Molecular Biotechnology: Principles and Applications - Glick B., Pasternak J. 2002

Molecular Biotechnology of Microbiological Systems
Human Molecular Genetics

Human health depends on a multitude of factors, including lifestyle, biology, environmental conditions, and the quality of healthcare. Over recent decades, significant progress in the Diagnosis, Treatment, and Prevention of infectious diseases has brought The impact of genetic factors into sharper focus, particularly in developed countries. For instance, statistical data from Canada show that up to 5% of the population under the age of 25 suffer from hereditary defects leading to disabilities, while more than 50% develop a condition with a hereditary component to varying degrees over their lifetime. Currently, over half of all pediatric clinic visits are related to Genetic Disorders.

There are over 1,000 known human genetic disorders. The majority of these conditions are extremely rare (~10^-5), though some occur relatively frequently (~10^-4). Many Monogenic Disorders are caused by Mutations in a single Gene, whereas A number of complex pathologies, such as Cancer, result from mutations across multiple genes. When a complete, precise, and consistent description of a disease's symptoms (its phenotype) is available for a single-gene condition, its genetic nature can be determined by analyzing its inheritance pattern across multiple generations within families. There are four primary inheritance patterns: autosomal dominant (Fig. 20.1), autosomal recessive (Fig. 20.2), X-linked dominant (Fig. 20.3), and X-linked recessive (Fig. 20.4). The term "autosomal" refers to the 22 pairs of non-sex Human Chromosomes (autosomes), whereas "X-linked" indicates that the gene is localized on the X chromosome. A trait is termed dominant when the presence of just a single mutant allele of the gene is sufficient for the disease to manifest, whereas in recessive disorders, both alleles must be defective. Because males carry only one X chromosome in their Cells' nuclei, the majority of X-linked genes—regardless of whether they are dominant or recessive—lead to the manifestation of the disease.

Pedigree analysis is extremely useful for establishing the inheritance pattern of a specific condition; however, it provides no information about the disease-associated gene, the biological basis of the disorder, or—in the case of autosomal conditions—the chromosomal Location OF THE gene. Furthermore, it is not always possible to determine whether a disease is strictly hereditary. First, not all individuals carrying a defective gene exhibit symptoms of the disease (incomplete penetrance). Second, symptoms (the phenotype) can range from mild to severe (variable expressivity). Third, the exact same phenotype can be caused by defects in entirely different genes (genetic heterogeneity). Fourth, in some cases, alternative forms (alleles) of the same gene can lead to different phenotypes. Fifth, due to The small size of families affected by the studied condition, data must be gathered from A large number of pedigrees to draw definitive Conclusions about The Nature of the disease. Successfully establishing a correlation between a normal or pathological phenotype, on the one hand, and its corresponding genotype, on the other, largely depends on identifying and isolating (cloning) the specific gene in question. Knowing The nucleotide sequence of a gene makes it possible to determine the normal function of its product, how that function is disrupted by mutation, and the extent to which various mutations in different exons contribute to the disease phenotype. Once a gene is cloned, experiments can be designed to establish the Functions of the gene product, its interactions with other molecules, and the Nature of the metabolic disruption caused by the mutant product. In addition, by comparing The nucleotide sequences of normal and mutant genes, diagnostic tests can be developed to detect specific mutations. The more comprehensive our understanding of the functions of a disease-causing gene, the more effective treatment strategies we can devise.

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Fig. 20.1. Autosomal dominant inheritance. Squares represent males, and circles represent females; filled symbols denote affected family members, and open symbols denote unaffected individuals. A horizontal line connecting a square and a circle indicates a mating pair. Vertical lines lead to their offspring, shown in birth order from left to right. Roman numerals (I, II, and III) designate generations, and Arabic numerals (1, 2, and 3) designate family members within each generation. A two-part alphanumeric code (e.g., II-3) is used to precisely identify any individual family member. The hallmark features of autosomal dominant inheritance are: 1) disease symptoms appear in successive generations given complete penetrance (i.e., when every genotype manifests phenotypically); 2) males and females are affected with equal frequency.

Fig. 20.2. Autosomal Recessive Inheritance. Hallmark features: 1) unaffected parents can have affected children; 2) males and females are affected with equal frequency; 3) if both parents are affected, all their children will also be affected (this scenario is not depicted in the illustration).

Fig. 20.3. X-linked dominant inheritance. Hallmark features: 1) assuming complete penetrance, affected individuals are present in every generation; 2) all daughters of an affected male are affected, while all his sons are unaffected; 3) subsequent generations frequently exhibit a "father-to-daughter-to-grandson" inheritance pattern; 4) the number of affected females may exceed the number of affected males.

Fig. 20.4. X-linked recessive inheritance. Hallmark features: 1) unaffected parents can give birth to affected children; 2) there is no direct father-to-son transmission of the disease; 3) affected males outnumber affected females.

Cloning human genes does not always follow a strictly predefined sequence of Procedures. Rather, researchers rely on a toolkit of diverse Methods and approaches applied according to specific circumstances. For example, the initial phase of searching for a disease-associated gene is guided by existing knowledge about its product. As a rule, identifying genes linked to various disorders requires genetic and physical maps, the construction of which ultimately contributes to mapping the complete nucleotide sequence of The Human Genome. A genetic (linkage) map indicates the relative positions of specific sites (loci) along a chromosome. To construct comprehensive linkage maps, loci on every chromosome must be represented by frequently occurring alleles that are easily identifiable. A physical map is a set of ordered DNA clones spanning an entire chromosome or a specific chromosomal region. In practice, these clones overlap to form a continuous sequence of fragments known as a contig. The length of a region covered by a contig is measured in Base Pairs. A physical map composed of contigs serves as the foundation for building the ultimate physical map, which represents the complete nucleotide sequence of the chromosome.¹⁾

¹⁾ The complete nucleotide sequence of each human chromosome has already been determined. Furthermore, a significant proportion of genes have been identified and their structures elucidated. (J.C. Venter et al., Science, 16 Feb, 2001, v. 291, No 5507, p. 1303; E. Lander, Nature, 2001, v. 409, No 6822, p. 860). — Editor's note.