The ___________ population is at a ________ higher risk of T-S than the general population.
100-fold higher risk (1 in 3600 vs. 1 in 360,000 in gen.pop.)
Describe the phenotype of Tay-Sachs disease.
CNS neurons progressively destroyed
early onset (infancy), fatal
normal till 3-6mo. old —> fatal by age 3-4yrs
muscle weakness, decreased attentiveness, increased startle response
neurodegeneration: seizures, vision & hearing loss, diminished mental function, paralysis
Characteristic: “Cherry-Red Spot” on the eye
Some other high risk groups for T-S?
French Canadian (Quebec)
Old Order Amish (Pennsylvania)
What is the biological category of TS disease?
Lysosomal Storage disease
Patients with T-S disease are unable to degrade _________ leading to a ______-fold buildup of this ___________ in swolen ________.
In particular, the lysosomes in what kind of cell become swollen with sphingolipids?
Neurons in the brain and spinal cord
What protein is deficient in T-S disease and what does it normally metabolize?
Hexosaminidase A (HexA) metabolizes G_M2
HexA is a ________ of alpha/beta subunits which are encoded for by ______ and ______ repsectively.
HEX A & HEX B genes
What is another name for Tay-Sachs disease?
G_M2 gangliosidosis Type I
What is G_M2 gangliosidosis type II?
What are the symptoms of this disease?
Another lysosomal storage disorder
Symptoms the same as T-S disease. Neurological
What is the key biochemical difference between T-S disease and Sandhoff disease?
In TS the HEXA gene is unable to make the alpha monomer needed for the alpha/beta heterodimer that degrades G_M2
In Sandhoff disease, the HEXB gene is unable to make the beta monomer needed for BOTH the alpha/beta heterodimer and the beta/beta homodimer (which degrades globosides)
Which chromosomes are the HEX A and HEX B genes found on?
15 and 5 respectively
What is the AB-variant of T-S disease?
The AB variant is an activator deficiency. GM2AP gene (chr 5) synthesizes the activator protein for the alpha/beta heterodimer that degrades G_M2 ganglioside.
The HEXA and HEXB genes are both normal, but their heterodimer product cannot be turned on.
How many different HEXA mutations are known (T-S disease) and what is the most common ?
over 100 mutations are known
the most common one (80% of cases) is a 4bp insertion (frameshift) in exon 11 of HEXA (chr. 15)
–> premature stop codon (null allele)
How does one screen for T-S disease?
- Enzymatic activity assay
- Carrier screening
- Prenatal screening
- DNA testing
In prenatal T-S screening, what sort of sample is tested and what sort of disease reduction has this accomplished?
Test cultured amniotic fluid cells (if both parents are carriers, otherwise don’t bother right?)
Thsi test has reduced T-S incidence by 95% over the past 3 decades.
How accurate is the DNA test for carriers of T-S disease mutations?
95% accurate in Ashkenazi Jewish pop.
60% accurate in General Population
Is T-S always fatal?
There are juvenille and adult-onset forms of the disease. These pts. have reduced HexA activity.
Describe the phenotype of alpha1-Antitrypsin Deficiency (ATD)
Late onset (esp. non-smokers) 80-90% penetrance
Emphysema (esp. smokers),
increased risk of liver carcinoma
(accumulation of misfolded alpha1-AT protein in the liver)
What is the prevalance of alpha1-ATD and what is a high-risk group for this disease?
carrier freq 4%
more common in those with Northern European ancestry
is an underdiagnosed disease
What are the alleles associated with alpha1-ATD?
Mutant (2) Z and S
Z = bad S = less bad
What are the function levels of the various genotypes of alpha1-ATC?
Z/Z 10-15% of normal alpha1-AT activity
Alpha1-antitrypsin (ATT or SERPINA1) is made in the liver and secreted into plasma. What does it do?
SERPINA1 (serine proteas inhibitor - serpins) is a suicide substrate
It binds to and inhibits specific serine proteases. In particular, SERPINA1 likes elastase, which is released by neutrophils in the lung.
If there is not enough SERPINA1, then too much elastase accumulates and this will destroy connective tissue (elastin) in the lung –> alveolar damage & emhysema
How many different mutant alleles are there that cause alpha1-ATD and which are the most common ones? Which chromosome does the SERPINA1 gene live on?
about 20 possible mutant alleles. S & Z are most common.
SERPINA1 lives on long arm of chr. 14
What are the particulars of the worst allele? (the Z allele)
Z allele codes for a Glu342Lys resulting in a misfolded protein.
This protein then gets stuck in the ER of liver cells. Thus the liver is damaged (cirrhosis in addition to the lung damage). Since the protein aggregates in the liver, it never makes it to the lungs to do its job.
What are the particulars of the S allele? Why is it “less bad” than the Z allele?
S allele codes for Glu264Val which results in an unstable protein. This protein is functional but less effective –> milder symptoms
How might one go about screening for alpha1-antitrypsin deficiency?
