The guppy curveback and medaka wavy as a model for heritable spinal curvature
As a PhD student in my lab, Dr. Kristen Fay Gorman developed the guppy mutant, curveback, as a model for human idiopathic scoliosis (IS), and she is continuing this research as an independent PI. Among humans, this debilitating deformity affects 3%-4% of the global pediatric population, and imposes a substantial cost on the healthcare system. Our research introduces the first genetic animal model for idiopathic-type scoliosis (i.e. known to be genetic in nature, but the 'cause' is unknown), with unique morphological and developmental parallels to human IS. Prior to the introduction of curveback, research regarding the causes of curvature and the factors that affect progression of curvature has relied on induced curves in animals such as rabbits, goats, chickens, dogs, mice and rats, and there is some question as to the extent that induced curves can inform the cause of IS. Due to the lack of a non-induced animal model and a high degree of phenotypic variability in human pedigrees, little is known about the basis of idiopathic-type curvature in humans. Our research will provide valuable insight into the basic biological nature of idiopathic-type scoliosis. The fact that humans and fish share many genes with similar tissue and temporal expression characteristics is well established, making them a valuable asset for understanding the genetics of diseases that affect humans. Ideally, the identification of genes associated with idiopathic-type scoliosis in curveback could allow for effective screening and early curve detection in IS. However, the more immediate question is what are the biological processes that underlie this type of deformity?
Parallels between curveback and human IS include:
- Curvature that develops after birth (guppies are born live, with a fully ossified skeleton) and generally does not progress after sexual maturity.
- Variability for curve magnitude, curve shape, and rate of curve progression (all within a family).
- Extreme curvature is most common in "adolescents."
- A tendency for some curves to resolve to normal or nearly normal before maturity.
- The propensity for a curve to stabilize or progress is highly variable.
- Although males and females have a similar rate of onset, females are ~ 5 times more likely to develop severe curves.
- Vertebral bodies become distorted at the apex of severe curves.
- The deformity is most likely caused by multiple genes.
Our research will help elucidate the basic biological processes involved in complex heritable spinal curvature, thereby providing a better understanding of factors that maintain spinal stability.
Our characterization of the parallels to human IS that are observed in the curveback phenotype will provide the foundation for further research into not only the cause(s) of idiopathic-type scoliosis, but also into risk factors that explain why some curves progress to severity and others do not.
Our gene mapping efforts combine curveback with a closely related teleost, medaka. The medaka fish has a mutant with a similar morphological deformity to curveback, referred to as wavy. The advantage to medaka is that many modern genomic tools are available. We are in the process of mapping the genes that cause curvature in both of these species. One important question is whether these genes are the same in two closely-related species. Dr. Julian Christians, of the Department of Biological Sciences, SFU is helping with the QTL mapping and analysis of the vertebral mutants.
We have received funding for this project from the Scoliosis Research Society (Exploratory Grant in 2005), and the National Institutes of Health (NIH R21 in 2008). Kristen has also received a fellowship from Graduate Women in Science Program (GWIS 2008). Roozbeh Ahmadi, an undergraduate working in our lab, has been supported by the BC Clinical Genomics Network.
Above: The guppy curveback. L denotes lordosis, and K denotes kyphosis. The insert of a CT scan shows no vertebral fusion or breaking associated with the curve. A normal guppy and a CT insert of normal vertebrae is shown below.
Unlike quadrupeds, the human and guppy spines are aligned so that vertebrae are stacked in a cranial (head) to caudal (tail) direction. In both humans and guppy, biomechanical force is applied in a cranial-to-cuadal direction. In humans, cranial force is applied from the weight of the head and gravity, and caudal force is applied from standing and the bipedal motion. In guppies, cranial force is from swimming into the dense medium of water, and caudal force is from the tail-beat motion. Thus in both humans and guppies, curvature is oriented so that it is similar to a column that has buckled. In contrast, force does not interact with the vertebrae in a cranial-caudal direction in quadraped animals. This may explain why spinal curvature is common in many teleosts, while animal genetic models such as rat or mouse have not exhibited idiopathic spinal curvature.
Above: The numbers in the upper left corner of each photo denote the magnitude of curvature, based on a qualitative scale used for scoring the phenotype (scale on ruler is in mm). Each fish is scored from birth, through sexual maturity, and into adulthood. At maturity, each fish is either selected for breeding, or photographed on a light table and preserved for further genetic work. Despite their curves, these fish are otherwise healthy. We currently have over 3000 inbred curveback photographed and preserved, all originating from a single cross. This pedigree will be used for fine-mapping genes for presence and progression of curvature.
Above: Whole mount skeleton from a male with a slight curve. Skeletons are prepared by clear and stain; this involves digestion of the musculature with enzymes and staining of bone with red and cartilage with blue dyes. Computed tomography 3-D movie showing spine of curveback female. Note that the vertebrae are well-formed, in that they are not fused or cracked. The vertebrae show distortion from the normal hour-glass shape, as seen in human idiopathic scoliosis. Thanks to the University of Texas Digimorph lab