Mutated Cilia Genes Shown to Lead to Scoliosis

    Researchers at Princeton University and the University  of Toronto report that in zebrafish irregular fluid flow through the spinal column brought on by gene mutations is linked to a type of scoliosis that can affect humans during adolescence. Found in humans and zebrafish, these mutated genes damage the cilia that line the spinal canal and help move the fluid, leading lead to a curvature of the spine.

    The scientists found that when they repaired the mutated cilia genes, they restored cerebrospinal fluid flow and could prevent spinal curves from developing. If translatable to humans, the study could lead to a nonsurgical approach for treating the condition known as idiopathic scoliosis, which has no known cause and affects roughly three out of every 100 adolescents. The study (“Zebrafish Models of Idiopathic Scoliosis Link Cerebrospinal Fluid Flow Defects to Spine Curvature”) is published in Science.

    “This is the first hint of a biological mechanism for idiopathic scoliosis,” said Rebecca Burdine, Ph.D., associate professor of molecular biology at Princeton, and a senior author of the study. “We hope this research will open up new areas of inquiry as to how the disruptions to normal cerebrospinal fluid flow can lead to spinal curvature.”

    Dr. Burdine’s lab conducted the study in collaboration with a team led by senior author Brian Ciruna, Ph.D., an associate professor of molecular genetics at the University of Toronto and a senior scientist at the Hospital for Sick Children in Toronto.

    “Traditionally, theories regarding the biology behind idiopathic scoliosis have revolved around defects in the bone, cartilage, or neuromuscular activity,” noted Dr. Ciruna. “The finding that defects in cerebrospinal fluid flow may be contributing to scoliosis came as a surprise. It is not a theory that had been put out there previously.”

    The study is the first to link spinal curvature to mutations in genes that govern motile cilia, which stick out from cells and make synchronous whip-like motions to push fluid through narrow passages such as the spinal column.

    Researchers in the Burdine laboratory had observed that mutant genes that disrupt cilia motility produce spinal curves in zebrafish as adults, although the work had not been published. “I’ve presented this finding for years, but didn’t have a way to link this work to human disease,” added Dr. Burdine. “Collaborating with Brian’s group helped us make this link.”

    Previous research by Ciruna’s lab revealed that mutations in a gene found in zebrafish and humans called protein tyrosine kinase-7 (ptk7) causes spinal curvature during a period of rapid growth that corresponds to adolescence in zebrafish. Published in the journal Nature Communications in 2014, the findings suggested that the mutant fish could serve as a model for studying the condition. The researchers knew that the ptk7 gene plays a role in helping cells orient in the correct direction during embryonic development, but they didn’t know that it also governed the formation of motile cilia.

    To explore how ptk7 mutations lead to spine curvature in zebrafish, Curtis Boswell, a graduate student at the University  of Toronto, examined the brains and spinal cords of fish with mutated ptk7. In brain ventricles, which sit at the top of the spinal cord, the motile cilia were sparse and malformed and the fish developed a brain-swelling condition called hydrocephalus, which is associated with loss of cilia function. Using fluorescent dyes to track the flow of cerebrospinal fluid through the ventricles, the researchers saw that the flow was irregular and slower than normal.

    When the researchers introduced a nonmutated version of the ptk7 gene specifically into tissues harboring motile cilia, the hydrocephalus disappeared, the cerebrospinal fluid began to flow normally and the spine straightened.

    “We demonstrated that if we could restore gene function in the motile ciliated tissues, we could restore cerebrospinal fluid flow, and we could actually prevent scoliosis in these mutants,” Dr. Ciruna said.

    The next step will be to understand the mechanisms by which disrupted cerebrospinal fluid flow causes the spine to curve, according to Dr. Burdine.

    “Now that we can study idiopathic scoliosis in zebrafish,” she said, “we can begin to identify molecular pathways that are involved in spine curvature, and hopefully, find therapeutic targets to address this condition.”