Genetics & Rare Diseases

What rare diseases teach us about common ones

Think of the urea cycle as a river. A normal river flows to where it empties, similar to the process the body uses to rid itself of harmful ammonia via the urea cycle.

Think of the urea cycle as a river. A normal river flows to where it empties, similar to the process the body uses to rid itself of harmful ammonia via the urea cycle.

I recently presented at Spotlight Health 2016, the health-focused portion of the Aspen Ideas Festival, about how studying and treating rare diseases can inform innovative treatment approaches for more common medical conditions. Our Division of Genetics and Metabolism sees more than 8,000 patients a year with rare conditions, such as urea cycle disorders and Down syndrome. Through decades of analyzing these diseases and treating children who have them, we have developed therapies that apply not only for the small numbers of patients who have rare diseases but also for more common conditions caused by environmental factors leading to a similar physical response.

For instance, we’ve demonstrated that the stress of cardiopulmonary bypass during surgery to correct congenital heart disease creates conditions similar to a critical blockage in the urea cycle, specifically the biochemical creation of citrulline, a key biochemical.

When that cycle is unable to flow, or continuing the river analogy, becomes dammed up due to a genetic defect, as in urea cycle disorders, or an environmental factor, such as the extreme stress of cardiopulmonary bypass, the body is unable to make enough citrulline which is critical for maintaining normal blood pressure. We’ve shown that replacing that citrulline can correct a lot of these problems whether caused by rare genetics or the cardiac OR.

Applying rare disease treatment approaches to more common diseases is not limited to urea cycle disorders. Work by my colleague Carlos Ferreira, MD, demonstrates how a rare genetic calcifying arterial disease (generalized arterial calcification in infancy, GACI) causes the same calcium buildup and blockages as chronic kidney disease. Dr. Ferreira hypothesizes that life-saving drugs developed for use in GACI could help patients with long-term kidney disease by averting organ damage and eventual failure caused by the buildup of calcium crystals.

The more we learn about these rare diseases, the more we come to appreciate the tremendous implications our findings have for patients with the rare disorders and potentially hundreds of thousands of others.

About the Author

Marshall Summar, MD
Research interests: The interactions between common genetic variations and the environment.

How a rare disease treatment could impact millions

Post-mortem image shows significant narrowing of the artery in an infant with GACI due to buildup of calcium crystals between the vessel wall’s inner and middle layers. Inset: Normal non-calcified artery. Patients with GACI lack the protein ENPP1, which is responsible for creating pyrophosphate. Pyrophosphate plays a critical role in preventing calcium crystallization and accumulation.

Post-mortem image shows significant narrowing of the artery in an infant with GACI due to buildup of calcium crystals between the vessel wall’s inner and middle layers. Inset: Normal non-calcified artery. Patients with GACI lack the protein ENPP1, which is responsible for creating pyrophosphate. Pyrophosphate plays a critical role in preventing calcium crystallization and accumulation.

One of the first patients I ever saw with generalized arterial calcification of infancy (GACI) was actually the third child with this condition born to the same parents. GACI is a rare genetic disease, occurring in 1 of 200,000 live births. Unfortunately, as is common in GACI, two of the family’s children previously succumbed to the disorder within the first 6 weeks of life.

GACI causes calcium to build up in the arteries, causing critical blockages that reduce blood flow to organs leading to diminished function, including stroke, heart attack, and death.

Etidronate, a pyrophosphate analog developed to treat osteoporosis, has shown limited success at replacing the pyrophosphate for patients with GACI. However, more than 55 percent of children with GACI still die before their first birthday.

We need more effective solutions. Several treatment options are in development, including the administration of ENPP1 bound to an antibody, which has shown to provide a marked survival improvement in a mouse model of the disease.

These new solutions could translate to more effective treatment of GACI but also other conditions causing calcification in the arteries, particularly the calcium buildup associated with long-term kidney disease. A treatment that potentially reduces morbidity for the estimated 20 million plus Americans with chronic kidney disease would have tremendous health and economic benefits.

Developing more targeted therapies for GACI could allow this to be the outcome for many more patients, both children with GACI and potentially also patients affected by chronic kidney disease.

