Mutation :
A change in the DNA at a particular Locus in an organism . The Term is used Loosely to include Point Mutations involving single gene change , as well as chromosomal change .Mutation Based on Location of Mutation :
1- Germ line mutations : are mutations occurring in gametes . 2- Somatic mutations : are mutations occurring in any cell in the body except germ cells . 3- Autosomal mutations : are mutations within genes located on autosomes (chromosomes 1- 22 ) . 4- X- Linked mutations : are mutations within genes located on the X- chromosome .Point Mutation : Involve the Addition or Deletion or Inversion or Substitution of a few bases with in genes
Point Mutation :
A-Silent Mutation : Triplet Codes for same amino acid : GCA GCC both code for alanine (amino acid). (Figure 2). B- Neutral Mutation : Triplet codes for different but functionally equivalent amino acid :AAA AGA changing basic Lysine to basic Arginine ( at many positions , will not alter Protein Function ) . C- Missense Mutation : a mutation that changes a codon specifying an amino acid to a codon specifying a different and non functional amino acid . D- Nonsense Mutation : a mutation that changes a codon specifying an amino acid to atermination codon . CAA UAA changing from a codon for Glycine to a termination codon .Mutation Based on Type of Molecular change :
Mutation Based on Type of Molecular change :
Frame Shift Mutation : Any Addition or Deletion of base pairs that is not a multiple of 3 result in a frame shift in DNA segments that code for Proteins . ( Figure 2 ) . gene Mutation can arise Spontaneously or they can be Induced : Spontaneous Mutations : arise from a variety of sources including : 1-Errors in DNA replication . 2-Spontaneous Lesions . 3-Transposible genetic elements . Induced Mutations : are produced when an organism is exposed to Mutagenic agent or Mutagen ( Radiation , Chemical agent ) .Thalassemia : Is a group of inherited blood diseases that affect a person's ability to produce hemoglobin in red blood cells . A person with thalassemia does not have enough hemoglobin or red blood cells, which can cause mild to severe anemia .
Hemoglobin : is a protein found in red blood cells . It carries oxygen to all parts of the body . Normal hemoglobin( also called hemoglobin A) has 4 protein chains, two alpha globin chains and two beta globin chains, and iron containing chemical group called heme .
Genetics : The genes that code for the globin chains of hemoglobin are found on chromosome 11 and 16 . Each person has ( 4 alpha globin genes located on chromosome 16 ) that code for alpha globin chains of hemoglobin( two from each parent ) and each person has ( 2 beta globin genes located on chromosome 11) controlling the production of beta globin chains of hemoglobin( one from each parent ) . This keeps the production of protein chains equal . Thalassemia occurs when a globin genes fails( mutation occurs) and the production of globin protein chains is thrown out of balance .
The genes that code for the globin chains of hemoglobin are found on chromosome 11 and 16 .
Types of Thalassemia : Thalassemia are classified into alpha or beta according to the affected globin genes alpha or beta . Alpha Thalassemia or α- Thalassemia : Alpha thalassemia occurs when one or more of the 4 alpha globin genes varies or missing. ( absence or deficiency of alpha chain synthesis) . Classification of alpha Thalassemia : 1- Silent carriers : People with only one gene affected and have no sign of illness . 2- Alpha Thalassemia Trait or ( Minor ) : People with two genes affected and have mild anemia and are considered carriers . 3- Hemoglobin H disease : People with three genes affected and have moderate to severe anemia . 4- Alpha Thalassemia major or ( Hydrops fetalis ) : Babies with all four genes affected, could not survive and usually died before birth
Alpha Thalassemia major or ( Hydrops fetalis ) :
Beta Thalassemia or β- Thalassemia : Beta Thalassemia occurs when one or both of the two beta globin genes needed for making the beta globin chain of hemoglobin are variantClassification of Beta Thalassemia : 1- Beta Thalassemia minor or ( Trait ) : People with only one gene affected, a person is a carrier and has mild anemia .Smaller red blood cells that are lighter in colour due to lack of hemoglobin . 2- Beta Thalassemia Intermediate : People with two genes variant and a person may have moderate anemia, and bony deformities due to bone marrow trying to make more blood cells to replace defective ones . 3- Beta Thalassemia Major ( Cooley's anemia ) : Complete absence of two beta globin genes, enlarged spleen, lightly coloured RBCs, severe anemia Beta Thalassemia major treatment by blood transfusion every month and iron chelating and bone marrow and stem cells transplant .Blood film in normal person and in β- Thalassemia major patient showing Hypochromia and Microcytosis : Normal
Thalassaemia
Blood film in β-Thalassemia major
Major
Inheritance of Thalassemia :1- If both parents have normal hemoglobin : all their children will be normal 100% . ( Figure 10 )
Both types of Thalassemia are inherited in the same manner Autosomal Recessive Inheritance :
2- If only one parent has Thalassemia trait ( also called carriers ) : 50% chance of having a child with thalassemia trait . and 50% chance of having a normal child ( Figure 11) none of the couples children will get thalassemia major .
