Linkage analysis in families with aortic aneurysm

Aneurysms and dissections are the major diseases affecting the aorta. They lead
to substantial morbidity and mortality in case of rupture. In general AA is
considered to be a complex disorder, in which both environmental and genetic
factors contribute to its development. But there are families in which the
condition follows a mendellian inherited pattern. Identification of the genetic
defect in these families will greatly contribute to the understanding of the
pathology of AA formation. In addition, this will be of great value for the
family members. With minimal strain, they can then be screened for having the
genetic predisposition. Subsequently, they can either be reassured, or be
admitted in an inndividualized surveillance program (see burden and risks).

The aim of the study described in this protocol is the identification of genes
involved in aortic aneurysm development. This will be achieved by
characterization of large AA families, including pedigree investigations,
diagnostic measurements (echocardiograms and abdominal ultrasounds), linkage
analysis and sequencing of candidate genes.

Patients
AA patients that visit their specialist (vascular surgeon or cardiologist) will
be asked whether an AA runs in their family. This a general question, because
of the inherited nature of AA. If the family has at least 3 known cases of AA,
the patient is informed about the study and receives patient information from
the specialist. He-she is asked if the specialist can provide contact
information to the researcher. If the indexpatient agrees, the researcher will
invite the patient for a consult at the Medical Genetics department in the UMC
Utrecht. The study will be explained in detail, and informed consent is
obtained. With help of the patient, an initial pedigree will be constructed.
Subsequently, the indexpatient is asked to inform his-her family members about
the study, and to give them the patient information. If they are interested in
participating, they can contact the researcher. They will be invited for a
consult at the Medical Genetics department in the UMC Utrecht. The study will
be explained in detail, and informed consent is obtained. Family members of
whom it is unknown whether they have an AA will be invited for an ultrasound
examination or for an echocardiogram in the hospital from the indexpatient (for
now the Antonius hospital and the Erasmums MC respectively). With all the
obtained information, the pedigree will be finalized.

Methods
Once the pedigree is finalized, blood will be obtained from all family members
whom have given informed consent and DNA will be isolated. We will use
Illumina*s SNP-based linkage panel V, which includes 6056 SNPs, to conduct
linkage analysis using an *affected-only* approach. Both parametric and
non-parametric analysis will be carried out with the Merlin and Simwalk
programs. Haplotypes of LOD scores>2 will be fitted to the full family pedigree
using Cyrillic to check for the consistency of shared haplotypes of affected
family members. Subsequently, the boundaries of the locus with the best
solution will be determined. We will determine which genes in the locus are
good candidate genes based on the current pathophysiological knowledge, and
these will be sequenced. If possible, all genes in the locus will be sequenced.

Burden and risks are minimal (1x blood withdrawal, diagnostic test).
Identification of the causal gene in AA families will be of great value for the
family members. With minimal strain, they can then be screened for having the
genetic predisposition. Subsequently, they can either be reassured, or be
admitted in a specialized surveillance program. This program will be carefully
composed by the specialist, based on the age of the patient, and the family
history of AAs (age of onset, age of rupture). For example: patients of 40
years and older with the genetic predisposition will be invited for a yearly
ultrasound examination. If the diameter of the aorta does not increase within
5 years, together, the specialist and patient can decide to extend the
intervals between the examinations. If the aorta enlarges, shorter intervals
between the examinations may be required. If surgery is required, based on
diameter or on velocity of growth, proper action can be undertaken.
The knowledge of having the genetic predisposition, or of having an AA, will
have psychological and medical consequences. This will be emphasized during the
consult with the researcher, prior to giving informed consent (in which the
patient has to indicate whether he/she wants to know if he/she has the genetic
predisposition). Furthermore, during ultrasound examination, other disorders
can be discovered. The knowledge of this may also have psychological and
medical consequences. The patient is informed about this, and asked if he/she
wants to know about these potential other disorders. If not, the patient can
not be included in the study.