Chromosomal translocations

Translocation takes place when a piece from one chromosome is transferred to the other. It can occur in the oocyte or sperm during their production or at the time of conception. In some cases, modified genetic material is inherited from the mother or the father.

The translocation is balanced when pieces of chromosomes are rearranged but no genetic material is gained or lost in the cell. A carrier usually has no symptoms. The problem is having healthy children. Couples where one of partners carries translocations have to face the increased risk of miscarriages and infertility. This results from the distribution of chromosomes to reproductive cells during cell divisions which can result in sperm or ova with abnormal amount of chromosomal material. Disorders of chromosome numbers in reproductive cells can also result in giving birth to a child with a syndrome of congenital defects, e.g. with Down syndrome which is the consequence of translocation t(14;21) carried by one of parents. There are two main types of translocations: RECIPROCAL and ROBERTSONIAN.

There are two main types of translocations:

When two pieces of two different chromosomes break off and exchange places.

Robertsonian translocation occurs when two acrocentric chromosomes (type of chromosome with a very short arm on one side (p) and a very long arm on the other (q) break at their centromeres (the constrictions of a chromosome) and cross connect to each other. The most common Robertsonian translocation in humans is a translocation between chromosomes 13 and 14. Due to the abnormal chromosome created from the combination of the long arms (q), the carriers have only 45 chromosomes in each cell of their bodies. When reproductive cells are created, occurrence of such translocation in one of parents can cause disorders of the number of complete chromosomes in offspring.

There can also be other situations – inversions (where a fragment of chromosome is in an inverted orientation), deletions (where a part of chromosome is missing), duplications (where a fragment of chromosome occurs twice) and insertions (where any part of a chromosome appears in the place where it shouldn’t be).

PGD for translocations

PGD diagnostics for translocations involves the examination for the genetic material of embryos for the presence of abnormal number of chromosome fragments of complete chromosomes involved in translocation. It allows checking whether there are any disorders in the amount of chromosomal material in the embryo. It has to be stressed that those who carry chromosomal translocations, are also at higher risk of disorders in the number of other chromosomes not linked to chromosomes involved in translocation. PGD testing for chromosomes involved in translocation should be supplemented by comprehensive PGD-NGS diagnostics of all chromosomes.

PGD Preimplantation diagnostics allows examining cells collected from embryos for the presence of translocations of various types.