Information on sperm chromatin (DNA) packaging using Chromomycin A₃ (CMA₃)

Many studies have detected anomalies in the nucleus of ejaculated spermatozoa. Different mechanisms lead to these abnormalities. Three factors have been identified which cause DNA damage in the germ line: oxidative stress, deficiencies in chromatin packaging and abortive apoptosis.

The formation of mature spermatozoa is a unique process. This process is characterizes by meiosis, mitosis, changes in cytoplasmic structure and replacement of histones which bind to DNA. The histones are first replaced by transition proteins then by arginine-rich protamines and the formation of chromatin-stabilizing disulphide bonds. The presence of protamines leads to extremely compacted and stable chromatin. The high degree of chromatin aggregation protects the mature sperm against physical and chemical damage. Furthermore due to the tight packaging afforded by the protamines, any modification or absence of these proteins could lead to an anomaly in the packaging process of the sperm nuclei and influence the sperm quality and fertilizing ability.

It is only within the ooplasma of an activated oocyte that the sperm chromatin becomes decondensed as a result of the cleavage of the disulphide bonds and the substitution of the protamines by oocyte derived histones. Spermatozoa with abnormal sperm chromatin packaging and incomplete chromatin condensation more often display DNA strand nicks, more single than double stranded DNA and posses chromosomal abnormalities.

Abnormal sperm chromatin packaging results in defective decondensation and DNA activation during fertilization which leads to delayed formation of the male pronucleus and/or first division. This will cause embryonic wastage or poor quality embryos and lower pregnancy rates. Researches have illustrated the importance of paternal influence in early embryo development and have linked increased chromosomal damage to repeated spontaneous abortions. Sperm decondensation defects may result in unrecognized de-arrangements of spermatozoa DNA regardless of sperm morphology.

 With a cut off value of, 40% abnormal chromatin packaging a fertilization rate of at least 70% can be expected. A value >60% will be indicative of a significantly lower fertilization rate (35%). The implantation in this last group is also significantly lower than the group of <40% (15% versus 32%).

The outcome of ejaculated human spermatozoa where DNA packaging is abnormal can be improved by employing sperm preparation techniques and intra cytoplasmic sperm injection with aggressive immobilization of the sperm.

References

1. Esterhuizen AD, Franken DR, Lourens JGH, et al. (200) Sperm chromatin packaging as an indicator of in vitro fertilization rates. Hum Reprod., 15, 657-661
2. Esterhuizen AD, Franken DR, Becker PJ, et al. (2002) Defective sperm decondensation: a cause for fertilization failure. Andrologia., 34, 1-7
3. Sakkas D, Mariethoz E, Manicardi G, et al. (1999) Origin of DNA damage in ejaculated human spermatozoa. Rev Reprod., 4, 31-37
4. Larson KL, DeJonge CJ, Barnes Am, et al. (2000) Sperm chromatin structure assay parameters as predictors of failed pregnancy following assisted reproductive techniques. Hum Reprod., 8, 1717-1722