Handbook of Genetic Counseling/Balanced Translocation

Balanced Translocation

Introduction and Contracting

• Can you explain in your own words what you understand about why you were referred to genetics?
• What are your main concerns for ______ at this time?
• Do you have any specific questions you would like us to make sure we try to answer?

Outline session

• Take medical and family history
• Review the chromosome results with you
• Explain more about why we want to do chromosome testing on you (parents)
• Dr Viskochil will examine him
• We will then make sure to discuss any further questions or concerns

Medical/Developmental History

• follow form that I started filling out and confirm info

Family History

• 3 generation pedigree
• include standard questions don't forget miscarriages as they occur more frequently in people with balanced translocation

Explain Chromosomes and Genes (using sample karyotype)

• When the lab looked at _____ 's cells under a microscope they found a change in the chromosomes
• The change they found was called a translocation

What is a translocation?

• Transfer of chromosomal material between chromosomes

Why do translocations happen?

• We do not really know why - (Sort of. It happens because it is possible. And its possible because of the inherent nature of the genetic material.)
• Estimated that about 1 person in 500 has a chromosome translocation
• We know that our chromosomes seem to break and rejoin quite often, and it is only sometimes that it leads to problems
• These changes are out of our control, and nearly always translocations happen in either the egg or sperm cell before they join together
• Therefore they are not likely to be caused by anything during the pregnancy

How Translocations Occur

• Sometimes the chromosomes break during cell division and are usually repaired
• Sometimes two fragments may break off from two different chromosomes, and "swap" places
• This is called a reciprocal translocation (this is the type of translocation he has)

Show Karyotype and Explain Specific Break Points

• Sometimes helps to see the actual karyotype
• 46 means that the usual number of chromosomes is present
• Y chromosome is what makes him male
• T stands for translocation, where a piece of chromosome 2 and 21 exchanged places
• The rest just tells us more exactly where the chromosomes broke
• So arrows are pointing to where the chromosomal material exchanged places

Balanced Translocations

• In most cases, there is no loss or gain of chromosomal material during the exchange process
• When the lab looked they didn't see any missing or extra pieces of chromosomes and they therefore think that this might be a balanced translocation

What Are the Effects

• Often people will have a balanced translocation and not even know because they don't experience any problems (this occurs in majority of cases of new balanced rearrangements Warburton 1991)
• Sometimes, apparently balanced translocation carriers show some characteristics or symptoms (Warburton showed that frequency of abnormality was 6.1% but confidence interval includes 3% in apparently balanced translocations)
• One explanation for this finding is that the break may have occurred in the middle of a gene that is important in growth or development and the gene no longer provides correct instructions because it is disrupted
• We do not know if this translocation is the explanation for the characteristics and health problems in ______

What will results from parental chromosomes tell us

• In order to determine what the translocation might mean it is important to do chromosomes on all parents of children with a translocation
• If a parent has the same translocation and they do not have any of the same symptoms or characteristics as the child then we cannot conclude that the translocation is the cause
• If neither parent has the translocation than it may explain some or all of the characteristics and health problems in ______, but would be difficult to know for certain
• Parental chromosomes will also be important because it will help us determine what risks might be to future offspring

What can happen in offspring of people with balanced translocation

• If one of the parents carries a balanced translocation, it is possible for a child of theirs to acquire a bit of extra chromosome material or a bit missing
• Either of these would be known as an UNBALANCED translocation
• This extra and or missing pieces of chromosome can unfortunately cause serious mental, physical and medical challenges in the child who inherits them
• Sometimes they cause a miscarriage, but not always
• However, it is also possible for these same parents to have a child with normal chromosomes, or a child with the same balanced rearrangement of their chromosomes as the parent
• Having a balanced translocation of your chromosomes does not mean you cannot have normal healthy children
• Factors that influence risks (not applicable in this case)
• Ascertainment - A family ascertained as a result of a previously affected child is seen to be at increased risk of recurrence since the imbalance has been proved viable.
• Risk of having an affected child is lower when a family is ascertained following for example repeated spontaneous miscarriage since in these cases the imbalance is more likely to be inviable
• Each time they have a child, there are several possibilities.
  • they may have a child with normal chromosomes
  • they may have a child with the same balanced rearrangement of the chromosomes as the parent
  • they may have a child with an unbalanced rearrangement of the chromosomes which may cause mental and physical challenges
  • the pregnancy may end in a miscarriage

What sort of problems do unbalanced translocations cause?

• Can be difficult to predict because it depends on:
• Which parts of chromosomes are missing or duplicated (aneuploidy for 4p, distal 4q and 9p for example all have a high degree of viability therefore relatively high risk)
• how large of pieces they are (if large less likely to be viable due to more extra or missing information)
• There are not usually the same breaking points for each translocation because it is random so we don't have any other children to compare this exact translocation to
• Nearly always have learning problems though if translocation is unbalanced

Tests in Pregnancy for Translocation Carriers

• amniocentesis This is usually done from 14 weeks in the pregnancy and the result normally takes about 2 weeks.
• Chorionic villus sampling (CVS) usually done between 10-12 weeks in the pregnancy, and the results take about 2 weeks.

Psychosocial Considerations

• Information can be difficult to understand because translocations are not an easy concept
• Don't expect them to remember everything, we will provide a summary in a letter we will write
• People who have a balanced translocation of their chromosomes do not have a disease that can be "caught" by other people
• Guilt about possibly passing this on (not under our control can't cause or prevent)
• What do other family members know and do you plan on telling them
• Either parent considering having more children because this may alter their plans or could be upsetting to think they may have a recurrence risk if they didn't think so before

References and Patient Resources

• patient literature about translocations
• Chromosome deletion outreach support site
• Unique a UK rare chromosome disorder support group (similar to chromosome deletion outreach)
• information on translocations more advanced
• Dorothy