Genetic Diseases. Notions of Genetic Pathology, Chromosomal Abnormalities, Diseases By

“Genetic diseases. Notions of genetic pathology, chromosomal abnormalities, diseases by multifactorial etiology,

genetic counseling”

for students of IV courses

havebeen elaborated MD, PhD, associate professor,Mariana Sprincean

Introduction

In an effort to identify individuals and families who might benefit from a genetic evaluation, it is necessary to have a basic understanding of some general genetic concepts. In this lesson the students will review the basics about chromosomes and chromosome abnormalities. There are sections on single gene and multifactorial disorders. The students will learn about traditional patterns of inheritance, imprinting, uniparental disomy, and other phenomena that result in nontraditional patterns of inheritance.

It is important to clarify at the outset the difference between chromosome abnormalities and single gene disorders. To understood this difference help the methods of laboratory diagnosis. Chromosome abnormalities are errors that result in an abnormal chromosome number or an abnormal chromosome structure. These abnormalities lead to the loss or gain of chromosome material- and we use the methods of chromosome analysis. All the genes within these chromosomes, however, are normal. Most chromosome abnormalities are sporadic with a small to negligible risk of recurrence. Of those that are familial, the risk of recurrence is usually less than 15%.

In the case of single gene disorders, the error lies in a mutation or change within the DNA sequence. For diagnosis of single gene disorders we using molecular genetics analyses. Errors that occur within a gene can result in absent, deficient or abnormal protein products. The chromosomes, or karyotype, of a patient with a single gene disorder are expected to be normal, 46,XX or 46,XY. Therefore, chromosome studies are not recommended for patients who are thought to have a single gene disorder such as cystic fibrosis or muscular dystrophy.

Abnormalities in chromosome structure occur when one or more chromosomes break and, during the repair process, the broken ends are rejoined incorrectly. Individuals who inherit a balanced chromosome rearrangement are physically and intellectually normal; however, they are more likely to produce chromosomally abnormal gametes. Individuals with balanced translocations often come to clinical attention following the birth of a child with a chromosome abnormality. They are also more likely to have miscarriages and may be identified if a chromosome study is done to determine the cause of the pregnancy losses.

Reference has been made to the dramatic explosion knowledge which has led to the recognition of over 6000 single gene traits and disorders. The majority of these are individually extremely rare. Some, however, are relatively common and their management in families has presented a major challenge for clinical genetics and closely allied specialities.

The cloning of the relevant genes, identification of their mutational basis and isolation of their protein products serve to illustrate important genetic principles and represent major scientific achievements.

Preconception and prenatal risk assessment tools have been developed to identify factors that might increase a woman's chances of having an adverse pregnancy outcome. Most tools include questions about a woman's age, health, nutrition, social situation, previous pregnancies and her use of cigarettes, alcohol and drugs. We would like to suggest that by adding a few additional questions the usefulness of these survey tools can be expanded.

The decision to opt for prenatal diagnosis is often difficult and is influenced by a couple's feelings about the type of information that is provided through testing and the available options they feel are appropriate considering their moral, ethical and religious beliefs. No prenatal test can guarantee a healthy child and there is no treatment or cure available for the vast majority of conditions for which testing is possible. Structural defects, such as renal agenesis, may not be apparent on ultrasound examination when early testing (CVS or EA) is performed. Many conditions can be identified in the second trimester by detailed ultrasonography in conjunction with amniotic fluid alpha fetoprotein screening or fetal karyotyping.

The availability of prenatal diagnostic tests may be limited in your area, and testing options will change over time. It is for this reason that we recommend you establish a working relationship with the genetics or prenatal diagnostic clinic staff in your area. They will provide you with information regarding the availability of, and the risks associated with, specific prenatal tests. Genetics or prenatal center staff are also available to consult with your patients should they have additional questions or concerns.

