Chapter 5 - Genetic Disorders

Genetic disorders are far more common than is widely appreciated. The lifetime frequency of genetic diseases is estimated to be 670 per 1000.[1] Included in this figure are not only the

"classic" genetic disorders but also cancer and cardiovascular diseases, the two most common causes of death in the Western world. Both of these have major genetic components.

Cardiovascular diseases, such as atherosclerosis and hypertension, result from complex interactions of genes and environment, and most cancers are now known to result from an

accumulation of mutations in somatic cells ( Chapter 7 ).

The genetic diseases encountered in medical practice represent only the tip of the iceberg, that is, those with less extreme genotypic errors permitting full embryonic development and live

birth. It is estimated that 50% of spontaneous abortuses during the early months of gestation have a demonstrable chromosomal abnormality; there are, in addition, numerous smaller

detectable errors and many others still beyond our range of identification. About 1% of all newborn infants possess a gross chromosomal abnormality, and approximately 5% of individuals

under age 25 develop a serious disease with a significant genetic component. How many more mutations remain hidden?

The draft sequence of the human genome is complete and much has been learned about the "genetic architecture" of humans. Some of what has been revealed was quite unexpected.[2] For

example, we now know that less than 2% of the human genome codes for proteins, whereas more than one half represents blocks of repetitive nucleotide codes whose functions remain

mysterious. What was totally unexpected was that humans have a mere 30,000 genes rather than the 100,000 predicted only recently. Quite remarkably, this figure is not much greater than

that of the mustard plant, with 26,000 genes! However, it is also known that by alternative splicing, 30,000 genes can give rise to greater than 100,000 proteins. In addition, very recent

studies indicate that fully formed proteins can be sliced and stitched together to give rise to peptides that could not have been predicted from the structure of the gene.[2A] Humans are not so

poor, after all. With the completion of the human genome project, a new term, called genomics, has been added to the medical vocabulary. Whereas genetics is the study of single or a few

genes and their phenotypic effects, genomics is the study of all the genes in the genome and their interactions. DNA microarray analysis of tumors ( Chapter 7 ) is an excellent example of

genomics in current clinical use.[3] However, the most important contribution of genomics to human health will be in the unraveling of complex multifactorial diseases (discussed later) that

arise from the interaction of multiple genes with environmental factors.[4]

Another surprising revelation from the recent progress in genomics is that, on average, any two individuals share 99.9% of their DNA sequences. Thus, the remarkable diversity of humans is

encoded in about 0.1% of our DNA. The secrets to disease predisposition and response to environmental agents and drugs must therefore reside within these variable regions. Although small

as compared to the total nucleotide sequences, this 0.1% represents about 3 million base pairs. The most common form of DNA variations in the human genome is the single nucleotide

polymorphism (SNP). Typically, the SNPs are biallelic (i.e., only two choices exist at a given site within the population), and they may occur anywhere in the genome—within exons, introns,

or intergenic regions. Less than 1% of SNPs occur in coding regions. These could of course alter the gene product and give rise to a disease. Much more commonly, however, the SNP is just

a marker that is co-inherited with a disease-causing gene, due to physical proximity. Another way of expressing this is to say that the SNP and the genetic factor are in linkage disequilibrium.

Much effort is ongoing to make SNP maps of the human genome so that we can decipher genetic determinants of disease.[5] Just as genomics involves the study of all the DNA sequences,

proteomics concerns itself with the measurement of all proteins expressed in a cell or tissue. Currently, progress in proteomics is lagging behind genomics, because the methodology to

identify hundreds of distinct proteins simultaneously is not fully developed, but much effort continues.

Although genomics and proteomics are revealing a treasure-trove of information, our ability to organize and mine such a vast array of data is not yet fully developed. To simultaneously

analyze patterns of expression involving thousands of genes and proteins has required the parallel development of computer-based techniques that can manage vast collections of data. In

response to this, an exciting new discipline called bioinformatics has sprouted. This has involved biologists, computer scientists, physicists, and mathematicians, a true example of a

multidisciplinary approach in modern medical practice.[6]

Much of the progress in medical genetics has resulted from the spectacular advances in molecular biology, involving recombinant DNA technology. The details of these techniques are well

known and are not repeated here. Some examples, however, of the impact of recombinant DNA technology on medicine are worthy of attention.