About the Author

Carlos Ferreira LopezCarlos Ferreira Lopez, M.D.
Geneticist Specialist

Personalized sequencing tailors genetic tests for each patient

Changes or errors in an individual’s DNA are often at the root of many disorders. Personalized Sequencing is a fast, cost-effective way to look at a region of the genome without repeat tests and blood draws.

Changes or errors in an individual’s DNA are often at the root of many disorders. Personalized Sequencing is a fast, cost-effective way to look at a region of the genome without repeat tests and blood draws.

Until recently, doctors and patients had two choices for ordering genetic sequencing panels to identify underlying causes of disease—Individual Gene Testing (single genes and gene panels) or Whole Exome Sequencing.

Individual gene testing is the standard testing modality. Physicians identify a single gene to analyze for change or mutation. If results are negative, they order another individual test, requiring a repeat visit and another blood draw. The process is repeated again and again based on likely candidate genes for a specific disease or symptom. If a physician is very lucky, it takes only a few rounds of tests to find the culprit. More likely, however, the number of individual tests grows large, taking months of patients’ time and increasing healthcare costs significantly. By contrast, Whole Exome Sequencing includes sequencing and analyses of 25,000 genes. It is more expensive when compared with individual gene testing and takes three to six months to complete. When complete, the results often can be more than the doctor and patient bargained for: Potentially revealing a genetic problem that is unrelated to the patient’s current symptoms. A 3-year-old with seizures also may come up positive for BRCA1, the breast cancer gene. Knowing that doesn’t help understand what causes the seizures or how to best treat them. In this model, you receive everything you could ever want. Because there is so much information, however, the results are difficult to interpret or to inform treatment decisions.

We’ve come up with a different way: Personalized Sequencing Panels, a precision medicine initiative at Children’s National Health System. We offer physicians a menu of genetic regions from which to choose when they order a sequencing analysis. While a medical exome is still sequenced, we only analyze a subset of genes that the physician and geneticist think are the most likely targets, which reduces the cost and time for analysis compared to Whole Exome Sequencing. Targeting regions in this approach shortens our turnaround time for results to two or three weeks. If the first identified region shows nothing, we can return to data we’ve already collected for a second look.

We’ve been using the model for 18 months and have tested more than 1,000 patients this way. Eighty percent of physicians prefer to “create their own test” using our menu of options. Rather than bringing a one-size-fits-all test to the patient, we bring the patient their very own personalized test.

About the Author

Sean Hofherr
Laboratory Medicine Specialist

Analysis of a progressive diffuse intrinsic pontine glioma: a case report

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What’s Known
Despite multiple clinical trials testing an assortment of new treatments, the survival rate for diffuse intrinsic pontine glioma (DIPG) remains abysmal, with most children succumbing to the pediatric brainstem tumor within 12 months of diagnosis. Focal radiation therapy, the primary treatment approach, has not improved overall survival. While the majority of DIPG tumors grow within the brainstem, metastases can occur elsewhere in the brain. Due to recent availability of tissue, new data are emerging about the biologic behavior of tumors, details that could be instrumental in constructing optimal treatment strategies.

What’s New
An otherwise healthy 9-year-old girl developed weakness in the left side of her face; magnetic resonance imagining revealed T2/FLAIR hyperintensity centered within and expanding the pons. Despite various treatments, her pontine lesion increased in size and new metastases were noted. The team led by Children’s National Health System researchers is the first to report comprehensive phenotypic analyses comparing multiple sites in primary and distant tumors. All tumor sites displayed positive staining for the H3K27M mutation, a mutation described in more than two-thirds of DIPGs that may portend a worse overall survival. Persistence of mutational status across multiple metastatic sites is particularly important since the effectiveness of some therapeutic approaches relies on this occurring. mRNA analyses, by contrast, identified a small number of genes in the primary tumor that differed from one metastatic tumor. This divergence implies that a single biopsy analysis for mRNA expression has the potential to be misleading.

