Marcus Pembrey

Publisher Summary

This chapter discusses the new techniques for analysis of DNA that can be used to detect specific mutations, or to at least track the transmission of the chromosome region that carries the mutant gene within the family. Some areas of medical genetics have moved rapidly from being largely academic to having, in addition, great practical importance in the management of a family at risk of further affected children. One such area is genetic linkage. Another area is the matter of genetic heterogeneity within what appears to be a single disease. A crucial distinction in deciding on the strategy used for genetic prediction is whether the mutations affect the same gene locus. Thus, genetic heterogeneity is primarily divided into allelic heterogeneity where a single gene locus is involved and nonallelic heterogeneity where separate gene loci—often on different chromosomes—are involved.

INTRODUCTION

It is now more than 100 years since Mendel’s death, and the word ‘gene’ has long been used by biologists, physicians and the informed public, yet direct analysis of human genes and the way they are arranged has only been possible in the last seven years or so. It is not surprising, therefore, that although most physicians have some idea of the different patterns of Mendelian inheritance, autosomal dominant, autosomal recessive and X-linked, they may have rather limited information on what genes actually are and the mechanism by which they dictate the structure of proteins.

Contemplation of how the activity of all the genes is orchestrated during embryological and fetal life to allow the development of a healthy newborn, let alone the continued short-term regulation of gene activity in metabolic homeostasis, is an awe-inspiring business. So awe inspiring, in fact, that it tends to inhibit the casual visitor to the land of molecular biology from even attempting to extract those principles that are relevant to his or her clinical practice. Some of the difficulties, of course, stem from our fragmentary knowledge of what actually goes on and, contrary to what one might imagine, it is an area where more detail can make it easier, not harder, to understand. There is, however, a simplification and that is that the study of genetic defects that show simple Mendelian inheritance, as most of the inborn errors of metabolism do, allows one to concentrate on a single gene locus. Admittedly the activity of other genes may modify the outcome, but for practical purposes the differences between those who have the disease and those who do not almost certainly resides in differences in the nucleotide sequence of the particular gene involved. Partial inherited disorders, such as many of the congenital malformations or diabetes mellitus, for example, present great difficulties because the genetic influence can vary from one case to another. Indeed the whole subject of multifactorial inheritance appears to be full of paradoxes if one tries to consider the genetic component separately from the environment in which the genes are operating.

This review will confine itself to the simpler task of single genes and the mutations in them that cause so-called monogenic disorders. This still means a variety of different mechanisms, for not only are different proteins encoded in the genome in a variety of ways but there are a number of different types of mutation that can occur. I will also confine myself to ‘constitutional’ mutations and not consider somatic mutations such as appear to underlie malignant transformation, for example.

Another purpose of this review is to outline the way in which the new techniques for analysis of DNA can be used to detect specific mutations, or if this is not practical to at least track the transmission of the chromosome region that carries the mutant gene within the family. Some areas of medical genetics have moved rapidly from being largely academic to having, in addition, great practical importance in the management of a family at risk of further affected children. One such area is genetic linkage discussed in the next chapter. Another is the matter of genetic heterogeneity within what appears to be a single disease, and here it will be argued that a crucial distinction in deciding on the strategy used for genetic prediction is whether the mutations affect the same gene locus. Thus genetic heterogeneity is primarily divided into allelic heterogeneity, where a single gene locus is involved, and non-allelic heterogeneity where separate gene loci, often on different chromosomes, are involved.

In discussing the different types of mutation that can occur I will draw heavily on the haemoglobinopathies, because much of Nature’s known repertoire can be illustrated within this single group of disorders affecting the adult haemoglobin molecule. Only time will tell whether the mutations that disrupt the genes encoding key enzymes in man’s metabolic pathways will turn out to be ‘more of the same’ or whether entirely novel disruptive mechanisms to the synthesis of normal proteins will be discovered. To date there seem to be two main consequences of gene mutations: the synthesis of an abnormal protein that may have an altered function, or the reduced or complete absence of synthesis of a particular protein. There are other possibilities, such as the primary excess synthesis of a gene product, the production of a normal protein at the wrong time during development or in an inappropriate tissue, but clinically significant examples of these are wanting at the present time.

THE STRUCTURE AND FUNCTION OF GENES