Docstoc

GENETICS OF CONGENITAL ADRENAL HYPERPLASIA

Document Sample
GENETICS OF CONGENITAL ADRENAL HYPERPLASIA Powered By Docstoc
					               DR TREVOR COLE
             CONSULTANT CLINICAL
                 GENETICIST
         BIRMINGHAM WOMEN’S
               HOSPITAL
        EDGBASTON, BIRMINGHAM




  GENETICS OF CONGENITAL
   ADRENAL HYPER PLASIA

Transcript prepared by Aarn & Vanessa Huisoon, edited by Sue Elford and checked for accuracy by
Dr Cole




CAHG 2007
                       THE GENETICS OF CAH



            Chromosomes, DNA, and Genes
                                                            Gene
                Nucleus
         Cell              Chromosomes




                                                                             Protein

    Adapted from Understanding Gene Testing , NIH, 1995




Geneticists now tend to be more proactive in the approach to counselling in diseases
such as CAH.

While the majority of the audience had undergone blood testing to identify the genetic
abnormality causing their or their child’s condition, only about half had seen a clinical
geneticist.

Dr Cole gave an overview of the principles of genetic inheritance and the genetics of
CAH. Everyone carries on average two or three genetic abnormalities of some sort.
In recessive conditions these do not cause a problem unless you have a child by a
partner who has a similar abnormality. This is called autosomal recessive inheritance.

Autosomal means that the condition can affect both sexes, while recessive means that
both copies of the genes (for example in CAH) are affected, and a faulty gene is
inherited from each parent. In autosomal recessive inheritance, there is a one in four
chance of a child suffering from deficiency of this gene if both parents are unaffected
carriers. In the UK about one in 35 to one in 50 people carries an abnormal gene for
CAH.
    Autosomal Recessive Inheritance

                                                               Noncarrier individual

                                                               Non -affected carrier

                                                               Affected individual




          ! Two germline mutations (one from each parent)
            to develop disease
          ! Equally transmitted by men and women




Once it is known that both parents are carriers of an abnormal gene, every child they
have has this one in four chance of being affected. This is random, so that it is
possible for a couple of carriers to have all normal children or all affected children or
any combination of affected and unaffected children.

Before considering a further pregnancy, it is important to know exactly what the
parents’ gene abnormalities as this makes it possible to test for the disease in the
foetus at a very early stage (by chorionic villus biopsy at about 11 weeks). CAH can
also be detected by an amniocentesis, although this is not usually performed until the
pregnancy is further developed (at about 18-20 weeks). However, as both tests are
associated with a small risk of miscarriage, many parents decide they do not want
prenatal testing, especially if it would not change any decisions they make about the
pregnancy.


                     Prenatal Diagnosis : -
                   CVS versus Amniocentesis
            !   CVS                               !   Amniocenetesis

            !   Early result 11 to 12             !   Lower miscarriage
                weeks (fetal sex 24                   rate (about 1 in 150)
                hours)

            !   Higher miscarriage                !   Late result - after 20
                rate ( about 1 in 75)                 weeks
One of the possible decisions in early pregnancy is whether to take dexamathasone to
reduce virilisation of a CAH female foetus. This needs to be started as soon as
possible in pregnancy, but only 1 in 8 foetuses will actually need it. This is not
known until the diagnosis is made; by chorionic villus biopsy. Dexamethasone
treatment is controversial. It is not predictable how much it will reduce virilisation in
any one pregnancy, and has potential for side effects for mother: hypertension,
diabetes, proteinuria and headaches, weight gain (cushingoid face), odema, striae and
mood fluctuations. However, to date all these effects have proved to be reversible.

There is also speculation that dexamethasone treatment could cause unwanted foetal
concerns, such as; it may reduce birth size and the sudden withdrawal could cause
foetal loss and it is not 100% effective. There is also a possibility that cardiovascular
disease, diabetes and behaviour patterns may affect the baby in later life, although
there is no proof to date of these effects.

The slide below shows the results of an 8 year study by Dr Maria New in the USA on
the effects of dexamethasone treatment in CAH.




                    DEXAMETHASONE THERAPY
                NEW ET AL 1995 (8 YEARS EXPERIENCE)

            !   239 PRENATAL TESTS

            !   37 / 239 AFFECTED PREGNANCIES

            !   21 / 37 FEMALE

            !   13 / 21 FEMALES TREATED

            !   TREATED CASES NO OR LESS VIRILISED



A parent affected with CAH will always pass on one copy of the abnormal CAH gene
to their child. That child will only have CAH if their other parent also carries an
abnormal copy of the CAH gene and passes it on to the child. Because about 1 in 50
people carry this abnormal gene, the risk of a CAH child having an affected baby
themselves is about one in 100. Because carriers of abnormal genes are relatively
common, partners of CAH patients are offered screening for the four common CAH
mutations. This reduces the risk of an affected child being born, but it is not possible
to completely exclude this risk.
                      CAH RISK TO THE NEXT
                          GENERATION
                     (slightly higher risk if use 1 in 35 carrier risk)
                                     AFFECTED CHILD
                               1 x 1/50 X 1/2 = 1/100 ( OR 1%)



                               SIBLING OF AN AFFECTED CHILD
                               2/3 X 1/50 X 1/4 = 1/300 (OR 0.3%)




                              UNCLE OR AUNT OF AFFECTED CHILD
                                1/2 X 1/50 X 1/4 = 1 / 400 (OR 0.25%)




A sibling of an affected child has about a one in 300 chance of having an affected
child themselves. This again can be reduced by testing the sibling for the abnormal
genes that the parents carry (preferably when they are old enough to understand the
consequences of this test). If they are positive, their partners can also be tested.

In discussion, the distant prospect of gene therapy was highlighted. While widely
publicised, gene therapy has not to date cured any genetic disease. The situation with
CAH is similar to that with cystic fibrosis, where gene therapy has been promised for
years but is still a long way from being a reality. The main problem is getting the
replacement normal gene into the correct cells (i.e. the adrenal cortex), which has not
been possible.