• Molecular basis of human disease: Two general strategies have been used to isolate and characterize involved genes ( Fig. 5-1 ). The functional cloning, or classic, approach has

been successfully used to study a variety of inborn errors of metabolism, such as phenylketonuria and disorders of hemoglobin synthesis. Common to these genetic diseases is

knowledge of the abnormal gene product and the corresponding protein. When the affected protein is known, a variety of methods can be employed to isolate the normal gene, to

clone it, and ultimately to determine the molecular changes that affect the gene in patients with the disorder. Because in many common single-gene disorders, such as cystic fibrosis,

there was no clue to the nature of the defective gene product, an alternative strategy called positional cloning, or the "candidate gene," approach had to be employed. This strategy

initially ignores the biochemical clues from the phenotype and relies instead on mapping the disease phenotype to a particular chromosome location. This mapping is accomplished if

the disease is associated with a distinctive cytogenetic change (e.g., fragile-X syndrome) or by linkage analysis. In the latter, the approximate location of the gene is determined by

linkage to known "marker genes" or SNPs that are in close proximity to the disease locus. Once the region in which the mutant gene lies has been localized within reasonably narrow

limits, the next step is to clone several pieces of DNA from the relevant segment of the genome. Expression of the cloned DNA in vitro, followed by identification of the protein

products, can then be used to identify the aberrant protein encoded by the mutant genes. This approach has been used successfully in several diseases, such as cystic fibrosis,

neurofibromatosis, Duchenne muscular dystrophy (a hereditary disorder characterized by progressive muscle weakness), polycystic

kidney disease, and Huntington disease. In addition to this step-by-step approach to cloning single genes, cDNA microarray analysis allows simultaneous detection of thousands of genes and

their RNA products. When normal and diseased tissues are analyzed in this fashion, changes in the expression levels of multiple genes can be detected, thus providing a more comprehensive

profile of genetic alterations in diseased tissues.

• Production of human biologically active agents: An array of ultrapure biologically active agents can now be produced in virtually unlimited quantities by inserting the requisite

gene into bacteria or other suitable cells in tissue culture. Some examples of genetically engineered products already in clinical use include soluble TNF receptor for blocking TNF in

treatment of rheumatoid arthritis, tissue plasminogen activator for the treatment of thrombotic states, growth hormone for the treatment of deficiency states, erythropoietin to reverse

several types of anemia, and myeloid growth and differentiation factors (granulocyte-macrophage colony-stimulating factor, granulocyte colony-stimulating factor) to enhance

production of monocytes and neutrophils in states of poor marrow function.

• Gene therapy: The goal of treating genetic diseases by transfer of somatic cells transfected with the normal gene, although simple in concept, has yet to succeed on a large scale.

Problems include designing appropriate vectors to carry the gene and unexpected complications resulting from random insertion of the normal gene in the host genome. In recent wellpublicized

cases, gene therapy in patients with x-linked SCID (severe combined immunodeficiency, Chapter 6 ) who lack the common g chain of cytokine receptors had to be put on

hold because the transduced gene inserted next to a host gene that controls proliferation of cells. The resulting dysregulation gave rise to T-cell leukemia in the patient.

• Disease diagnosis: Molecular probes are proving to be extremely useful in the diagnosis of both genetic and non-genetic (e.g., infectious) diseases. The diagnostic applications of

recombinant DNA technology are detailed at the end of this chapter.

Figure 5-1Schematic illustration of the strategies employed in functional and positional cloning. Functional cloning begins with relating the clinical phenotype to biochemical-protein

abnormalities, followed by isolation of the mutant gene. Positional cloning, also called candidate gene approach, begins by mapping and cloning the disease gene by linkage analysis, without

any knowledge of the gene product. Identification of the gene product and the mechanism by which it produces the disease follow the isolation of the mutant gene.

Figure 5-2Schematic illustration of a point mutation resulting from a single base pair change in the DNA. In the example shown, a CTC to CAC change alters the meaning of the genetic

code (GAG to GUG in the opposite strand), leading to replacement of glutamic acid by valine in the polypeptide chain. This change, affecting the sixth amino acid of the normal b-globin

(bA ) chain, converts it to sickle b-globin (bS ).

Figure 5-3Single-base deletion at the ABO (glycosyltransferase) locus, leading to a frameshift mutation responsible for the O allele. (From Thompson MW, et al: Thompson and Thompson

Genetics in Medicine, 5th ed. Philadelphia, WB Saunders, 1991, p 134.)

Figure 5-4Four-base insertion in the hexosaminidase A gene in Tay-Sachs disease, leading to a frameshift mutation. This mutation is the major cause of Tay-Sachs disease in Ashkenazi

Jews. (From Nussbaum, RL, et al: Thompson and Thompson Genetics in Medicine, 6th ed. Philadelphia, WB Saunders, 2001, p. 212.)

Figure 5-5Point mutation leading to premature chain termination. Partial mRNA sequence of the b-globin chain of hemoglobin showing codons for amino acids 38 to 40. A point mutation

(CÕU) in codon 39 changes glutamine (Gln) codon to a stop codon, and hence protein synthesis stops at the 38th amino acid.

Figure 5-6Three-base deletion in the common cystic fibrosis (CF) allele results in synthesis of a protein that is missing amino acid 508 (phenylalanine). Because the deletion is a multiple of

three, this is not a frameshift mutation. (From Thompson MW, et al: Thompson and Thompson Genetics in Medicine, 5th ed. Philadelphia, WB Saunders, 1991, p. 135.)