Objectives

The students will be able to know:

The basics of genetic pathology.
Classification of genetic pathology
Mutation (type of mutation, mechanism)
Pedigree Analysis (symbols, risk of hereditary)
Autosomal dominant condition. Characteristic.
Autosomal recessive condition. Characteristic.
X-linked inheritance. Characteristic.
Multifactorial conditions. General characteristics of diseases with genetic predisposition.
Congenital anomalies. Epidemiology and classification.
Teratogen, teratogenic agents, the critical period of exposure is during organogenesis
Differentiate between chromosomal, single gene, and multifactorial disorders.
Describe what might cause nontraditional patterns of inheritance.
Identify the influence that new mutations and susceptibility genes have on general health and well being.
Human karyotype. Classification of chromosome abnormalities;
Numerical abnormalities;
Structural abnormalities;
Down’s syndrome (trisomy 21): incidence, clinical features, recurrence risk;
Turner’s syndrome (45,X): clinical features, chromosome findings;
Klinfelter’s syndrome (47,XXY): clinical features, chromosome findings;
Trisomy 13 (Patau’s syndrome) and trisomy 18 (Edward’s syndrome) clinical features;
Portions aneuploidy (cri-du-chat(5p-), Wolf-Hirschhorn (4p) syndromes);
Microdeletion syndromes ( the Angelman and Prader-Willi Syndromes, DiGeorge and Shorintzen syndromes);
The fragile X syndrome: incidence, clinical features, the molecular defect.
Citogenetic measures (kariotyping, Barr test, FISH);
Methods of preparation of metaphase chromosomes:
Standard methods of chromosome’s preparations from different cells and tissues (peripheral blood, chorion villi samples, amniocytes);
Practical indication for investigation of sexual chromatin and human chromosomes.
Methods of DNA extraction of different cells and tissues (peripheral blood, chorion villi samples, amniocytes);
Schema and main components of PCR;
Preparing the polyacrylamide and agarose gel;
English genetics terms glossary.
Singel gene (mendelian) inheritance. Definitions, pedigree symbols and constructions , features- new mutations, reduced penetrance, variable expressivity, variation in age of onset,
Genetic heterogeneity, phenocopy, variation in severity dependent on sex.
Biochemical genetics: disorders of aminoacid metabolism-Phenylketonuria, disorders of steroid metabolism-congenital adrenal hyperplasia, lipoprotein metabolism- familial hypercholesterolaemia.
Neurofibromatosis- incidence, clinical features, mode of inheritance, mapping, NF1, NF2.
Cystic fibrosis- incidence, clinical features, confirmation of diagnosis, mapping, mutations in the CF gene, gene product, clinical applications.
Duchenne muscular dystrophy- incidence, clinical features, mode of inheritance, confirmation of diagnosis, carrier females, mapping, mutations in the DMD gene, gene product, clinical applications.
Sindrom Marfan- incidence, clinical features, mode of inheritance, confirmation of diagnosis, mapping, clinical applications.
Sindrom Ehlers-Danlos- incidence, clinical features, mode of inheritance, confirmation of diagnosis, mapping, clinical applications.
Hemophilia A and B- incidence, clinical features, mode of inheritance, confirmation of diagnosis, carrier females, mapping, mutations in the gene, gene product, clinical applications.
Primary prevention.
Secondery prevention.
Sceening program

: AFP- alfa fetoprotein,
human chorionic gonadotropin,
sceening for fetal anomalies using ultrasound,
other screening programmes

Diagnostic fetal interventions (obtaining fetal cells).
Studies performed on fetal cells: cytogenetics, biochemical test, molecular analysis.
Carrier testing.
Presymptomatic diagnosis.
Ethical considerations.

GENETIC DISEASES

Genetic diseasesrepresent pathological conditions determined by genetic factors, occurring as a consequence of errors (mutations ) of the hereditary material.

A genetic disorder results from a chromosomal abnormality or defective gene:

- numerical and structural chromosomal abnormalities,

- single gene disorder,

- multifactorial, or polygenic disorders: isolated and multiple congenital malformations, etc.

A congenitaldisease or malformationis any abnormality present at birth, even if not detected until after birth and encompasses all abnormalities caused by disturbed prenatal development, regardless of their nature.

Congenital defects are found in 2 to 3% of newborns. And 2 to 3% of developmental defects not recognized at birth become apparent as the child grows.

Major malformations are found in 25 to 50% of spontaneously aborted embryos, fetuses and stillborns.

Some geneticdisease, including single gene disorders are inherited.

A mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease.

Still other genetic disorders are due to problems with the number of packages of genes called chromosomes. In Down syndrome, for example, there is an extra copy of chromosome 21.