Inheritance of Thalassemia :
3- If both parents have Thalassemia trait ( carriers ) : 25% chance of having a child with Thalassemia major 50% chance of having a child with Thalassemia trait ( carriers ) 25% chance of having a normal child ( Figure 12)
Inheritance of Thalassemia :
4- If a Thalassemic major marries a Thalassemic carrier : in each pregnancy there is a : 50% chance that the child will be Thalassemic major and a 50% chance that it will be a carrier . (Figure 13)
Inheritance of Thalassemia :
Thalassemic Mother
Normal Father
All children will be carriers
Inheritance of Thalassemia :
5- If a Thalassemic major married a normal : all the children will be carriers 100% . (Figure 14)
6- If one Thalassemic major marries another thalassemic major : all their children will be Thalassemics major 100% . ( Figure 15 )
Inheritance of Thalassemia :
Somatic Mutation or Cancer :
Cancer is a genetic disease that arises from mutations in genes controlling many aspects of cellular function . All cancer cells share 2 fundamental properties : 1- abnormal cell growth and division ( cell proliferation ) 2- and apropensity to spread and invade other parts of the body ( metastasis ) . ( figure16)Difference between cancer and other genetic diseases :
1- cancer is caused by mutations that occur predominantly in somatic cells. Only about (5 - 10% ) percent of cancers have ahereditary component. 2- cancers rarely arises from a single mutation, but from the accumulation of many mutations , as many as six to twelve . The mutations that lead to cancer affect multiple cellular functions, including repair of DNA damages, cell division, apoptosis, cellular differentiation, majority behavior, and cell to cell contact.Karyotype of cancer cell showing :
Translocations , Deletion , Aneuploid characteristic features of cancer cell. ( figure 17)Genetic Control of Metabolism :
Metabolism occurs by sequences of chemical reactions , each step of which is catalyzed by a specific enzyme . each enzyme is , in turn , specified by one or more genes . The genetic control of metabolic pathway may thus be diagrammed as in ( Figure 18) :When Mutations occur in genes, leads to the enzyme is not produced or canot function . Genetic disorders that result from loss of enzyme activity resulting from mutation in genes encoding these enzymes are called Inherited Metabolic Disorders. Example of Inherited Metabolic Disorders The metabolism of Phenylalanine and Tyrosine in Humans . ( Figure 19) .
Phenylalanine
PhenylketonuriaPhenylalanine hydroxylase
Tyrosine
Albinism(Tyrosinase –negative type) Tyrosinase
3,4Dihyroxy phenylalanine
-hydroxyphenylpyruvic acid Melanin
(Dark pigment in skin and hair)
-hydroxyphenylpyruvic Acid oxidase Tyrosinemia
2,5 Dihydroxyphenyl pyruvic acid (homogentisic acid)
Homogentisic acid oxidase
Alkaptonuria
Acetoacetic acid + Fumaric acid
Co2 + H2O
Phenylpyruvic acid
Albinism(Tyrosinase –positive type) (Toxic to Central Nervous System)
Tyrosine Tyrosinosis Transaminase
Inherited human disorders with defects in phenylalanine – tyrosine metabolism
1- Phenylketonuria: (PKU) :
The Failure of Brain to develop in infancy . PKU results from arecessive Mutation that causes aloss of Phenylalanine Hydroxylase activity which converts Phenylalanine to Tyrosine , the first step in the catabolism of Phenylalanine . As Protein is consumed Phenylalanine accumulates in the blood of individuals with PKU , some times up to 100 times the normal Level . as a result , metabolic derivatives of Phenylalanine , such as Phenylpyruvic acid , are formed . some of the derivatives are toxic to the Central Nervous System and produce irreversible Brain damage .
2-Alkaptonuria :
Is caused by a recessive Mutation that results in the Loss of activity of the Enzyme Homogentisic acid Oxidase . In the presence of this enzyme ( in normal people ) Homogentisic acid is normally oxidized and eventually converted to carbon dioxide and water . ( Figure 19 ) . In the absence of this enzyme Homogentisic Acid accumulates and is excreted in the Urine and turns urine black when exposed to air . Alkaptonuria with many symptoms illustrated in ( Figure 20 ) .Alkaptonuria
3- Albinism :In certain types of Albinism , the enzyme Tyrosinase is inactive or Lacking , resulting in ablock in the Pathway of conversion of Tyrosine to the Dark-colored Pigment Melanin 4- Tyrosinosis : Result from the lack of the Tyrosine Transaminase . 5- Tyrosinemia : Result from the lack of the enzyme called p- Hydroxyphenylpyruvic acid oxidase .
