Genetic and chromosomal conditions

Pregnancy

Frebruary, 2024.

The term genetic risk is generally used to broadly define the risk of passing on an inherited disease caused by a change in our genetic information.

But did you know that there are different types of conditions that involve our DNA depending on whether it’s a “misspelling” in our genetic code, or a missing or extra piece of chromosome? And that not all genetic conditions are necessarily hereditary?

 A little reminder about our genetics…

Our body is made up of billions of cells, each containing a nucleus where our chromosomes are located. Our chromosomes are divided into 23 pairs, one of which is a pair of sex chromosomes (usually XX or XY). They are numbered from 1 to 22 according to their decreasing size. Chromosomes are made up of condensed strands of DNA that are wound around themselves. These strands are composed of a unique combination of the 4 letters of the genetic code (A, T, C, G). This DNA sequence can be compared to an instruction manual that guides the functioning of our body’s cells.

What do we mean when we talk about a chromosome abnormality?

 Extra or missing chromosomes (aneuploidies)

When one copy of a chromosome is duplicated, it is called a trisomy because we end up with three copies of that chromosome instead of two. Trisomy 21 (also known as Down syndrome) is the presence of an extra copy of chromosome number 21. In the same way, monosomy is when a copy of a chromosome is missing. This is particularly the case for Turner syndrome (also called monosomy X) in which the only sex chromosome present is an X chromosome.  

Some of these conditions can be compatible with life but lead to the potential presence of congenital malformations and/or developmental issues. Generally, this includes trisomy 21 and monosomy X, as well as trisomies 18 and 13 and other abnormalities affecting the sex chromosomes. 

Most of these chromosome abnormalities are not compatible with the development of a full-term pregnancy and are therefore not viable, which means that these conceptions could lead to implantation failure or early miscarriage 

For most of these abnormalities, the mechanism involved is called non-disjunction, where the egg or sperm cell contributes an extra or missing chromosome to the embryo. While all pregnancies are at risk of a chromosome abnormality, the main risk factor is maternal age. The reserve of oocytes (eggs) carried by females is determined from the embryonic stage and decreases in quality and quantity from puberty onwards. The older we get, the less our body will be able to repair potential chromosome errors and therefore the higher the risk of a chromosome abnormality. 

Routine non-invasive prenatal screening assesses a pregnancy’s risk of having a common viable aneuploidy (trisomy 21, 18 and 13) with the possibility of adding risk assessment for abnormalities of the sex chromosomes (X, Y). Some technologies will also make it possible to estimate the risk for other aneuploidies (but these are generally not compatible with life).

If a previous pregnancy has been diagnosed with a chromosome abnormality, invasive prenatal diagnostic options (chorionic villus sampling or amniocentesis) are also available.

Small fragments of extra or missing chromosomes (microdeletions and microduplications)

Sometimes only a portion of a chromosome is extra or missing, rather than the entire chromosome.

These abnormalities are usually viable but can cause fetal loss or a syndrome with physical and/or developmental differences. This is the case, for example, with DiGeorge syndrome, which is associated with a deletion of a small part of chromosome 22. Unlike whole chromosome abnormalities, the risk of having a pregnancy with microdeletions and microduplications is not influenced by maternal age. 

Some non-invasive prenatal screening technologies offer the possibility of assessing a pregnancy’s risk of a microdeletion syndrome. This type of screening is usually not included in standard recommendations because the individual risk of each syndrome is low and does not increase with maternal age.

If a result indicates an increased risk, it is less likely to be a true positive. If a risk of a specific microdeletion syndrome is known, pre-implantation genetic testing on embryos resulting from IVF (PGT-SR or PGT-M) or invasive prenatal diagnosis (chorionic villus sampling or amniocentesis) are available.

Changes in the structure of chromosomes (chromosomal rearrangements)

The most common rearrangements are translocations, where genetic material is exchanged between different pairs of chromosomes. When the change does not result in a loss of DNA, it is unlikely to cause any health consequences and so an individual can be a carrier of a translocation without knowing.

However, when we pass on this type of so-called “balanced” rearrangement to our children, it is possible that it becomes “unbalanced”, i.e., the embryo ends up with too much or too little of a part of a chromosome (complete or partial aneuploidy). Again, as there is variation in the amount of chromosomal material, this can lead to significant risks to conception, including implantation failures, early miscarriages, and the risk of having a child with a chromosomal syndrome. These variations can be found in both men and women. 

Non-invasive prenatal screening tests are not able to detect a balanced structural change in chromosomes. Chromosome analyses (karyotypes) are required in both partners, especially when a couple experiences recurrent miscarriages, to assess for a silent chromosomal rearrangement that could be the cause. If a known rearrangement is present in one of the biological parents, pre-implantation genetic testing on embryos resulting from IVF (PGT-SR) or invasive prenatal diagnosis (chorionic villus sampling or amniocentesis) are available.

What do we mean when we talk about genetic abnormality? 

Spelling mistakes in the DNA (genetic variants or mutations)

Genetic abnormalities are usually caused by a spelling mistake in the DNA instructions, for example missing letter(s), extra letter(s), or a letter replaced by another, which affects the instructions. Depending on the position and nature of the change in the DNA, the health consequences can vary: some changes called polymorphisms will have no consequences, while others will cause potentially severe genetic conditions such as cystic fibrosis, thalassemias, or neurofibromatosis 

Triplet repeat expansions in the genetic code

Some genetic conditions are caused by too many repeats of DNA triplets. For each of these diseases, there is a threshold for the number of repetitions beyond which symptoms can appear. Sometimes, an anticipation phenomenon is added, which means that the number of repetitions (and therefore the severity) can increase as it passes from one generation to the next – this is the case for Steinert’s myotonic dystrophy, Huntington’s disease and Fragile X syndrome. The inheritance of these conditions is exclusively dominant but can be autosomal or X-linked. 

 In the case of subtle genetic abnormalities, there are no clinically validated non-invasive screening tests to assess their overall risk for pregnancy. If a couple or individual is at risk of passing on a specific genetic condition to their offspring, targeted preimplantation diagnostic options on embryos derived from in-vitro fertilization (M-PGD) as well as invasive prenatal diagnostic options (chorionic villus biopsy or amniocentesis) are available. 

Are all genetic and chromosomal abnormalities hereditary?

Most chromosomal changes are sporadic, accidental events that do not originate from either biological parent and occur randomly, also called ‘de novo‘. For an individual with a chromosome abnormality, they will not necessarily pass this on to their children; this depends on the specific case. Chromosomal rearrangements, on the other hand, are hereditary changes that can be transmitted in a balanced or unbalanced way, from generation to generation. 

Some genetic variants are passed down from generation to generation, while others occur accidentally without the biological parents being carriers. There are three main modes of inheritance of these genetic conditions: autosomal recessive, autosomal dominant, and X-linked. The term autosomal refers to inheritance that is not influenced by chromosomal sex, i.e., male and female individuals will have the same mode of inheritance. The inheritance of X-linked varies according to the sex of the carrier. In recessive conditions, if both biological parents are carriers, they have a 25% risk of having an affected child. In dominant conditions, one affected parent would have a 50% risk to have an affected child.  

As you can see, there is a wide variety of genetic and chromosomal causes that can cause diseases, which are not always inherited. To help you navigate the range of tests available and their indications, you can consult our following article on genetic testing. If you have any concerns about a personal or family genetic or chromosomal history, please do not hesitate to contact our team of genetic experts.

Claire Bascunana, Genetic Counsellor (MSc, CCGC)

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Genetic and chromosomal conditions

Par Claire Bascunana Temps de lecture: 6 min
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