomarker Work In Related Fields
Dr. Angela Genge, McGill University, Montreal, provided a thorough review of the efforts being made in the ALS community to develop biorepositories which will serve as a resource for scientists seeking to develop biomarkers and therapeutics. A biorepository is simply a library of biological specimens, images, and cells collected from ALS patients and then made available to research scientists. Following years of failure to develop effective therapeutics for ALS, Dr. Genge and other researchers realized that they simply had to do better. They learned that drug development for ALS means finding answers quickly, long before research enters phase III clinical trials – which ultimately, Dr. Genge notes is a long, drawn out process.
The ALS community has established and continues to develop a number of these ‘biobanks’. One such example is CALSNIC, a long-term, or longitudinal, study which focuses on clinical data collection such as cognitive testing results and imaging studies. Other initiatives include ANSWER ALS which collects stem cells for scientists and industry to use for studies on new potentially therapeutic molecules. Another major initiative, CAPTURE ALS, seeks to bring together the resources of all ALS clinics in Canada. Dr. Genge urges clinicians and laboratories to come together to form the necessary partnerships to establish biorepositories, clinical data and imaging resources. Dr. Genge says that these projects will allow scientists with important ideas to have a resource to develop their ideas for treatments.
For all of these initiatives, Dr. Genge emphasizes the importance of maintaining ‘open science’. This means that at McGill no claim will be made for intellectual property rights on any specimen. Further, all specimens will be shared openly with scientists and industry. Dr. Genge has found that because of their interest, industry sometimes offers a financial partnership in order to use these resources. Also, to encourage industry partnerships, McGill allows all intellectual property issues to ultimately remain with industry.
Dr. Gülin Öz Discusses Biomarker Work In Related Fields
Dr. Gülin Öz , Professor of Radiology at the University of Minnesota Medical School spoke of the potential for using magnetic resonance imaging scans (MRS) to uncover many of the chemical secrets of the brain. In reference to APBD Dr. Oz addressed the issue of applying this technology to monitor progress in APBD clinical trials. At present there is not yet a way to measure the accumulation of polyglucosan bodies in the brain using MRS, however, in discussion, she noted that at least theoretically we may one day be able to do so. During the presentation Dr. Oz further reviewed many applications of MRS technology for measuring brain health and function, all of which may one day be applied as markers for APBD studies.
Dr. Berge Minassian Discusses APBD Gene Therapy
Two highly promising treatment modalities for decreasing polyglucosan body levels (the cause of APBD) and alleviating APBD symptoms were discussed by Dr. Berge Minassian (UT Southwestern Medical Center). Clinical trials to make these treatments available to patients will require reliable biomarkers, tools to help researchers measure the therapeutic effectiveness of treatments under development. Dr. Minassian discussed the first treatment, gene replacement therapy. He explained that monogenic diseases (diseases with a single mutation) such as APBD may be treated by replacing the diseased gene. This approach has been applied to neonatal mice as proof of principle and has been shown to work well by decreasing polyglucosan levels. Turing to a second treatment method under investigation, Dr. Minassian described that symptoms of APBD result from an imbalance between glycogen synthase, which initiates glycogen production, and branching enzyme, which forms the final product, a healthy, soluble glycogen molecule. Utilizing a technique known as antisense oligonucleotides, glycogen synthase activity can be targeted and decreased resulting in reduced levels of polyglucosan body production.
Dr. Felix Nitschke Discusses a Potential Cerebrospinal Fluid-Derived Biomarker for APBD
To move the development of these promising treatments forward, Dr. Minassian discussed two experimental biomarker projects which can be used in clinical trials to measure the effectiveness of these treatments. One project involves measuring the presence of polyglucosan bodies in the retina of APBD patients through advanced ophthalmological examination. A second investigational biomarker study for advancing APBD therapeutics seeks to measure glycogen levels in the cerebrospinal fluid (CSF), the fluid that bathes the spinal column. The concept behind this investigation is to utilize the CSF as a surrogate to measure metabolic or pathological changes in the brain. Dr. Felix Nitschke (UT Southwestern Medical Center), is a long-term collaborator with Dr. Minassian on Lafora disease and APBD. Dr. Nitschke reviewed the complex process by which the small amounts of glycogen in the CSF can be quantified. Dr. Nitschke’s work has demonstrated, importantly, that glycogen derived material exists in the CSF and possibly increases in the presence of APBD. This promises to open a whole new technique for biomarker application in APBD clinical trials.
Or Kakhlon, PhD Discusses Therapeutic Candidates and Biomarkers for APBD
Dr. Or Kakhlon, Department of Neurology, Hadassah-Hebrew University Medical Center, lucidly described the basis for therapeutic development in APBD patients. Glycogen, which is the storage form of glucose (sugar), is formed with the aid of two enzymes, glycogen branching enzyme (GBE) and glycogen synthase (GS). In cases where GBE is deficient, the glycogen molecule produced will appear as an elongated, insoluble molecule known as a polyglucosan body that accumulates over time. The result is either Andersen’s Disease in children or APBD in adults. Promising therapeutic approaches, Dr. Kakhlon explained, will attempt to alter the relative activity of GS and GBE production. Normal levels of GS accompanied by deficient GBE production, as happens in APBD, will create the polyglucosan body. Any therapeutic that activates GBE or inhibits GS will push towards the proper activity balance between these two enzymes resulting once more in the production of normal glycogen.
The key for one therapeutic that has been investigated, guaiacol, is to create a mild GS inhibition which helps to tilt production back towards normal glycogen. On the other hand, GBE activation also will reduce polyglucosan production in favor of normal glycogen. This has been observed in a lipid (fat) known as TGM5. For the most common mutation that causes APBD, it has been shown that TGM5 stabilized the mutated glycogen branching enzyme (GBE) making it more effective and therefore shifting the balance back towards normal glycogen production.
Another potential new therapeutic under investigation by Dr. Kakhlon is compound A. This formulation was found during screenings of chemicals that have the potential to reduce polyglucosan levels. It is now proven that compound A reduces polyglucosans in APBD modeling mice and in the specific cells that were studied.
Turning his focus to biomarker research, Dr. Kakhlon spoke of a study undertaken to observe the presence of brain glycogen. The study concluded the presence of glycogen by measuring isotopically-labelled glucose. The study is based on the understanding that APBD patients, besides producing polyglucosan bodies, also produce an abnormal surplus of glycogen in the brain. The excess glycogen present in older symptomatic animals is measurable by MRI (magnetic resonance imaging) as has been shown by Dr. Kakhlon in his pre- clinical studies. This new non-invasive approach for measuring increased glycogen levels associated with APBD by use of MRI has produced results that are unequivocal.
D. Goldman Shares Her APBD Story — A Patient’s Perspective
D. Goldman — a member of the APBDRF’s Board of Directors — was the opening presenter at the APBDRF’s Biomarker Workshop held in June 2020. She shared her APBD story and spoke from her heart. Goldman shared, “I have hopes and aspirations to see my two children, who are in their twenties, get married, flourish in their careers, and have children. I want to spare future generations from the horrible consequences of APBD. I remain hopeful that the scientists and researchers will find a treatment or a cure to help make that happen.”