A healthy pregnancy starts with a healthy embryo: The value of reproductive genetics in the fertility setting

By Jaysen Knezovich 3 years ago
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 Our genome in a nut shell

Imagine that the human genome was an encyclopaedia collection. It would contain 23 books, with two copies of each, to give a total of 46 books. Opening a book would reveal pages with paragraphs – where each paragraph can be broken down into sentences, words, and individual letters. If this collection of books was our genome, it would contain about 22,000 paragraphs, and 6 billion individual letters.  Now, imagine that you could shrink all 46 books to fit into a library that is one tenth the size of a human hair. Nature has managed this feat – packing 46 chromosomes, 22,000 genes, and 6 billion letters of DNA into each one of the trillions of cells that make up our body.

 

Passing on genetic information

In order to reproduce, we need to pass on genetic information. The way in which we do this, is via the sperm and egg. Each sperm and egg cell should contain half of the genetic information from each biological parent. When making sperm and eggs, a cell containing 46 chromosomes divides into two daughter cells (to make sperm or eggs). Each pair of chromosomes is divided, so that each daughter cell receives one copy of each chromosome. When a sperm cell carrying 23 chromosomes fertilises an egg cell carrying 23 chromosomes, the chromosome pairs unite and an embryo is created with 46 chromosomes (23 pairs).

 

Mistakes can happen

While nature has perfected packaging our genetic material into individual cells, when it comes to making more copies of this information and passing it on, errors can occur. This is particularly so when splitting the chromosome pairs during the production of sperm and eggs. If the mechanism that separates chromosome pairs during egg production mistakenly places two copies of the same chromosome in one egg, the other egg  will not receive a copy of this chromosome. When a sperm cell containing one copy of that chromosome fertilises the egg with two chromosomes, the resulting embryo will contain an extra chromosome (three copies, instead of two). Conversely, if the egg that is missing that chromosome is fertilised by a sperm cell containing one copy of that chromosome, the resulting embryo will have a missing copy of that chromosome (only one copy). Missing or extra copies of chromosomes can have two outcomes: (1) the abnormal amount of genetic information is not compatible with life, and the embryo (or baby) will fail to develop and result in a miscarriage, or (2) the abnormal amount of genetic information allows for development, but will result in a genetic condition such as Down Syndrome.

Genome 8

Time is of the essence

The mechanism that sorts chromosome pairs when making sperm or eggs, is prone to increased errors the older one gets – particularly in women. In fact, the passing on of an extra or missing chromosome comes from the egg, 90% of the time. It is for this reason, that when a woman falls pregnant, the chances that she will have a child with a condition like Down Syndrome or suffer a miscarriage, increases with age. This is why doctors always take the age of the mother into consideration.

 

Why do patients seek fertility treatment?

About one in every six couples of reproductive age, will experience fertility issues. There are many causes of infertility. One of the reasons why couples struggle to fall pregnant, is due to a high incidence of chromosome abnormalities in the embryos they produce.

Couples who suffer from infertility, often seek advice from fertility clinics. In the fertility setting, embryos are typically generated outside of the body, by obtaining sperm and eggs from the couple. The sperm is used to artificially fertilise the eggs, which will give rise to embryos. Once an embryo has developed to a certain stage, it can then be transferred to the patient’s uterus – where it will hopefully result in a pregnancy. This process is commonly described as in vitro fertilisation (IVF).

The selection of which embryo to transfer can be tricky. Most of the time, trained embryologists select embryos based on how they look. However, not all good looking embryos are genetically normal, and not all poor looking embryos mean that they’re genetically abnormal (and don’t have the potential to give rise to a pregnancy). So, decisions on which embryo to transfer to a patient’s uterus based only on what it looks like, does not rule out the possibility of a chromosomally abnormal embryo.

What can be done to prevent miscarriages and genetic conditions?

Because IVF allows embryos to be generated outside of the body, and each cell of the embryo contains its library of genetic information, it is possible to access and assess the genetic make-up of each embryo. Having the genetic information of each embryo, provides more information to the decision on which embryo(s) to transfer. A technology called pre-implantation genetic screening (PGS) is able to determine whether an embryo has inherited extra or missing chromosomes – which would result in a miscarriage or genetic condition, if it was transferred. Thus, by screening embryos with PGS, the chances of a miscarriage or having a child with a genetic condition, is vastly reduced.

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About

 Jaysen Knezovich

  (3 articles)

Jaysen obtained his PhD in Human Genetics from the University of the Witwatersrand, South Africa, in 2014, which was part of a split-site doctorate with the University of Cambridge, UK. Jaysen’s primary area of research has been investigating the effects of environmental factors on epigenetic signatures in gametes. Jaysen is a Medical Biological Scientist registered with the Health Professions Council of South Africa (HPCSA). Jaysen also holds a Postgraduate Diploma in Management (PDM) which he obtained from Wits Business School in 2014. Jaysen holds an honorary lectureship position at the University of the Witwatersrand, where he lectures on epigenetics and reproductive genetics, and supervises a number of postgraduate degrees. In August 2013, Jaysen was approached by Genesis Genetics Inc. to open a laboratory in Johannesburg, South Africa, to offer reproductive genetic testing services. Following the acquisition of Genesis Genetics South Africa by Next Biosciences in April 2016, Jaysen was appointed as its Managing Director, where he manages reproductive genetics services.