(Figure21 ) Albinism
( Figure 1)Human cell nucleus contain 46 chromosomes arranged in 23 pairs, 22 pairs autosomes and one pair sex chromosomes . The chromosomes can be separated into groups, based on their length and the position of the centromere.
Human Chromosomes
Karyotype : is a test to examine chromosomes in a sample of body cells, which can help identify genetic disease . This test can: Count the number of chromosomes Look for structural changes in chromosomes In a karyotype, chromosomes are arranged and numbered by size, banding pattern and centromere position from largest to smallest. This arrangement helps scientists quickly identify chromosomal alterations that may result in a genetic disorder.
Karyotype Test
Nucleus
Normal human male karyotype
Figure 1 Human chromosome groups
Clinical Indication for Chromosome Analysis : Problems of growth and development : development delay, mental retardation , multiple malformation . Fertility problems , couples with a history of infertility or recurrent miscarriage . Prenatal diagnosis of serious congenital diseases such as Down Syndrome especially in fetus of mothers ˃ 35 years of old Sex determination .Many hematologic and lymphoid malignancies (e.g., leukemia, lymphoma ) are associated with chromosomal abnormalities, which can help diagnose the disease andSpontaneous abortions and stillbirths and newborn deaths.* * Preparation of Karyotype : 1- Sample Collection : A - Prenatal Diagnosis : During pregnancy the sample can either be :
B - Postnatal Diagnosis :
Amniotic fluid collected during an amniocentesis
A piece of the placenta collected during a chorionic villi sampling test ( CVS )Blood sample
Bone marrow
( Figure 2 )
2 - Growing Cells : In order to have enough cells to analyze , the dividing cells are grown in special media or a cell culture . This media contains Phytohaemagglutinin that enable the cells to divide and multiply
3- Culturing cells can take 3 to 4 days for blood cells and up to a week for fetal cells . 4- Cell Division is arrested at Metaphase by the addition of colchicine ( prevents mitotic spindle fibers forming ) . 5- Hypotonic solution is added to cause the cells to swell and to separate the individual chromosome before fixation . 6 - Staining the Chromosomes : Giemsa dye G-banding ( Giemsa banding )
Preparation of Karyotype ( figure 3 )
G-banding ( Giemsa banding ) produces alternating light and dark bands , which are characteristic for each chromosome pair and which reflect different degrees of chromosomal condensation With G-banding :dark bands : appear to •(Rich in A and T base pairs ) •contain relatively few active genes •replicate late in S phase of cell cycle•contain more condensed chromatinLight bands : •( rich in G and C base pairs )•contain 80% for the active genes•replicate early in S phase .•have less condensed chromatin .There are 46 chromosomes that can be grouped as 22 matching pairs and 1 pair of sex chromosomes (XX for a female and XY for a male). The size, shape, and structure are normal for each chromosome.
Normal Human Karyotype:
There are more than or less than 46 chromosomes. The shape or size of one or more chromosomes is abnormal. A chromosome pair may be broken or incorrectly separated.
Abnormal:
The Result of Karyotype Test :
Normal female karyotype 46,XX
Application of Karyotype : Diagnosis of genetic disease because its usedto detect :A – Abnormalities in chromosome number B – Abnormalities in chromosome structure. C - Molecular diagnosis of cancer .
A- Abnormalities in chromosome number : Abnormalities in chromosome number resulted from nondisjunction nondisjunction : a chromosome pair fails to separate at anaphase of either the first or second meiotic division . This produces a sperm or oocyte that has two copies of a particular chromosome, or none, rather than the normal one copy . When such a gamete fuses with its partner at fertilization the zygote has either 45 or 47 chromosomes instead of the normal 46 .
( Figure 4) Nondisjunction of chromosomes in meiosis and fertilization
The causes of meiotic nondisjunction : It occurs at increased frequency with increasing maternal age ( women 35 years or old ). (figure 5) Exposed to toxins , viruses and radiation Familial tendency Spindle abnormality ( doesnt attach or pull correctly ) that cause nondisjunction . Result from gametes in which a homolog pair do not extensively cross over during meiosis 1 . Reduced frequency of recombination . Delayed movement of chromosome at anaphase (anaphase lag )
( figure 5 )
1- Down Syndrome ( Mongolism ) : A child with Down Syndrome has 47 chromosomes instead of 46 chromosomes because Trisomy for chromosome 21 ( three copies of chromosome 21 autosome ). ( Figure 6 and 7 ).