TABLE 5-1-- Autosomal Dominant Disorders

System Disorder

Nervous Huntington disease

Neurofibromatosis *

Myotonic dystrophy

Tuberous sclerosis

Urinary Polycystic kidney disease

Gastrointestinal Familial polyposis coli

Hematopoietic Hereditary spherocytosis

von Willebrand disease

Skeletal Marfan syndrome *

Ehlers-Danlos syndrome (some variants) *

Osteogenesis imperfecta

Achondroplasia

Metabolic Familial hypercholesterolemia *

Acute intermittent porphyria

*Discussed in this chapter. Other disorders listed are discussed in appropriate chapters of this book.

Autosomal recessive disorders include almost all inborn errors of metabolism. The various consequences of enzyme deficiencies are discussed later. The more common of these conditions

are listed in Table 5-2 . Most are presented elsewhere; a few prototypes are discussed later in this chapter.

TABLE 5-2-- Autosomal Recessive Disorders

System Disorder

Metabolic Cystic fibrosis

Phenylketonuria

Galactosemia

Homocystinuria

Lysosomal storage diseases *

a1 -Antitrypsin deficiency

Wilson disease

Hemochromatosis

Glycogen storage diseases *

Hematopoietic Sickle cell anemia

Thalassemias

Endocrine Congenital adrenal hyperplasia

Skeletal Ehlers-Danlos syndrome (some variants) *

Alkaptonuria *

Nervous Neurogenic muscular atrophies

Friedreich ataxia

Spinal muscular atrophy

*Discussed in this chapter. Many others are discussed elsewhere in the text.

X-Linked Disorders

All sex-linked disorders are X-linked, almost all X-linked recessive. Several genes are encoded in the "male-specific region of Y"; all of these are related to spermatogenesis.[9] Males with

mutations affecting the Y-linked genes are usually infertile, and hence there is no Y-linked inheritance. As discussed later, a few additional genes with homologues on the X chromosome

have been mapped to the Y chromosome, but no disorders resulting from mutations in such genes have been described.

X-linked recessive inheritance accounts for a small number of well-defined clinical conditions. The Y chromosome, for the most part, is not homologous to the X, and so mutant genes on the

X are not paired with alleles on the Y. Thus, the male is said to be hemizygous for X-linked mutant genes, so these disorders are expressed in the male. Other features that characterize these

disorders are as follows:

• An affected male does not transmit the disorder to his sons, but all daughters are carriers. Sons of heterozygous women have, of course, one chance in two of receiving the mutant

gene.

• The heterozygous female usually does not express the full phenotypic change because of the paired normal allele. Because of the random inactivation of one of the X chromosomes

in the female, however, females have a variable proportion of cells in which the mutant X chromosome is active. Thus, it is remotely possible for the normal allele to be inactivated in

most cells, permitting full expression of heterozygous X-linked conditions in the female. Much more commonly, the normal allele is inactivated in only some of the cells, and thus the

heterozygous female expresses the disorder partially. An illustrative condition is glucose-6-phosphate dehydrogenase (G6PD) deficiency. Transmitted on the X chromosome, this

enzyme deficiency, which predisposes to red cell hemolysis in patients receiving certain types of drugs ( Chapter 13 ), is expressed principally in males. In the female, a proportion of

the red cells may be derived from marrow cells with inactivation of the normal allele. Such red cells are at the same risk for undergoing hemolysis as are the red cells in the

hemizygous male. Thus, the female is not only a carrier of this trait, but also is susceptible to drug-induced hemolytic reactions. Because the proportion of defective red cells in

heterozygous females depends on the random inactivation of one of the X chromosomes, however, the severity of the hemolytic reaction is almost always less in heterozygous

women than in hemizygous men. Most of the X-linked conditions listed in Table 5-3 are covered elsewhere in the text.

TABLE 5-3-- X-Linked Recessive Disorders

System Disease

Musculoskeletal Duchenne muscular dystrophy

Blood Hemophilia A and B

Chronic granulomatous disease

Glucose-6-phosphate dehydrogenase deficiency

Immune Agammaglobulinemia

Wiskott-Aldrich syndrome

Metabolic Diabetes insipidus

Lesch-Nyhan syndrome

Nervous Fragile-X syndrome *

*Discussed in this chapter. Others discussed in appropriate chapters in the book.

There are only a few X-linked dominant conditions. They are caused by dominant disease alleles on the X chromosome. These disorders are transmitted by an affected heterozygous female to

half her sons and half her daughters and by an affected male parent to all his daughters but none of his sons, if the female parent is unaffected. Vitamin D-resistant rickets is an example of

this type of inheritance.

Date: 2016-04-22; view: 593