Testing and Quality Issues in Huntington's Disease Flashcards
Describe predictive (presymptomatic) testing for HD.
Predictive testing referrals are only accepted from clinical genetics departments.
The CAG repeats can be sized in an asymptomatice individual who wishes to determine if he or she carries the HD mutation because of a family history of the disease.
Predictive testing can be offered to individuals at 50% or 25% prior risk.
Predictive testing involved comprehensive genetic counselling and informed consent to be established prior to molecular genetic testing.
BGPs recommend having molecular genetic confirmation of an HD diagnosis in an affected family member. For example DNA from an affected family member, and/or a copy of the molecular genetics report. If no report is available then a caveat must be included in the report outlining this. For example, there could be another diagnosis in the family that is not HD.
Describe prenatal testing and preimplantation genetic diagnosis (PGD) for HD.
All referrals for prenatal testing must come from the clinical genetics department.
CAG repeat sizing methods are utilised if the relevant parent already knows their status. A positive control sample from the parent must be included in the PCRs.
Direct testing of the low risk parent should be avoided if possible as their is a risk of detecting alleles in the intermediate size range.
As with all prenatal testing there is the possibility of maternal cell contamination due to tissue sampling. If there is MCC then this may result in a false negative result. Therefore it is essential to check for MCC by testing both fetal and maternal DNA with markers to check for this contamination. MCC results must be included in the report.
Preimplantation genetic testing (PGD) may be an option for couples who wish to avoid transmitting the disease but without revealing their own status, or to avoid termination of a pregnancy. This is only offered in a limited number of specialised centres.
Exclusion testing may also be possible in the prenatal testing scenario.
Describe exclusion testing.
If a person with an affected grandparent wants to know if her or she has an increased risk of developing HD but their linking parent does not wish to know his or her status then an exclusion test can be proposed. These days it is almost exclusively used as a prenatal test.
Referrals are only accepted from clinical genetics departments and individuals require comprehensive genetic counselling.
In the pre natal setting exclusion testing can be utilised when the foetus is at 25% risk, and the future parent is at 50% risk, but does not want to be informed of their HD status. Using the direct method in such cases would be equivalent to performing an unauthorised predictive test on the at risk parent.
Exclusion testing uses polymorphic markers closely linked to the HTT gene.
Exclusion testing is based upon identifying the grandparental origin of the 4p16.3 region in the foetus. If one of the 2 4p16.3 haplotypes from the affected grandparent is found in the foetus then the foetus has an HD risk of approximately 50% and a termination of this pregnancy may be considered by the parents. If the foetus has not inherited the 4p16.3 haplotype from the affected grandparent then this risk is decreased to almost 0%.
Exclusion testing requires DNA from at least 1 grand parent that is a parent of the individual at 50% HD risk. A diagnosis of HD should have been confirmed previously by molecular testing in at least 1 family member and ideally in the affected grandparent.
An important feature of exclusion testing is that analysis should not reveal which of the affected grandparental haplotypes is associated with the HD mutation. Informative markers from the family will ideally be established before prenatal testing is required.
Other factors to bear in mind with exclusion testing are that the risk of recombination across the HD gene is approximately 2%. Testing may not be informative if there are insufficient markers available for the family and there is a very small risk of double recombination that would result in an affected individual with the low risk haplotype. This must be included as a caveat in the report to the clinician.
Parents of a foetus that has inherited the 4p16.3 haplotype from the affected grandparent may decide to terminate the pregnancy based on these results. This means that there is a 50% chance that they will be terminating an unaffected pregnancy.
Describe some of the quality issues that affect HD testing/
1) . It is essential to use sizing controls when carrying out any of the HD tests in order to compare the test cases and to determine an accurate allele size.
2) . CMGS BGPs give useful recommendations for testing and reporting HD cases.
3) . The lab takes part in external quality assurance schemes through NEQAS and EMQN to test the accuracy of genotyping, correct interpretation and reporting. Typically, 3 samples are distributed to the lab annually along with the referral details. The samples are then tested in the lab and the reports are returned to the scheme organisers who will score them. Labs are expected to participate in an available scheme fro each disease service they provide.