( figure 8 ) 47 chromosomes Trisomy for chr. 13 Nondisjunction
2- Patau’s Syndrome3- Edwards Syndrome : ( figure 9 ) Trisomy for chromosome 18
Nondisjunction and sex chromosomesNondisjunction and autosome chromosomes
(Figure7)
(Figure10)
4- Turner Syndrome : abnormal Female with 45 chromosomes , the sex chromosome is X chromosome only . ( Figure 10 and11 ).
5- Klinefelter Syndrome : Abnormal male with 47 chromosome, the sex chromosomes are XXY . ( Figure 10 and 12 )
6- Triple X Syndrome : Female with 47 chromosomes, the sex chromosomes are : XXX ( figure 10 and 13) .
7- Polyploidy : A cell with extra sets of chromosomes . The total number of chromosomes 69 chromosomes . The causes is error in cell division ( meiosis or mitosis ) and multiple fertilization Triploidy detected at amniocentesis 3 copies of each chromosome in every cell lead to spontaneous abortions and stillbirths and newborn deaths .
( figure 14 )
B - Abnormality in Chromosome Structure : Structural abnormalities results from chromosomal breakage and abnormal reunion . 1- Deletion 2- Duplication 3- Inversion 4- Translocation 5- Ring chromosome 1- Deletion : Cri Du Chat Syndrome : The syndrome associated with deletion of part of short arm of chromosome 5 . ( Figure 15 ) .
In Chronic Myelogenous Leukemia a reciprocal translocation between chromosome 9 and chromosome 22 forming Philadelphia chromosome It results in one chromosome 9 longer than normal and one chromosome 22 shorter than normal , the latter is called the Philadelphia chromosome This translocation is designated t ( 9 ; 22 )
A- Chronic Myelogenous Leukemia
Philadelphia Chromosome
4 - Translocation : The transfer of a segment of one chromosome to another chromosome . Examples of cancers associated with Translocation :
(Figure 26 )
( Figure 16 )
Philadelphia chromosome
The diagnostic Karyotype abnormality for Chronic Myelogenous Leukemia shows reciprocal translocation t ( 9 : 22 ) ( Figure 17 )In most (approximately 90%) of the cases of Burkitt's lymphoma, a reciprocal translocation between chromosome 8 and chromosome 14 This translocation is designated t ( 8 ; 14 ) leading to abnormal growth of lymphoid tissue and development of Burkitts lymphoma .
B- Burkitt Lymphoma
( figure 18 )
Here is an actual karyotype of a cell from the tumor of a patient with Burkitt's lymphoma. The long (q) arm of the resulting chromosome 8 is shorter (8q−) than its normal homologue; the long arm of translocated chromosome 14 longer (14q+). ( figure 19 )
Breast Cancer : Resulted from : 1- Mutation in genes of : (figure 20) a. BRCA1 (BReast Cancer gene one ) Located on chromosome 17
b. BRCA2 (BReast Cancer gene two) Located on chromosome 13 2- Abnormalities in chromosome number 3- Abnormalities in chromosome structure ( figure 21and 22 )
Breast Cancer ( figure 21 )
Karyotype of breast cancer cell showing translocations, deletions , and aneuploidy characteristic features of cancer cells .Several chromosomal changes ; abnormalities in chromosome number and structure .
Fig. 1. DNA repair and cancer. In many breast cancer cells, DNA double-strand breaks are inaccurately repaired. This results in gross genome rearrangement which results in cancer. Human Heredity Principles and Issues Eighth Edition Fig. 1 shows chromosomes of a breast cancer cell illustrating this point. Note that the cell is aneuploid, e.g., chromosomes 9 and 12 are present in three copies and the colors indicate that many chromosomes are the products of breakage and ligation from two different chromosomes. Consistent with this observation, the breast and ovarian cancer susceptibility genes BRCA1, BRCA2 and other tumor suppressor genes are involved in accurate DNA double-strand break repair. The lack of these gene functions results in inaccurate DNA repair, producing gross chromosome rearrangements that contribute to the development of cancer. Therefore, understanding the mechanisms of DNA double-strand break repair will contribute greatly to our understanding of cancer.
( figure 22)
5 - Ring chromosome : A chromosome loses telomeres and the ends fuse forming a circle .
( Figure 23 ) Mechanism of ring chromosome :
Human Karyotype from bladder carcinoma showing ring chromosome in addition to other chromosomal abnormalities . ( Figure 24 )* Abnormal numbers or sizes of chromosomes can lead to many genetic disorders , some are shown in this table , these disorders can be revealed by karyotype analysis .
Abstract