Sequence Specific oligonucleotide probes can distinguish btwn M, S & Z alleles.
Provides accurate prenatal diagnosis.
What are the Ecogenetic factors associated with alpha1-antitrypsin deficiency?
Smoking = bad (duh)
In particular, in response to the damage caused by smoking, the body sends more neutrophils to the lung
neutrophils release elastase
—> more severe lung damage than just the normal amount of unchecked elastase.
What treatments are being developed for alpha1-antitrypsin deficiency?
Delivery of human SERPINA1 to the lungs by either intravenous infusion or aerosol inhalation
What do we call the existence of multiple mutant alleles of a single gene?
What do we call a population with a higher-than-expected risk for a particular autosomal recessive disease?
What do we call a person who carries two different mutant alleles for the same gene?
Can you give a specific example of this?
Ex: alpha1-antitrypsin deficiency genotype Z/S (or S/Z)
since Z is really bad and S is less bad, Z/S is middle bad
What is the technical term for when your parents are cousins?
…when parents share any degree of common ancestry
Describe the phenotype of a patient with PKU (phenylketoneuria).
if untreated during infancy:
profound intellectual disability
Neuro symptoms: seizure, tremor, gait disorders
Plasma levels: high phenylalanine, low tyrosine
“Mousey” odor due to high phenylalanine metabolites in urine and sweat
What is the incidence of PKU and what is the high risk population for this disease?
1 in 10,000
Norther Europeans are high risk
What is the primary molecules that are deficient in PKU?
inability to metabolize phenylalanine
PAH gene encoding phenylalanine hydroxylase is deficient Cofactor BH4 (tetrahydrobiopterin)
What does PAH do?
converts phenylalanine to tyrosine
what does BH4 do?
cofactor for 3 enzymes:
- PAH - phenylalanine –> tyrosine
- Tyrosine Hydroxylase - L-tyrosine –> L-DOPA
- Tryptophan Hydroxylase - tryptophan –> serotonin (pathway step)
When a patient is deficient for PAH (phenylalanine hydroxylase), what is the result?
In absence of PAH, phenylalanine is not converted to tyrosine.
Phe builds up and damages the central nervous system.
Phe will cause damage as the CNS is developing and interfere with mature brain function
Mechanism of damage in “unclear”
Describe a rare variant of PKU.
PAH is present and functional but its cofactor BH4 (tetrahydrobiopterin)
Since BH4 is a cofactor of PAH and the L-Dopa and Serotonin pathways, this disease variant has neurotransmitter imbalance in addition to the Phe accumulation.
What fraction of patients have the BH4 deficiency PKU variant?
Where does the PAH gene live?
Describe the gene mutations of the PAH gene that give rise to PKU.
partial or complete loss of function mutations
The PAH gene exhibits over_________ alleles. The fancy term for this is ____________ and most patients are ___________.
compound heterozygotes (two different mutant PAH alleles)
The severity of a patients PKU disease depends on what ?
The particular compound heterozygosity that they have.
Name two tests used to screen newborns for PKU.
- Guthrie test (Guthrie bacterial inhibition assay)
2. Mass Spectrometry
Describe the Guthrie test.
Thienylalanine inhibits growth of Bacillus subtilis (bactera)
If a patiens blood has a high level of phenylalanine, this growth retardation is overcome.
Thus, if a baby’s blood makes the bacteria grow = bad PKU
Describe the Mass Spectrometry test for PKU
mass spec. separates molecules in blood sample by weight and size. measures the amount of each molecule present
can measure many different molecules at one time.
What is important about timing the PKU test of a newborn?
At birth, a newborn’s PKU level is normal due to maternal PAH gettin’ the job done.
Within a few weeks the maternal PAH is gone and the newborn’s PKU kicks in.
Thus, the test screening is performed right at birth and then repeated within a week or two.
If screened too early (only) then cases of PKU can be missed.
If not screened soon enough again after birth, irreversible CNS damage will occur.
If a mother is a PKU patient and she is pregnant, what does she need to do to protect her baby and why?
Maintain a low-phenylalanine diet throughout pregnancy. If the maternal phenylalanine blood levels are high, they will leak into the baby and cause birth defects. Risk of miscarriage will be high.
High maternal Phe levels will cause damage even if the baby is not a PKU patient themselves.
Why does T-S disease affect mostly the central nervous system?
The ceramide oligosaccharides that TS is deficient in processing are found in all cell types, but are most abundant in the brain.
In particular ceramide oligosaccharides are found in the dendrites and axon terminals of neurons where they function as receptors for various glycoprotein hormones and bacterial toxins; they are also involved in cell differentiation and cell-to-cell interaction.
What does the alpha/beta HexA cleave?
The terminal N-acetylglactosamine from the GM2 ganglioside.
What does the diagnosis of GM2 gangliosidosis rely on when performing an enzyme activity assay?
reduced or absent HexA activity and normal to elevated HexB