Hereditary transmission and teratological influence of certain factors during early periods of ontogenetic development, determines appearance of genotypic and phenotypic fatal changes to the fetus, and then to the newborn.

Teratogen refers to any agent that causes a structural abnormality following fetal exposure during pregnancy. Seldom, if ever, have teratogens been identified following designed epidemiologic studies. Usually an increased prevalence of a particular birth defect leads to the discovery of a teratogenic agent. For instance, Minamata disease, an encephalopathy that mimics cerebral palsy, was first recognized in towns surrounding MinamataBay in postwar Japan. This localized area of increased prevalence led to an investigation that pointed to methyl mercury, which was discharged into the bay by a local factory, as the offending agent. Ingestion of contaminated fish during pregnancy was the primary cause for this devastating condition. Similar results followed the ingestion of grain laced with methyl mercury, which was used as a fungicide, in Mexico and Iraq.

The sudden appearance of several cases of a rare disorder can also raise suspicions of teratogenicity. Cases of phocomelia in the early 1960's in Germany and Australia led to the identification of thalidomide as a human teratogen. Thalidomide was used to treat morning sickness, resulting in exposure at the stage in development when the embryo is most vulnerable, the first trimester.

Teratogenic agents include: infectious agents (rubella, cytomegalovirus, varicella, herpes simplex, toxoplasma, syphilis, etc.); physical agents (ionizing agents, hyperthermia); maternal health factors (diabetes, maternal PKU); environmental chemicals (organic mercury compounds, polychlorinated biphenyl or PCB, herbicides and industrial solvents); and drugs (prescription, over-the-counter, recreational). It may appear as though there are more suspected teratogens than were apparent a generation ago. This may be because there has been an increase in the number of synthetic chemical compounds in use or possibly the clinical recognition of subtle malformations as teratogenic effects. Examples of the latter would be fetal alcohol syndrome, fetal hydantoin syndrome, fetal trimethadione syndrome, fetal warfarin syndrome and smoking associated with low birth weight infants.

There are no absolute teratogens; however, many agents can exhibit teratogenic effects under certain circumstances. The dose and the time of exposure to a particular agent often determines the severity of the damage and the type of defect that occurs. The dose response is obvious: the greater the dose, the greater the effect. The time of exposure is another important concept, as certain stages of embryonic and fetal development are more vulnerable than others. In general, the embryonic stage (first trimester) is more vulnerable than the fetal period (second and third trimesters). Thalidomide provides a classic example. The critical period of exposure is during organogenesis (the formation of the organs) from the 35th-8th day after the last menstrual period. The specificity of the malformations is linked to the time of exposure: 35-37 days, no ears; 39-41 days, no arms; 41-43 days, no uterus; 45-47 days, no tibia; and 47-49 days, triphalangeal thumbs.

The types or severity of abnormalities caused by a teratogenic agent is also dependent on the genotype of the pregnant woman and the genotype of the fetus (genetic susceptibility). For example, variation in maternal metabolism of a particular drug will determine what metabolites the fetus is exposed to and the duration of exposure. Differences in placental membranes, placental transport and biotransformation all affect fetal exposure. The genetic susceptibility of the fetus to a particular teratogenic agent will also have an effect on the final outcome.

Absolute risk refers to the rate of occurrence of an abnormal phenotype among individuals exposed to the agent. Chronic alcoholic mothers, for instance, have a 45% chance of having an infant with fetal alcohol syndrome. Relative risk refers to the ratio of the rate of the condition among the exposed and the nonexposed. A relative risk of 2 means that smokers have twice the risk of having a low birth weight baby as nonsmokers. Keep in mind that a high relative risk may indicate a low absolute risk if the condition is rare. For a rare condition with an incidence of 1 in 100,000, a high relative risk of 10 still leads to a very low absolute risk of 1 in 10,000. Finally, attributable risk is the rate of the condition that can be attributed to exposure to the agent. Ninety percent of phocomelia patients have a history of thalidomide exposure in utero.

It goes without saying that some pregnant women need to be on medication. However, there are strategies to prevent or decrease the risk of fetal abnormalities: the use of the lowest dose possible, the avoidance of combination drug therapies (for the treatment of seizure disorders), the use of a different agent (heparin instead of coumadin for thrombophlebitis), the avoidance of first trimester exposures (preconception diabetes or PKU control), and folic acid supplementation. Reassurance of a low risk or negligible risk, by itself, is often welcome.

