Sluggish cell division during development may help explain genital defects
Scientists have learned how a gene widely known for precisely positioning and sculpting various organs also controls the speed of cell division, a finding that could be useful for understanding the explosive growth of cancer cells or why increasing numbers of children are being born with genital and urinary tract malformations.
Writing in Nature Communications, researchers at the University of Florida say a gene memorably named Sonic hedgehog controls genital development by regulating a process known as the cell cycle — a biological event that regulates when, and how fast, cells divide to form hearts, brains, limbs and all the other complex structures needed to build an individual.
The findings in mice provide insight into the molecular mechanisms that underlie growth of urinary and reproductive organs in both sexes. Abnormalities of the genitalia and urinary tract are among the most common birth defects, according to the March of Dimes. Similarly, the ability of Sonic hedgehog to alter the time it takes to complete the cell cycle might also influence tumor growth in a wide range of cancers, including the most common form of skin cancer.
“The role of Sonic hedgehog during embryonic development is to set up the positional addresses of cells in everything from limbs to the spinal cord, telling cells where they are located and what they will become — a process known as patterning,” said senior author Martin Cohn, Ph.D., a Howard Hughes Medical Institute early career scientist and a member of the UF Genetics Institute and the College of Medicine. “We’ve shown Sonic hedgehog also controls organ growth by determining how long a cell spends preparing to replicate its DNA. The surprise is to find out how much patterning and growth are intertwined. An embryo has only a fixed amount of time to grow. Once we discovered that inactivation of Sonic hedgehog slowed down the cell cycle, it explained the big differences in growth and the structural defects we were finding in genitalia.”
The knowledge may help scientists understand why an increasing number of boys are being born with birth defects called hypospadias, which involve incomplete formation of the urethral tube, resulting in an abnormally placed urethral opening on the underside of the penis. About one in 250 children has a urethral tube defect, more than double the frequency of 30 years ago.
The cell cycle controls whether a cell continues to give rise to more cells or stop dividing and become specialized with a specific function to carry out. Humans begin life as a single fertilized egg cell that eventually gives rise to countless cells in an adult. As each cell divides it must proceed through a growth phase, replicate its DNA and divide again, or it can be instructed to stop dividing and perform a specific function.
When scientists deleted the Sonic hedgehog gene in specific tissues at different stages of external genital development, they discovered the cell cycle takes longer than it normally does — about 14.4 hours instead of the usual 8.5 hours for these cells. As a result, fewer cells are produced and genital growth is reduced by about 75 percent. The shape of the genitals is also altered.
“In this case, embryos wind up with underdeveloped, malformed genitalia, and the reason is that it takes nearly twice as long to complete the cell cycle, limiting the number of cells available to build the structure. What is surprising is that the number of cells seems to underlie the shape,” said Ashley Seifert, Ph.D, a postdoctoral associate in the department of biology in the College of Liberal Arts and Sciences and first author of the paper. “We did not just see a miniaturized version of the genitals, we observed patterning defects, from subtle changes to severe malformations.”
Researchers had thought malformations might be explained because important shaping genes that sculpt the genitalia would be controlled by Sonic and thus turned off in its absence, Seifert said. But instead, scientists found many of these key genes were still expressed in the right places, forcing the scientists to look elsewhere for the cause of the defects. The search led them to the cell cycle.
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