Genetics And Pathologies Of The Ear

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How many pairs of chromosomes are in humans?

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23 pairs (total of 46)

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Where is Mitochondria located?

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Mitochondira are in the cytoplasm of the cell

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How much genetic material is in the mitochondria of each cell?

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A small amount of genetic material-37 genes (out of 30,000)

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What is Mitochondria responsible fore?

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Mitochondria are responsible for the energy production for the cells

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What is contained in mitochondria?

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Mitochondria contain thousands of copies of a circular chromosome composed of genes inherited only from the mother

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What good are genes?

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Genes code the production and function of proteins, which form the structural and regulatory elements of the body

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What are genes composed of?

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4 chemical bases of DNA:
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)

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What is mitosis?

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process of cell division in SOMATIC cells that results in the forming of two new cells

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What are somatic cells?

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Somatic cells are non-sex cells, such as skin

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Mitosis phases

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1) Interphase-resting
2) Prophase- centrioles will help seperate the cell
3) Metaphase
4) Anaphase
5) Telophase
--Centrioles will start pulling the cell apart; structures start lineing up.

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What is Meiosis?

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Meiosis is the process of cell division in sex cells that halves the number of chromosomes to produce haploid gametes (sex cells)-for reproduction

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What are haploid gametes?

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Sex cells

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Meiosis produces...?

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Meiosis produces haploid cells with different combinations of the parent genes- it is not identical!

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When does meiosis begin and end in females?

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Meiosis begins in the fetus and is completed with sperm fertilizing the oocyte

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When does meiosis begin and end in males?

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Sperm production begins at puberty and continues throughout the lifespan

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Meiosis I

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Meiosis I produces 2 haploid cells with 23 chromosomes each

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Meiosis II

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Meiosis II produces 2 copies of the haploid cells, for a total of 4 haploid cells

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Apoptosis

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programmed cell death

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Mendel's First Law

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Law of segregation-
the distribution of alleles of a gene into separate gametes during meiosis

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Mendel's Second Law

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For two genes on different chromosomes, the inheritance of one does not influence the chance of inheritance of the other

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Criteria for Autosomal Dominant Trait

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1) Males and females can be affected, and male-to-male transimission is possible
2) Males and females transmit the trait with equal frequency
3) Successive generations are affected
4) Transimission stops if a generation arises with no affected individuals

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Criteria for Autosomal Recessive Trait

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1) Males and females are affected
2) Affected males and females can transmit the trait
3) The trait can skip generations
4) Parents of affected individuals are heterozygous or homozygous with the trait

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Autosomes

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Chromosomes other than sex chromosomes. There are 22 pairs of automes in humans

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Sex chromosomes

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The X and Y chromosomes that are involved in sex determination

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Characteristics of Mitochondrial Inheritance

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1) All offspring inherit the trait
2) Both sexes affected equally
3) No transmission through a father
4) Reduced penetrance, variable expressivity
5) High rates of mutation because of lack of repair enzymes and free radicals generated by energy reactions

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When to suspect Mitochondrial dysfunction?

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1) There is no one identifying feature of mitochondrial disease
2) Patients can have combinations of problems whose onset may occur from before birth to late adult life

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Homoplasmy

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Homoplasmy is the presence of a mutation affecting all of the mitochondrial DNA (mtDNA) copies in a cell. Since there are hundreds or even thousands of mtDNA copies in every eukaryotic cell, mutations may either be present in all copies, homoplasmy, or affect only a fraction of them (heteroplasmy).

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Heteroplasmy

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Heteroplasmy is the presence of a mixture of more than one type of an organellar genome (mitochondrial DNA (mtDNA) or plastid DNA) within a cell or individual. Since every eukaryotic cell contains many hundreds of mitochondria with hundreds of copies of mtDNA, it is possible and indeed very frequent for mutations to affect only some of the copies, while the remaining ones are unaffected.