Questions for Future Research
Q: Because a small cohort of genes in the girl’s primary tumor were different from genes in portions of the metastatic tumor, would genomic and proteomic analyses provide additional details about this genetic evolution?
Q: How do site-specific differences in mRNA expression affect decisions about which therapies to provide and in which order?

Source: “Histological and Molecular Analysis of a Progressive Diffuse Intrinsic Pontine Glioma and Synchronous Metastatic Lesions: A Case Report.” J. Nazarian, G.E. Mason, C.Y. Ho, E. Panditharatna, M. Kambhampati, L.G. Vezina, R.J. Packer, and E.I. Hwang. Published by Oncotarget on June 14, 2016.

Why subtle cellular changes can result in dramatically different genetic disorders

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What’s Known
One single gene, lamin A/C, is to blame for a multitude of genetic disorders, such as premature aging and problems with nerves, the heart, and muscles. Uncertainties linger in the scientific and medical community about why subtle changes of this gene cause such dramatically different disorders, such as Emery-Dreifuss muscular dystrophy, a condition that can lead to progressive muscle weakness in childhood and heart problems by adulthood.

What’s New
The nuclear envelope is where attached regions are pulled from genetic circulation never to be used again. The process of attachment signals which parts of the genome the cell no longer considers useful. Discarding superfluous DNA keeps the cell focused on what matters more: Its future role. Proper cell differentiation hinges on “the coordinated execution of three key cellular programs,” the study authors write. Pluriopotency programs, which give primitive cells the remarkable ability to generate any cell type in the body, are inactivated. Exit from the cell cycle occurs, and cells stop dividing. Myogenesis is induced, ushering in formation of muscle tissue. Mutations in lamin A/C can disrupt this careful choreography with the cumulative effect of slowing exit from cell cycle, slowing exit from pluripotency programs, and poorly coordinating induction of terminal differentiation programs.

Questions for Future Research
Q: What are the regions of human genome that become attached to the nuclear envelope during the development of tissues other than muscle (such as fat, nerve, and heart)?
Q: Can medicines that influence epigenetic pathways help to reverse the inappropriate DNA-lamin associations in Emery-Dreifuss muscular dystrophy?
Q: Can the new knowledge of DNA-lamin associations during muscle cell differentiation help to inform stem cell therapies?

Source: “Laminopathies Disrupt Epigenomic Developmental Programs and Cell Fate.” J. Perovanovic, S. Dell’Orso, V.F. Gnochi, J. K. Jaiswal, V. Sartorelli, C. Vigouroux, K. Mamchaoui, V. Mouly, G. Bonne, and E. P. Hoffman. Science Translational Medicine. April 20, 2016.
researcher using ice bucket in lab

Spatial and temporal homogeneity of driver mutations in diffuse intrinsic pontine glioma

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What’s Known
Needle biopsies help to guide diagnosis and targeted therapies for diffuse intrinsic pontine gliomas (DIPGs), which make up 10 percent to 15 percent of all pediatric brain tumors but carry a median survival of 9 to 12 months. This dismal survival rate compares with a 70 percent chance of children surviving other central nervous system tumors five years post diagnosis. In DIPG, tumors appear in the pons, an area of the brain that houses cranial nerve nuclei. Surgical options are limited. Spatial and temporal tumor heterogeneity is a major obstacle to accurate diagnosis and successful targeted therapy.

What’s New
The team sought to better define DIPG heterogeneity. They analyzed 134 specimens from nine patients and found that H3K27M mutations were ubiquitous in all 41 samples with oncogenic content, and always were associated with at least one partner driver mutation: TP53, PPM1D, ACVR1 or PIK3R1. These H3K27M mutations are the initial oncogenic event in DIPG, writes the research team led by Children’s National Health System. “Driver” mutations, such as H3K27M, are essential to begin and sustain tumor formation. This main driver partnership is maintained throughout the course of the disease, in all cells across the tumor, and as tumors spread throughout the brain. Because homogeneity for main driver mutations persists for the duration of illness, efforts to cure DIPG should be directed at the oncohistone partnership, the authors write. Based on early tumor spread, efforts to cure DIPG should aim for early systemic tumor control, rather focused exclusively on the pons.

