GENETIC COUNSELING FOR NEUROGENETIC DISORDERS
Dr. I.C. Verma*
Consultant Geneticist, Sir Ganga Ram Hospital. Email : icverma@airtelbroadband.in *
In the era of human genome project with rapid advances in molecular genetics and increase in the knowledge about genetic disorders, genetic counseling has become an integral part of management of any disorder with genetic or probable genetic etiology. Neurogenetic disorders are one of the common causes of referral for genetic counseling. In view of the availability of genetic diagnosis for many of these disorders prevention by prenatal diagnosis is possible. Genetic counseling though complex at some levels is the integral aspect of prevention. In the present communication, we will discuss basic principles of genetic counseling and counseling in neurogenetic disorders.
Definition
The American Society of Human Genetics (ASHG) defined genetic counseling as a communication process that deals with the human problems associated with the occurrence or the risk of recurrence of a genetic disorder in a family. The process involves an attempt by one or more appropriately trained persons to help the individual or family to:
  1. Comprehend the medical facts including the diagnosis, probable course of the disorder, and the available management.
  2. Appreciate the way heredity contributes to the disorder and the risk of recurrence in the specified relatives.
  3. Understand the alternatives for dealing with the risk of recurrence.
  4. Choose a course of action which seems to them appropriate in view of their risk, their family goals, and their ethical and religious standards and act in accordance with the decision.
  5. Make the best possible adjustments to the disorder in an affected family member and/or to the risk of recurrence of that disorder.
Non-directiveness in genetic counseling
The main principle of genetic counseling is non-directiveness which is the art of presenting facts without influencing decision. It promotes the autonomy or self determination and personal control of the client. To maintain the sense of psychological well being amongst the clients, genetic counseling has also been defined as a dynamic psycho-educational process centered on genetic information. With a therapeutic relationship established between providers and clients, clients are helped to personalize technical and probabilistic genetic information, to promote self determination and to enhance their ability to adapt over time. The goal is to facilitate client's ability to use genetic information in a personally meaningful way that minimizes psychological distress and increases personal control. It promotes understanding, decision making, personal control, adaptation to stress inducing events and reduces psychological distress.

The main principle of genetic counseling is non-directiveness which is the art of presenting facts without influencing decision.

As it is evident genetic counseling is a complex process and ideally requires the following:
  • A correct diagnosis
  • A trained counselor with good knowledge of genetics and excellent communication skills
  • Psychosocial support to the family in coping up with the disorder and making decision with non-directive counseling
Moral, Social and psychological issues
All the available choices after counseling involve moral and social issues as each can generate an endless debate and there is no 'right' or 'wrong' choice. It is also difficult for people to understand the concept of prenatal diagnosis unless they realize the responsibility of controlling their own reproduction. There are issues of pre-symptomatic and prenatal diagnosis in late-manifesting neurogenetic disorders like Huntington's disease. There are issues related to termination of pregnancy both on ethical and religious grounds. The psychological trauma to the parents with affected children and affected individuals is tremendous and all efforts should be made to provide continued support.

1. Neurogenetic disorders:
There are a multitude of neurogenetic disorders (NGD) and the number is increasing due to identification of new genes. Counseling is easier if the inheritance is known but becomes complex if the etiology is multifactorial/polygenic. In the later situation, most of the times one has to resort to empiric risk figures for counseling.
  • Autosomal recessive: Majority of neurometabolic disorders (e.g., PKU, MSUD, Biotinidase deficiency, MMA etc), Lysosomal storage disorders (MPS other than Hunter syndrome, NCL, Gaucher's type 2 & 3, Niemann Picks disease etc), Neuromuscular disorders like SMA etc.
  • Autosomal dominant disorders: Huntington's disease, Myotonic dystrophy, Tuberous sclerosis, Neurofibromatosis, etc.
  • X-linked disorders: Duchenne muscular dystrophy, Fragile X syndrome, Menkes Kinky hair disease, Palezius Merzbacher disease, Ornithine transcarbamylase deficiency, Rett syndrome etc.
  • Mitochondrial Disorders: MERRF, MELAS, KSS etc., in single gene disorders, the risk of recurrence will depend on the type of inheritance. In an autosomal recessive disorder risk of recurrence is 25% for the sibling. In autosomal dominant disorder the risk of recurrence for the sib is 50% if one of the parents is affected. If it is a sporadic case (new mutation) risk of recurrence in sib is very low but gonadal mosaicism cannot be ruled out. In X-linked recessive disorders, risk of recurrence for boys is 50% where as females usually do not manifest.

For most of the common single gene disorders either enzyme based or molecular diagnosis is available though facilities for diagnosis of all disorders are not available in our country. Prenatal diagnosis is possible is by using the same techniques on fetal tissues Chorionic villus biopsy or amniotic fluid.

2. Chromosomal disorders:
Chromosomal disorders Majority of chromosomal disorders have associated mental retardation, e.g., Down syndrome, Edward syndrome (Trisomy 18), Patau syndrome (Trisomy 13). In all, de novo numerical and structural chromosomal abnormalities have a low risk of recurrence (< 1%). Risk is higher if either of the parents is carrying a balanced translocation. Prenatal screening for common chromosomal disorders has good sensitivity using maternal serum biochemical markers and ultrasonography. Definitive diagnosis can be provided by chromosomal studies on amniotic fluid, chorionic villus biopsy or cord blood sample.

3. Multifactorial disorders :
Common neurogenetic disorders in this group include Neural tube defects, Epilepsy, Autism, ADHD, Neuro Psychiatric illnesses like Schizophrenia, Idiopathic mental retardation etc.

Counseling and risk of recurrence in these disorders is based on empiric risk figures available in literature. Prenatal diagnosis is possible can only be offered in limited defects like NTDs.

4. Disorders for which both single gene and multifactorial / polygenic / chromosomal / syndromic etiologies are existing:
Common examples in this group include Lissencephaly, hydrocephalus, epilepsies, microcephaly, macrocephaly, holoprosencephaly, etc. This group probably poses the biggest challenge for the clinician.

A very careful family history and pedigree charting can give important clues. In addition, detailed examination is important to look for associated defects to make a syndromic diagnosis and decide about further investigations.
Prenatal Diagnosis
Prenatal diagnosis and selective termination of affected pregnancies is the only way to decrease the burden of many neurogenetic disorders. Prenatal diagnosis is increasingly becoming available for these disorders world over. Fortunately, many centers in India are also offering prenatal diagnosis for some of the neurogenetic disorders.

The pre-requisites for prenatal diagnosis are:
  • Confirmation of diagnosis in affected child or conformation of carrier status in parent(s) using cytogenetic molecular or biochemical techniques.
  • Counseling about risk of disease
  • Risk of the procedure is to be explained (i.e., chances of fetal loss)
  • Acceptability of medical termination of pregnancy should also be discussed

Definitive prenatal diagnosis for single gene disorders can be provided using molecular (Mutation detection/linkage) or biochemical tests like enzyme assays on appropriate fetal tissues. For chromosomal disorders fetal karyotype on Chorionic villus sample or amniotic fluid sample can be offered. It should be kept in mind that all invasive procedures have inherent risk of fetal loss and the family should also be counseled about this.

Structural abnormalities can be picked up by careful ultrasonography (neural tube defects, hydrocephalus, microcephaly). Recently fetal MRI has been used to detect structural brain malformations.
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