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Effect of Heteroplasmy

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1) Expressivity may vary among siblings based on the number of mitochondria with mutations passed to each
2) Severity depends on which tissues have cells with mitochondria mutations
3) Most severe mitochondrial disorders are heteroplasmy because homoplasmy may impair protein synthesis or energy production so much that embryo dies

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Genetic susceptibility to aminoglycoside otoxicity

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1) Matrilineal inhertiance
2) mitochondrial mutation
3) Mutation in combination with exposure to aminoglycosides results in rapid onset of hearing loss
4) Mitochondrial mutation A1555G in a rRNA gene

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Mitochondrial Deafness

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1) The A1555G mutation is also a frequent cause of deafness in Spanish and Asian populations WITHOUT exposure to aminoglycosides
2)A nuclear modifier gene on chromosome 8 was recently identified and is thought to account for the deafness in those individuals who do not have aminoglycoside exposure

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What is a modifier gene?

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A modifier gene is one that masks or modified the expression of another gene. Another term for this is epistasis. A modifier gene can suppress or enhance another genetic trait.

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Modifier gene for hearing loss?

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DFNM1

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DFNM1

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1) located on 1q24
2) inherited in a dominant pattern
3) is a suppressor of DFNB26
4) Identified in a Pakistani family in 2000
5) Individuals who were homozygous for DFNB26 mutations AND who had the DFNM1 gene were hearing

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Digenic inheritance

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1) Digenic inheritance includes two different genes at two different locations
2) Mutations in each of the two unlinked genes are present in one individual, and the combination of the two genetic traits causes disease phenotype that is not apparent if the individual only carries one of the gene alterations

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Digenic inheritance and hearing loss?

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the interaction of the GJB2 (connexin 26) and GJB6 (connexin 30) genes to cause deafness is thought to represent an example of digenic inheritance

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GJB6 (Connexin 30)

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Encoded by the GJB6 gene located on choromosome 13q11.12, just upstream of the GJB2 (connexin 26) locus.
Mutations in any two of the four alleles from connexin 26 or connexin 30 can result in deafness.
-specific interaction is unknown
*** Digenic inheritance

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Digenic inheritance study- Type I

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Type I is classified as: early onset, dominant, SNHL, cochlear, middle and high freq, sloping, bilateral, progressive, nonsyndromic hearing impairment, with two gene loci: 1p32 and 11q22-q24

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Digenic inheritance study- Type II

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Type II is classified as: middle age onset, dominant, SNHL, high freq dip, bilateral, progressive, non-syndromic hearing impairment while having only one of the chromosomal locations mentioned in Type I.

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Uniparental Disomy

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A very rare event in which a double dose of genetic material (a gene or a chromosome) is inherited from one parent, but none from the other

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Two types of UPD

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Isodisomy and Heterodisomy

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Isodisomy

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UPD
1) same chromosome is present in duplicate
2) occurs when failure of chromatids to separate occurs at second meiotic division

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Heterodisomy

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UPD
1) Two different chromosome homologs from one parent are present
2) Occurs when failure of normal disjunction occurs ate first meiotic division

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UPD Mode of transimission

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-Maternal or paternal
-occurs when a chromosome pair fails to divide correctly during meiosis
-several types of errors in chromosome segregation may be responsible

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UPD Mechanism Theory #1

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Trisomic Rescue- occurs when a fertilized ovum initially contains 47 chromosomes but loses one in the process of cell division; resulting daughter cells contain 46 chromosomes

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UPD Mechanism Theory # 2

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Gamete complementation- occurs when a disomic gamete (e.g. egg with 2 copies of a pair of chromosomes) is fertilized by a nullsomic gamete (e.g. sperm missing a chromosome from that pair)

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UPD Mechanism Theory # 3

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Somatic Recombination- can occur after fertilization in developing zygote/embryo causing mosaicism for cells with UPD and for normal cells

