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Finding a brain/body connection
For the first time, Whitehead Institute scientists have documented a direct link between deletions in two genes — fam57ba and doc2a — in zebrafish and certain brain and body traits in humans such as seizures, hyperactivity, enlarged head size, and obesity.
Mechanisms underlying brain to body connections are still not well understood. But now area fam57ba, gives intriguing hints into how metabolism and brain function are intertwined. It produces an enzyme which affects lipid production — believed to affect regulatate of our metabolism. The type of lipid, ceramide, also functions in various signaling pathways affecting synapses, even though its primary role is providing structure for cell membranes.
"Finding the molecular connections between a brain and a body phenotype is really a paradigm shift. It lets us think about the common control of these two aspects of phenotype—ß which could be useful in developing therapies for them."
Both genes reside in the 16p11.2 region of human chromosome 16. About 1 in 2000, or around 4 million people worldwide, have deletions in this region. Such deletions are associated with multiple symptoms including autism spectrum disorders, developmental delay, intellectual disability, seizures, and obesity.
Scientists have had difficulty teasing apart the relationship between specific traits and deletions in the 16p11.2 region, as it includes at least 25 genes, without any one-to-one map of gene to phenotype. Instead, multiple genes seem to create a web of interactions producing a variety of characteristics.
To solve this complex puzzle, Jasmine McCammon, a postdoctoral researcher in Sive's lab, used zebrafish to study the genetic/phenotype connections associated with human disorders. Like the human, the zebrafish has two copies of each gene, and scientists can remove the function of multiple genes to produce phenotypes recreating human symptoms.
The results from McCammon's initial screen of zebrafish indicated two genes in the 16p11.2 region could be key to brain development: fam57ba and doc2a.
fam57b encodes a 'ceramide synthase' a type of lipid. Ceramide comes from the Latin cera (wax) and amide, an organic compound derived from ammonia after replacing a hydrogen atom with a metal. It is seen in vernix caseosa, the waxy white coating covering the skin of newborn babies. Ceramides are highly concentrated in cell membranes as they help make up sphingomyelin, a major lipid in animal cell membranes, especially in the myelin sheath surrounding some nerve cell axons. Ceramides also participate in cell signaling, regulating differentiation, proliferation, and programmed cell death (PCD).
After deleting one copy of either fam57ba or doc2a, McCammon saw minimal effects. However, removing a copy of both genes revealed significant effects, supporting the concept of synergy exsting between them. Compared to normal (control) fish:
• fish with one copy of both genes, exhibit hyperactivity, increased seizures, increases in body and head size, and higher fat content.
• fish with both copies of just fam57ba removed grow much larger, with a higher fat content.
Study's results are published in the journal Human Molecular Genetics.
Although her findings use zebrafish and are far from any clinical application, McCammon was struck by how much people affected by deletions in these genes identified with her results.
"When I spoke with the parents of some kids with neurodevelopmental disorders, I was surprised how much the brain/body connection that we described resonated with them. They said that yes, their child has autism, but he also has really weak muscle tone. Or she has a gastrointestinal problem and that's been more problematic than her behavior issues. For me, it's been really revealing to talk to people who've actually experienced this as opposed to reading about statistics in journals."
For Sive, the two identified genes could be just the beginning. Sive: "Our data suggest that there may be other metabolic genes involved in human neuro-developmental disorders. This is a nascent field, and we're very interested in moving forward."
Deletion of the 16p11.2 CNV affects 25 core genes and is associated with multiple symptoms affecting brain and body, including seizures, hyperactivity, macrocephaly, and obesity. Available data suggest that most symptoms are controlled by haploinsufficiency of two or more 16p11.2 genes. To identify interacting 16p11.2 genes, we used a pairwise partial loss of function antisense screen for embryonic brain morphology, using the accessible zebrafish model. fam57ba, encoding a ceramide synthase, was identified as interacting with the doc2a gene, encoding a calcium-sensitive exocytosis regulator, a genetic interaction not previously described. Using genetic mutants, we demonstrated that doc2a+/- fam57ba+/- double heterozygotes show hyperactivity and increased seizure susceptibility relative to wild-type or single doc2a-/- or fam57ba-/- mutants. Additionally, doc2a+/- fam57ba+/- double heterozygotes demonstrate the increased body length and head size. Single doc2a+/- and fam57ba+/- heterozygotes do not show a body size increase; however, fam57ba-/- homozygous mutants show a strongly increased head size and body length, suggesting a greater contribution from fam57ba to the haploinsufficient interaction between doc2a and fam57ba. The doc2a+/- fam57ba+/- interaction has not been reported before, nor has any 16p11.2 gene previously been linked to increased body size. These findings demonstrate that one pair of 16p11.2 homologs can regulate both brain and body phenotypes that are reflective of those in people with 16p11.2 deletion. Together, these findings suggest that dysregulation of ceramide pathways and calcium sensitive exocytosis underlies seizures and large body size associated with 16p11.2 homologs in zebrafish. The data inform consideration of mechanisms underlying human 16p11.2 deletion symptoms.
Search topics: obesity phenotype seizures calcium body height ceramides embryo exocytosis genes heterozygote homozygote larva zebrafish brain head size gene interaction
Authors: Jasmine M. McCammon Alicia Blaker-Lee Xiao Chen Hazel Sive
This work was supported by Jim and Pat Poitras, Len and Ellen Polaner, and the Markell-Balkin-Weinberg Postdoctoral Fellowship.
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The Howard Hughes Medical Institute has developed a new tool at it's Janelia Research Campus, which allows light shone on a zebra fish brain to permanently mark active neurons. The tool, a fluorescent protein CaMPARI, converts calciumfrom green to red when it floods a nerve cell after firing. Image Credit: Howard Hughes Medical Institute