Tuesday, September 18, 2012

Sperm Sequencing Could Help Fight Infertility


I read a fascinating article and so decided to summarize it as best as I can, (there is a link below to the actual article):  

Not all sperm are created equal! The first genetic comparison of individual sperm cells has revealed just how diverse they can be. The technology used to study these tiny cells might also be used to study cancer and allow doctors to screen eggs for in vitro fertilization.  To investigate how much variety there is in one man's sperm, Stephen Quake, Jianbin Wang and their colleagues at Stanford University in California compared sperm cells from a single semen sample.
Analyzing the genes of individual cells is not an easy task. To perform genetic sequencing, one needs to amplify, or make lots of copies of the genes within a cell to have enough to analyze. The compounds needed for amplification produce chemical by-products that can make the analysis more difficult.  To overcome this difficulty, Quake's team shrank their experiment onto a microfluidic chip, a device roughly 3 cm, that was covered in tiny channels and valves. This size of the container meant that a smaller volume of amplification compounds would be necessary, interfering less with the sequencing stage of the experiment.  The group injected a liquid sample containing sperm cells from the ejaculate of one man into the channels of their chip. Valves were set up to separate 91 sperm cells into individual chambers. The team was then able to amplify the genes in each cell in preparation for sequencing.  
Our genomes contain 23 pairs of chromosomes. When a sperm cell is produced, by a process called meiosis, parts of the chromosomes get shuffled around, also known as recombination. It means that the genetic sequence of each resulting sperm cell varies slightly. It is thought that this process is partly responsible for sibling variation. Recombination also occurs in other body cells, for example, it helps immune cells to rapidly adapt and recognize new pathogens.
Quake's team was able to scan each of the 91 sperm cells' genomes to see if recombination had taken place at any of 1.2 million positions in the DNA. There are certain "recombination hotspots" where recombination often occurs, they say. The team's analysis revealed that chromosomes in some of the sperm cells had recombined in unexpected regions. The findings suggest that the process of genetic reshuffling is unique to each sperm cell. This further adds to the genetic diversity between siblings.
The microfluidic chip technique could be used to diagnose recombination problems, which are thought to be one of the causes of male infertility, says Wang. "Previous studies have shown that too much or too little recombination can cause infertility," he says. "This is a way to find out if recombination is a problem."
Wang hopes that the technique could also help in selecting eggs for in vitro fertilisation (IVF). Each egg cell is produced alongside three other non-functional cells that contain the same genetic code but do not develop into eggs (polar bodies). "We can use those cells to analyse an egg's genome, and screen for genetic diseases," says Wang. Currently, few eggs are screened in this way because existing techniques are so difficult, says Wang. "Our technique may make the process easier."


Original Article:
Harmon, Katherine.  "Sequencing of Single Sperm Could Reveal New Infertility Causes."  Scientific American.  July 19 2012. <http://www.scientificamerican.com/article.cfm?id=genome-sequence-single-sperm>

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