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Anticipation

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Anticipation is the clinical phenomenon that occurs when there is an earlier onset and/or increasing severity of a disease as the gene is transmitted from generation to generation
-Charcot-Marie-Tooth

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Charcot-Marie-Tooth Disease (CMT)

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The most common inherited peripheral neuropathy in the world, characterized by degeneration of peripheral nerves, resulting in distal muscle atrophy, sensory loss, and deformities of the hands and feet.
-Hearing loss occurs rarely
-cochlear and neural involvment (auditory neuorpathy)

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Mosaicism

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Mosaicism means there could be a situation where some of the cells have fewer than normal amount of chromosomes, while others have too many chromosomes
-it can also mean that a part/portion of the chromosome is missing

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How does Mosaicism occur?

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After fertilization, mitosis occurs - this is when the cell begins to divide
-- when mosaicism occurs, an error in cell division has occured

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Monosomy X- Mosaic Turner Syndrome

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-Caused by mosaicism
-Monosomy 45, X: having only one X chromosome in all cells
-Those with monosomy 45 X, had a more severe hearing loss and higher occurance of auricular anomalies and reoccuring OM

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Multifactorial inheritance

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-a trait, illness or syndrome determined by several genes and the environment
-traits inherited, learned or a combination of nature (genetics) and nurture (environment)

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Heterogeneity

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Several different genes result in one phenotype, e.g. deafness

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Phenocopy

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an environmental factor mimics a genetic condition that results in the same phenotype

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Pleiotropy

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One gene (or a pair of genes) causes multiple phenotypic effects on the body

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Genotype

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The specific genetic constitution of an organism: the allele combinations in an individual that cause a particular trait or disorder

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Phenotype

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The observable properties of an organism; the expression of genes in traits or symptoms

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Allele

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An alternate form of a gene

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Locus

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Place of a gene on a chromosome

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Homozygous

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having two identical alleles of a gene

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Heterozygous

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having two different alleles of a gene

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Homologous

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chromsomes with the same gene sequence

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Hemizygous

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the sex with half as many x-linked genes as the other; a human male

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Penetrance

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The percentage of individuals who possess a dominant gene and EXPRESS it

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Variable Expressivity

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Dominant genes- a genotype producing a phenotype that varies in severity among individuals

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Epistasis

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one gene masks or alters another's phenotype

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Genetic Code

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The genetic code is a set of rules, which maps DNA sequences to proteins in the living cell, and is employed in the process of protein synthesis

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Transcription of DNA to RNA to protein has four major stages

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1) Replication: The DNA replicates its information in a process that involves many enzymes
2) Transcription: The DNA codes for the production of messenger RNA (mRNA)
3) Processing: The mRNA is processed (essentially by splicing) and migrates from the nucleus to the cytoplasm
4) Translation: mRNA carries coded information to ribosomes. The ribosomes "read" this information and use it for protein synthesis.

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Three types of RNA

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Messenger RNA (mRNA)- copies the DNA strand
Transfer RNA (tRNA)- binds to amino acids and fits them to codons
Ribosomal RNA (rRNA)- forms the ribosomes where translation takes place

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Codon

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Codon is a sequential group of 3 mRNA bases that specifies a particular amino acid

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Transcription

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Sense strand- DNA strand that is transcribed
Antisense Strand- the DNA strand that is not used in transcription

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Control of Transcription

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Operons- genes which turn off or on transcription in bacteria through specific enzymes
Transcription Factors- activate transcription in eukaryotes by binding to DNA
RNA Polymerase (RNAP)- attracts complementary RNA nucleotides during transcription, builds the RNA chain

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Processing of mRNA

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Introns- noncoding regions of a gene, intervening sequences
Exons- coding regions of a gene code for amino acids
Ribozymes- small RNA molecules which cut out introns an join exons together

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Mutations

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Defined as a permanent change in the genetic material i.e. the nucleotide sequence or arrangement of DNA in the genome