lecture 12- human genetics 2 Flashcards
Genetic origins of disease
Over 5,000 human diseases and disorders are presently known to have genetic origins
-Recessive gene: PKU, sickle-cell anemia, Tay-Sachs disease, cystic fibrosis
-Single dominant gene: Huntington’s disease, neurofibromatosis
-Polygenic inheritance: cancer, heart disease, asthma, psychiatric disorders, behavior disorders
-Sex-Linked inheritance: male-pattern baldness, red-green color blindness, hemophilia, Duchenne muscular distrophy, fragile-X syndrome
-Chromosomal anomalies: Down syndrome (trisomy 21), Kleinfelter syndrome (XXY), Turner syndrome (XO)
Regulator gene defects: genetic male with female genitalia
-Unidentified (poorly understood) genetic basis: autism spectrum disorder (ASD), Attention Deficit Hyperactivity Disorder (ADHD) (most likely polygenic
Tay-Sachs disease
Is a fatal, autosomal recessive neurodegenerative disease of infancy and early childhood
Rare in most populations
Caused by mutations of HEXA gene on chromosome 15q23-q24.
First identified in 1969
tay sachs population studies
Population studies and pedigree analyses suggest that mutations may have arisen from small founder populations:
Carrier frequency is 1:25 in Ashkenazi Jews
Same mutation is found in Cajun population in southern Louisiana
Two different mutations are common in French Canadians
Higher carrier frequency in Irish-American and Pennsylvania Dutch communities compared with general population
Incidence in unscreened Jewish populations is 1 in 3,900 births
tay sachs population screening continued
Internationally, screening has reduced the incidence of Ashkenazi Jews with TSD-affected children by more than 90%
Australian high-school-based preconception genetic screening programs help young people through screening and education
Aim is to optimise reproductive choices for participants
Choosing a partner not at risk
IVF, Donor gametes, adoption
Selective terminations
Child-free
Plans to extend the program into the more general Ashkenazi Jewish community
Huntington’s disease is a progressive neurodegenerative disorder- onset, prevalence
Autosomal dominant
Huntingtin gene isolated in 1993, on chromosome 4 at 4p16.3
> 40 repeats of CAG (cytosine, adenine, guanine) trinucleotide in coding region of gene
Mean onset ~40 yrs, death 12-15 yrs afterwards
Prevalence 5-10 per 100,000
hungtingtons disease symptoms and therapeutic research
Symptoms include Cognitive deterioration Personality change, memory loss and depression Choreic and slow movement Therapeutic research Mechanisms of neuronal death Cell replacement therapy
Chorea and bradykinesia characterise movement control in HD: involuntary
Involuntary ‘dance-like’ choreic movement
Abates in advance stages of disease, when akinetic and bradykinetic movements become clearer
voluntary
Voluntary movement
Bradykinetic (finger, saccades, gait)
Inconsistent and inefficient movement
Abnormal co-contractions of muscles
Prolonged EMG bursts
Less efficient and more variable movements in handwriting
Sequential and simultaneous movement difficulties
Broader sub-cortical and cortical damage in HD, compared with Parkinson’ Disease
Neuronal and astrocyte loss in the basal ganglia (caudate, putamen, global pallidus and other areas)
Selective degeneration of GABAergic neurons of striatum
As disease progresses, greater cortical atrophy occurs
Broader sub-cortical and cortical damage in HD, compared with Parkinson’ Disease
continued..
The gene contains an expanded trinucleotide repeat (CAG) that ranges from 9-35 in healthy adults, and from 36 to 180 in HD
Alleles with 36-39 repeats show “reduced penetrance”-only some individuals will go on to develop clinical symptoms
Individuals with juvenile onset (Westphal variant) usually have expansions >55 repeats and develop HD before 20 years
Onset appears to be earlier when the transmission is from the father
Schizophrenia
Fundamental and characteristic distortions of thinking, perception and affect that are inappropriate or blunted
Psychopathological thoughts
Hallucinations, paranoid or bizarre delusions, and disorganised speech and thinking leading to significant social and occupational dysfunction
Onset typically occurs in young adulthood
Affects ~ 0.5% of population
Course of schizophrenic disorders can be continuous or episodic
Not due to depression or manic symptoms unless it is clear that schizophrenic symptoms antedate the affective disturbance; not due to drug intoxication or withdrawal
Schizophrenia continued
Genetic risk for schizophrenia is more likely to be continuous than categorical
The last 5 years of genetic research has produced evidence that genetic risk for SCZ is largely polygenic
“The efforts to ground a categorical biomedical model of schizophrenia in Mendelian genetics have failed. The genetic risk for SCZ is widely distributed in human populations so that we all carry some degree of risk”
Fragile-X syndrome
FXS is the most common inherited form of intellectual disability
Expansion mutation of a CGG repeat sequence in the FMRI gene
Leads to silencing of the gene and absence of the gene product - the fragile X mental retardation protein
This protein is essential for synaptic plasticity, development of the shape of the brain, and cognitive development
Boys are typically more severely affected than girls
FMRI gene is on the X chromosome; girls have a second X chromosome (back-up)
Phenotype and genotype
Normal human development will only occur if a given gene is turned on and off at the correct time, in the right place and for the right length of time
Some genes are only turned on in a few cells and for only a few hours and then are switched off permanently (e.g. during development of the embryo)
Other genes are involved in the basic functioning of almost all cells almost all of the time
Regulator genes control the switching on and off of genes
External factors can affect the switching on and off of genes
There is a continuous interaction between the environment and the genotype
The range of reaction refers to all the phenotypes that could theoretically result from a given genotype, given all the environments in which it could survive and develop…
Range of reaction
The phenotype is the unique consequence of a particular genotype developing in a particular environment