Molecular And Biochemical Basis Of Genetic Diseases

Front 1

In cystic fibrosis, complete loss of function is generally ____(more or less)______ severe than partial loss of function.

Back 1

Less

Front 2

What are the three classes of mechanisms of mutations?

Back 2

Loss of function (point mutation / truncation / splice mutation)

Gain of function (much more RARE → inherited in DOMINANT manner)

Conditional mutation (activity of enzyme depends on temperature → EX: cardiac channels function differently based on temp, so fever would impact function.)

Front 3

Between loss of function and gain of function mutations → which is more rare? Which is inherited in a dominant fashion?

Back 3

Rare → gain of function
Dominant → gain of function

Front 4

Generally, in a _______ mutation, half the normal activity will suffice and carriers will have no clinical phenotype, although there may be a biochemical phenotype (i.e. the protein is NOT RATE-LIMITING). These types of mutations are normally inherited in a _______ manner. Give examples of diseases that fit this category.

Back 4

Loss of function
recessive

Examples: Tay Sachs; Cystic fibrosis; Sickle cell anemia

Front 5

Cystic fibrosis is caused by mutation in ________. There are several mechanisms that can lead to ineffective protein. Give examples of four.

Back 5

CFTR (cystic fibrosis transmembrane regulator)

Mutations can lead to:
• absent protein (point mutaiton, truncation, etc)
• defective processing (EX: delta 508 mutation → problem getting protein from Golgi to cell surface)
• defective regulation
• defective Chloride conductance (EX: R334W mutation)

Front 6

Metabolism disorders are almost always ________ inherited.

Back 6

Autosomal recessively
(i.e. there are so many proteins/enzymes involved → a little less activity tends to be OK)

Front 7

Enzymopahties are almost always ____(dominant or recessive)______. What are the three main pathological features of enzymopathies? What is the effect of a diffusible versus macromolecular substrate?

Back 7

Recessive

• Lack of products of the deficient enzyme
• Accumlation of upstream metabolites (substrate buildup)
• Shunting of accumulated metabolites into other pathways (ex: 5-alpha-reductase deficiency)

The substrate buildup itself may be toxic to the cell, but sometimes the excess substrate is broken down and THOSE products are what end up being toxic. Diffusible substrates (or substrate product) are more likely to have a more distant effect (EX: PKU, substrate is in liver but affects brain). Macromolecular substrates are more likely to affect the same organ (EX: hepatomegaly).

Front 8

Describe the biochemical pathway that results in hyperphenylalaninemia. Classical PKU is a deficiency of what? What is another cause of PKU and how can you distinguish between the two causes?

Back 8

Phenylalanine is converted to Tyrosine via the enzyme Phenylalanine Hydroxylase. BH4 is a cofactor in this conversion.

Classical PKU is a deficiency of phenylalanine hydroxylase.

A deficiency in any of the factors that synthesize//activate BH4 would also result in PKU, but it would also lead to ADDITIONAL (often neurological) SYMPTOMS. This is because BH4 is also the cofactor in the conversion of tyrosine to dopamine (and dopamine is then converted to NE and epinephrine). It is also the cofactor in the conversion of tryptophan to serotonin. Therefore, someone with a BH4 deficiency would show additional symptoms associated resulting from the loss of these 4 critical neurotransmitters.

Front 9

Is PKU newborn screening done via mutation analysis? Why or why not?

Back 9

No → there is a huge amount of allelic heterogeneity for this disease, which conforms very closely to ethnicity. There is no single mutation or even a small number of mutations that account for most of the disease. Therefore, the newborn screen involves strictly a metabolite screen (despite the genotype, everyone shares the increased levels of phenylalanine.)

Front 10

What are two treatment methods for PKU?

Back 10

Most important: Diet restrictions.

Also: A more recent treatment → administer high levels of BH4 to enable the endogenous PAH to work better.

Front 11

What is alpha 1 antitrypsin deficiency disease? What type of protein is alpha 1 antitrypsin? Where does the mutation most commonly occur in Caucasians? What are the two main clinical phenotypes and how does the mutation cause them? How does smoking affect the biochemical mechanism of this disease?

Back 11

Alpha 1 antitrypsin deficiency disease is an autosomal recessive condition that leads to chronic obstructive lung disease AND POSSIBLY cirrhosis of the liver.

Alpha 1 antitrypsin is a protease inhibitor that chiefly inhibits ELASTASE. Alpha 1 antitrypsin acts primarily in the lungs to inhibit elastase from degrading elastin and breaking down the lung parenchyma. The Z allele is the most common mutation in Caucasians. This particular mutation results in both a lung AND a liver manifestation of the disease. Alpha 1 antitrypsin is produced in the liver, so mutated “Z proteins” get stuck in the liver and lead to cirrhosis. (Note, a non-Z allele mutation would result in ONLY a lung manifestation.)

This disease is especially pronounced in smokers because smoke oxidizes the active site of alpha 1 anti-trypsin,. Any remaining good alpha 1 antitrypsin becomes out of commission, so the lungs get “chewed up” much faster.

Front 12

How is Duchenne muscular dystrophy inherited? What protein is lost? What does that protein do? What is the name of a less severe form of this disease? What type of mutation generally occurs, where does it occur, and how does the location of the mutation relate to phenotype?

