Mice that are missing a protein required for taking up ‘good’ cholesterol from the blood have an increased risk for heart disease, which is what you might expect. But you might not expect these female mice to be sterile.
Research done in 1999 found that female mice missing the receptor for good cholesterol, SR-BI, were practically sterile. Now, in 2011, new research has connected this to humans.
Women undergoing in vitro fertilization were tested for different forms of the human SR-BI gene, called SCARB1 (just a note: gene names are written in italics, while the proteins that these genes make are not). The researchers, led by Dr. Annabelle Rodriguez at Johns Hopkins Bayview Medical Center in Maryland, found that one form of the gene was linked to lower levels of progesterone, the major pregnancy hormone. These women had very low levels of progesterone despite being supplemented with the hormone as part of the IVF treatment. You can see a description of the study here and the paper abstract here.
In addition, this form of SCARBI was also linked to fertility. None of the women in the study who had at least one copy of the A form of the gene (the less frequent form) had a viable pregnancy. Of course, this number was fairly small since only 9 women in the study carried the A form. Hopefully, future research will show whether women with the A form are always unable to get pregnant.
Previous work by these researchers showed that when mice are engineered to have only the A form of SR-BI, they make half as much SR-BI protein as mice with the G form (the more common form of the gene). This shows that the mutation in the A form causes less of the protein to be made. This could mean there is a link between how much good cholesterol is taken up out of the blood (the job of the protein) and female fertility.
We’ve all heard plenty about good cholesterol and bad cholesterol. Bad cholesterol, the LDL form, can build up in your arteries and cause heart attacks and stroke. So, you should try to get more of the good cholesterol instead, the HDL form.
But what is cholesterol good for anyway?
Cholesterol is used by our bodies to make many hormones, including estrogen and progesterone. Progesterone is crucial for establishing pregnancy in mammals, including humans.
In a 2001 study, scientists at MIT were able to use a drug called probucol to restore fertility the female mice missing SR-BI. This drug is no longer approved for use in the United States because it can dangerously lower HDL levels in the blood. However, this is good news if your main problem is that you can’t lower blood HDL levels yourself. In the future drugs like probucol might be used to treat infertile women with the A form of SCARBI.