Questions for Future Research
Q: If a larger sample size were analyzed, what would it reveal about the true heterogeneity/homogeneity status of DIPGs?
Q: “Accessory” driver mutations are not absolutely essential but do help to further promote and accelerate tumor growth. What is their precise role?

Source: “Spatial and Temporal Homogeneity of Driver Mutations in Diffuse Intrinsic Pontine Glioma.” H. Nikbakht, E. Panditharatna, L.G. Mikael, R. Li, T. Gayden, M. Osmond, C.Y. Ho, M. Kambhampati, E.I. Hwang, D. Faury, A. Siu, S. Papillon-Cavanagh, D. Bechet, K.L. Ligon, B. Ellezam, W.J. Ingram, C. Stinson, A.S. Moore, K.E. Warren, J. Karamchandani, R.J. Packer, N. Jabado, J. Majewski, and J. Nazarian. Published by Nature Communications on April 6, 2016.

The role of NG2 proteoglycan in glioma

A large number of staffers contribute to the Children's National team effort to unravel the mysteries of DIPG. We photograph a few essential players in Dr. Nazarian's lab.

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What’s Known
Neuron glia antigen-2 (NG2) is a protein expressed by many central nervous system cells during development and differentiation. NG2-expressing oligodendrocyte progenitor cells have been identified as the cells of origin in gliomas, tumors that arise from the brain’s gluey supportive tissue. What’s more, NG2 expression also has been associated with childhood diffuse intrinsic pontine glioma (DIPG) an aggressive tumor that accounts for 10 percent to 20 percent of pediatric central nervous system (CNS) tumors. Radiation can prolong survival by a few months, but children diagnosed with DIPG typically survive less than one year.

What’s New
Researchers are searching for appropriate targets and effective drugs that offer some chance of benefit. A team of Children’s National Health System researchers investigated whether NG2 – which plays a critical role in proliferation and development of new blood vessels and promotes tumor infiltration – could be a potential target for cancer treatment. Of the various options, antibody-mediated mechanisms of targeting NG2 are feasible, but the size of antibodies limits their ability to cross the blood-brain barrier. “Due to its role in maintaining a pluripotent pool of tumor cells, and its role in tumor migration and infiltration, NG2 provides multiple avenues for developing therapeutics,” the research team concludes. “Moreover, the large extracellular domain of NG2 provides an excellent antigen repertoire for immunotherapeutic interventions. As such, further research is warranted to define the role and expression regulation of NG2 in CNS cancers.”

Questions for Future Research

Q: Because healthy oligodendrocyte progenitor cells are important for the child’s developing brain, how could further characterization of NG2 isoforms help prevent drugs from damaging those beneficial cells?

Q: Could NG2-binding peptides cross the blood-brain barrier to deliver anti-cancer therapies precisely to tumor sites?

Source: “The Role of NG2 Proteoglycan in Glioma.” S. Yadavilli, E.I. Hwang, R. J. Packer, and J. Nazarian. Published by Translational Oncology on February 2016.

Clinicopathology of diffuse intrinsic pontine glioma and its redefined genomic and epigenomic landscape

Dr. Nazarian's lab

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What’s Known
Fewer than 150 U.S. children per year are diagnosed with diffuse intrinsic pontine glioma (DIPG), one of the most lethal pediatric central nervous system cancers. Despite an increasing number of experimental therapies tested via clinical trials, more than 95 percent of these children die within two years of diagnosis. Molecular studies have yielded additional insight about DIPG, including that mutations in histone-encoding genes are associated with 70 percent of cases. Understanding mutations that drive tumors and the genomic landscape can help to guide development of targeted therapies.

What’s New: Frequently found genetic alterations prevalent in DIPGs

dipg-gene-mutations-and-biological-consequences

Source: “Clinicopathology of Diffuse Intrinsic Pontine Glioma and Its Redefined Genomic and Epigenomic Landscape.” E. Panditharatna, K. Yaeger, L.B. Kilburn, R.J. Packer, and J. Nazarian. Published by Cancer Genetics on May 1, 2015.