Back 12

Duchenne muscular dystrophy is an X-linked disorder associated with the loss of dystrophin, which is a structural protein that anchors a large protein complex to the cell membrane to maintain muscle membrane integrity → it links actin to the extracellular matrix. Dystrophin is a huge protein that comes from one of the largest genes we have.

Becker’s muscular dystrophy is a less severe form. Histological stains will show more dystrophin protein in the muscles of kids with this kind of muscular dystrophy.

Since this is a huge protein, the mutation that renders the protein ineffective is nearly always a large DELETION. A deletion towards the end of the protein leads to a LESS SEVERE phenotype (Becker’s) than one that occurs in the middle or at the front (Duchenne’s).

Front 13

(ONCE AGAIN): Loss of function of a tumor suppressor gene is inherited in a _______ manner, but at a cellular level acts in a _______ manner. Why? Give three examples of this.

Back 13

Autosomal dominant (by looking at the pedigree)
Recessive
Because a second hit is required

EXAMPLES:
1.) BRCA1 and BRCA2
2.) Retinoblastoma
3.) Familial adenomatous polyposis (FAP)

Front 14

What is haploinsufficiency. Give an example. What are the inheritance patterns of this?

Back 14

Haploinsufficiency would be an instance when a loss of function mutation resulting in 50% normal activity is NOT sufficient for normal activity. So a phenotype WILL result, and these types of conditions are shown to be inherited in an autosomal dominant manner.

Maturity onset diabetes of youth (MODY) is an example of this. (Early onset type II diabetes.) → mutation is usually in GLUCOKINASE.

Front 15

What protein is usually mutated in MODY? What is the result of a loss of this protein?

Back 15

Glucokinase

A loss of glucokinase restricts the cell from trapping Glucose. As a result, the beta cells perceive a lower concentration of glucose in the body than actually exists. So they don’t produce insulin at the right level of glucose.

This is a very fine balance, so heterozygots will be haploinsufficient.

Front 16

What is Acute Intermittent Porphyria? How is it inherited? What is the phenotype? What gene is mutated? What famous person had this?

Back 16

Autosomal dominantly inherited.

Due to a deficiency of porphobilinogen deaminase (PBGD), which is needed to make heme. The mutation alters regulation of gene expression by 50% for heterozygots. Normally you’re fine and asymptomatic, except when your body is strained and more heme is needed (ex: a drug is used that decreases heme concentrations (barbiturates)). The result is intermittent neurological symptoms.

King George III of England (The Madness of King George)

Front 17

Give an example of a metabolic disease with a semi-dominant inheritance pattern with a gene dosage effect.

Back 17

The LDL receptor binds LDL and brings it into the cell. A LDL receptor deficiency leads to familial hypercholesterolemia. Heterozygotes and homozygotes both have elevated cholesterol and develop premature heart disease. Heterozygotes have an intermediate phenotype, homozygotes are much more severe.

Front 18

What gene is mutated in osteogenesis imperfecta? What is the inheritance pattern?

Back 18

A collagen gene is mutated, resulting in brittle bones and pathological fractures.

This is autosomal dominant negative inheritance (NOTE: dominant negative inheritance is most often seen in multimeric proteins.)

Collagen is a multimer (three strands tightly woven together), coded by two different pro-collagen genes → so mutations in one strand “poison” the multimer. The pieces that aren’t assembled into the fiber (because they’re mutated) end up getting post-translationally modified and secreted into the extracellular space. These abnormal fibrils (frayed strands, rather than tight woven strands) are what cause the brittle bones.

AGAIN → most severe forms of this disease come from De Novo mutations.

Front 19

What are the two mechanisms that can cause osteogenesis imperfecta (i.e. how does “Type I” differ from “Type II, III, IV”)?

Back 19

TYPE 1:
The collagen is made up of two pro-alpha 1 strands and one pro-alpha 2 strand. So if you have one alpha 1 gene with a LOSS of FUNCTION mutation, you still have another one that can make normal pro-alpha 1 collagen strands. So the result will be REDUCED amounts of collagen. This is the most mild form.

TYPE II, III, or IV
Instead of a loss of function, the mutation leads to a “TOXIC” but functioning allele. (GAIN OF FUNCTION, Missense mutation.) For example, instead of a Gly-X-Y set up, with Gly required for the tight turn, you get a larger amino acid in place of the Gly, and that completely messes up the collagen. If this mutation occurs in pro alpha 1, you’ll have a 1:3 ratio of good to bad collagen, which is enough to mess you up. If you have a toxic mutation in a pro alpha 2, they you’re completely screwed, because there is only one gene that encodes for pro alpha 2.

Front 20

Huntington Disease results from a _____(gain or loss)_____ of function mutation.

Back 20

Gain

Note: in general, triplet repeat diseases are often due to a novel function of the mutant gene product due to a polyglutamine repeat (CAG) that acts to kill the cells in which they are expressed. (Specifically in the brain.)

Front 21

What other disease listed in lecture results from a triplet repeat? Where does the repeat occur and what effect does it have?

Back 21

Myotonic Dystrophy

The repeat expansion occurs in the 3’ untranslated region, which encodes a protein kinase. The expanded repeat “sops up” the RNA binding proteins that should be working on splicing other genes. So the effect is actually on the OTHER proteins in the cell that don’t get properly handled.