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Rare disease research
BETHESDA, Md.—An international team of National Institute of Health (NIH) researchers and collaborators has identified genomic mutations for Carey-Fineman-Ziter syndrome (CFZS). This very rare congenital myopathy, or inherited muscle disorder, is characterized by facial weakness, a small or retracted chin, a cleft palate and curvature of the spine (scoliosis), among other symptoms.
The researchers determined that CFZS is caused by mutations in the gene MYMK that encodes for the protein myomaker, which is necessary for the fusion of muscle cells (myoblasts) into muscle fibers (myotubes) during the development of an embryo and the regeneration of muscle cells after injury. According to the study published July 6, 2017, in Nature Communications, “Autosomal recessive mutations in MYMK (OMIM 615345) cause Carey-Fineman-Ziter syndrome in humans (CFZS; OMIM 254940) by reducing but not eliminating MYMK function.”
As explained by Dr. Irini Manoli, co-lead author and a physician scientist and staff clinician in the Medical Genomics and Metabolic Genetics Branch at the National Human Genome Research Institute, “This discovery will improve physicians’ ability to diagnose this disease and offer families accurate genetic counseling and treatment. In addition, this rare genetic syndrome provides novel insights into the effects of muscle development on craniofacial and skeletal bone formation.”
People affected with CFZS have sometimes been misdiagnosed with Moebius syndrome, another very rare disorder characterized by facial paralysis. Uncovering that cell-cell fusion deficits can lead to congenital myopathies (inherited muscle disease) opens a new path of exploration for therapies for CFZS and other muscular diseases and tools for regenerating muscle, according to Manoli.
With the objective of learning more about the genetics and clinical characteristics of Moebius syndrome and other congenital facial weakness disorders, the NIH consortium brought 63 people affected with Moebius syndrome and other inherited facial weakness disorders to the NIH Clinical Center, along with their families. Subjects underwent detailed multi-system evaluations, including brain and muscle imaging studies and muscle biopsies.
The researchers collaborated through the Opportunities for Collaborative Research at the NIH Clinical Center, a new funding structure enabling intramural and extramural researchers to work together on the NIH campus. Study collaborators included researchers at the National Institute of Neurological Disorders and Stroke, the NIH Clinical Center, Boston Children’s Hospital affiliated with Harvard University, Icahn School of Medicine at Mount Sinai, the University of Utah and the University of Otago.
After performing detailed phenotyping, the researchers used exome sequencing of blood DNA in affected siblings from three unrelated families, as well as a muscle biopsy in one of the affected individuals. To identify the genomic mutations associated with CFZS, three laboratories—led separately by Dr. Elizabeth Engle at Boston Children’s Hospital, Dr. Stephen Robertson from the University of Otago and Dr. John Carey at the University of Utah—analyzed exome sequence data from each of the three families. Of the genes showing mutations identified in each family, only the gene MYMK was common to all three. A knockout mouse model showed a complete absence of muscle development, resulting in early death of the newborn mice, leading researchers to believe that this gene was a promising candidate for additional studies.
Using CRISPR-Cas9 technology, a team led by Engle and Dr. Silvio Alessandro Di Gioia developed zebrafish with a mutated MYMK gene. The mutant zebrafish were smaller and had abnormal muscle development and jaw deformities, resembling the patient phenotype. After further functional studies, the researchers validated the severity of each of the genomic mutations. They then corrected the affected zebrafish’s muscles by injecting the normal human MYMK gene product into the mutant fish, which raises the prospect of developing an effective treatment for restoring MYMK function in muscles. In addition to treating CFZS, a potential drug could reduce progressive features of this disorder.
Only eight people in the world have been diagnosed with CFZS with MYMK mutations. The researchers believe that is partially because of the inability to recognize the syndrome’s distinguishing characteristics. Now that researchers have identified the genomic cause underlying it, they can add it to the diagnostic gene panels for congenital myopathies. “This will improve the speed and accuracy of diagnosis and add to the understanding of the spectrum of disease severity and outcome,” Manoli concluded.