The public awareness of hazardous agents in the home and the workplace, and awareness of the teratogenic potential of drugs or medications during pregnancy has led to the establishment of teratogen information services in most states or regions of the country. This has been facilitated by the availability of national databases (Reprotox, TERIS), the teratology society organizations (OTIS - Organization of Teratogen Information Services) and the birth defect/genetics centers in most states.

Teratogens, agents that cause fetal injury with exposure during pregnancy, are found at home or the workplace. The effect is related to type of agent, dose and duration and time of exposure. The first half of pregnancy is the most vulnerable. Public awareness is essential for prevention.

Phenotype and Genotype

Phenotype is the visible trait (all the characteristics physical, morphological, physiological, biochemical and behavioral) that results in a particular genotype.

Genotype represent the hereditary information, embodied in the genes contained in chromosomes. Genotype is the combination of alleles that an individual possesses.

Phenotype is potentially variable.

Genetics is the study of heredity and variation

The transmission of traits from one generation to the next is called heredity or inheritance.

However, offspring differ somewhat from parents and siblings, demonstrating variation.

Genetic information is transmitted at several levels

Molecular – AND molecule
Morfological – Chromosomes
Cell's - Genetic apparatus

Mutations

A mutation may be defined as a permanent change in the DNA.

Mutations that affect the germ cells are transmuted to the progeny and may give rise to inherited diseases. Mutations that arise in somatic cells are important in the genesis of cancers and some congenital malformations.

Mutations may be classified into three categories:

Genome mutations – involve loss or gain of whole chromosomes (giving rise to monosomy or trisomy)

Chromosome mutations – result from rearrangement of genetic material and give rise to visible structural changes in the chromosome.

Gene mutations – may result in partial or complete deletion of a gene or, more often, affect a single base. For example, a single nucleotide base may be substituted by a different base, resulting in a point mutation.

Classification of genetic disorders

Chromosomal disorders (chromosomal abnormalities):

Defect is due to an excess or a deficiency in whole chromosomes or chromosome segments (trisomy 21,Turner syndrome, Klinefelter syndrome)

Single gene defects (Monogenic diseases):

Caused by individual mutant genes

Multifactorial inheritance: Combination of multiple genes and environmental factors. (Complex disease: diabetes mellitus, Crohn’s disease, Multiple sclerosis)

Mitochondrial disorders

Somatic mutations (cancer)

Autosomal dominant disorders (neurofibromatosis, tuberous sclerosis, polycystic kidney disease, familiar polyposis coli, hereditary spherocytosis, Marfan syndrome, osteogenesis imperfecta, achondroplasia, familiar hypercholesterolemia)
Autosomal recessive disorders (cystic fibrosis, phenylketonuria, homocystinuria, hemochromatosis, sickle cell anemia, thalassemia, alkaptonuria, neurogenic muscular atrophies)
X-linked disorders - (Duchenne muscular dystrophy, glucose-6-phosphate dehydrogenase deficiency, hemophilia)

Genetic disorders

Multifactorial (common)

- “Environmental” influences act on a genetic predisposition to produce a liability to a disease.

- One organ system affected.

- Person affected if liability above a threshold.

Single gene (1% liveborn)

- Dominant/recessive pedigree patterns (Mendelian inheritance).
- Can affect structural proteins, enzymes, receptors, transcription factors.

Chromosomal (0.6% liveborn)

-Thousands of genes may be involved.

-Multiple organ systems affected at multiple stages in gestation.

-Usually de novo (trisomies, deletions, duplications) but can be inherited (translocations).

Human genome

46 chromosomes

22 pairs of autosomes

XY male

XX female

Chromosomes

Normal human cells contain 23 pairs of chromosomes
This includes one pair of sex chromosome XX or XY
During cell division we can identify chromosomes
Haploid: set of 23 chromosomes
Diploid: normal number of 46 chromosomes
Aneuploidy: less than an even multiple of 23 usually is 45 or 47 and rarely 48,49
Triploidy: 69 chromosomes
Mosaicism
Abnormal in deletion and translocation (balanced and unbalanced)

Chromosome abnormalities account for 50% of all spontaneous miscarriages and are present in 0,5-1,0% of all newborn infants.