Enhanced Sperm Selection: The HBA Test
How are sperm selected for IVF?
During IVF, individual sperm are selected to inject into eggs. Typically, sperm are selected based only on their appearance under the microscope. Ideally, sperm should be moving (to prove they are alive) and have a normal shape. Up until recently, however, there haven’t been any tests which tell us accurately about the health of sperm.
It is hoped that by injecting sperm with better health that this would produce better embryos and improve pregnancy rates in IVF.
What is hyaluronan?
Hyaluronan is a type of sugar known as a high molecular weight glycosaminoglycan. Hyaluronan is found in many parts of the body. Most importantly, hyaluronan is a key component of the group of cells that surround the egg (the cumulus oophorous). During the final stages of sperm maturation, the sperm develop the ability to bind (attach) to hyaluronan. Research has shown that hyaluronan binding is an important indicator of sperm health and maturity. It appears that the attachment of sperm to the hyaluronan surrounding the egg serves as a natural selection mechanism for mature sperm during normal conception.
Markers of sperm health and maturity
Beyond the parameters that we look at in a normal semen analysis, there are other markers of sperm health and maturity.
Cytoplasmic inclusions -During the final stages of sperm maturation, a normal sperm must get rid of excess cytoplasm. Defects in the normal development of sperm may result in excess cytoplasm being retained near the sperm head. This is known as cytoplasmic retention or inclusions. These are visible with a high powered microscope.
Sperm creatine phosphokinase (CK) Elevated levels have been shown in a number of studies to be associated with defective sperm function and lower pregnancy rates.
HspA2 Chaperone Protein – This protein is found in higher levels in normal sperm. Sperm with low levels are more likely to have chromosome abnormalities and DNA fragmentation.
Sperm Hyaluronan Binding
Sperm hyaluronan binding may be an indicator of which sperm are most likely to produce a viable pregnancy. For example, mature, hyaluronan-binding sperm are essentially free of cytoplasmic inclusions. This is of great value since cytoplasmic inclusions are extremely difficult to see when selecting sperm to inject into eggs in an in vitro fertilization cycle.
Studies have also found that in sperm selected for hyaluronan binding
- a 4-6 fold lower frequency of chromosome abnormalities.
- more than 99% of HA-binding sperm had high DNA integrity compared to about 55% in the unselected sperm
- lower CK levels
- higher HspA2 chaperone protein levels
How does the Hyaluronan Binding Assay work?
Men who are preparing to do in vitro fertilization with ICSI will be asked to collect a semen specimen in the same manner that they would for a conventional semen analysis. The semen is mixed with some media and placed on a special slide that has been coated with hyaluronan. Mature sperm will bind the hyaluronan (bound). These sperm will appear to have their heads stuck but with tails that show vigorous tail motion. Immature sperm will move freely (unbound).
We then calculate the percentage of bound sperm. This is the HBA score. A normal HBA score is greater than 70%. An abnormal HBA score is less than 70%.
For all men with an abnormal HBA score, when they have ICSI performed in the lab, the embryologists will use media containing hyaluronan to select healthy sperm for injection
HBA is an indicator of pregnancy success
A recent (2011) large, multicenter study of couples doing IVF found that men who had a high percentage of mature sperm in their ejaculate, as judged by a hyaluronan binding test, produced embryos that looked healthier under a microscope as compared to men with a low percentage of mature sperm.
Does sperm selected for ICSI according to hyaluronan binding produce embryos which are more likely to form a pregnancy? The answer is yes, but only for those men who had a low percentage of mature sperm to begin with. Not only were there more pregnancies, but there were also less miscarriages.
According to a recent multi-center double blind randomized clinical trial. Sperm selected in this way
- Did not show any difference in the % of fertilization, but
- The embryos show less fragmentation
- A higher percentage of embryos will reach blastocyst stage
- Clinical pregnancy rate will be higher
- Miscarriage rate will be lower
These improvements are not seen when the HBA score is >70% because these men do well without the HBA test.