Pathology: Genetics IIIpg. 1
Jenny Seibert
Deleted “Distruption” Slide: Disruption is an interruption of a normally proceeding pathway. The tissue that was first there was perfectly normal, everything was proceeding as scheduled and then something came in and interfered with its development. It's something usually external to the fetus.
There are two examples. In this first one, the colon is atretic. At the part of the bowel that is dilated, it is occluded and the rest of the bowel that is distal to the occlusion is thin and underdeveloped. This happens when there is low blood flow to an area that is developing. Small intestinal atresia or colonic atresia, these happen when intrauterine blood flow has been impaired. The gut is more susceptible because it is the first place blood flow is limited (in order to be shunted to more important body parts) if there is massive loss of blood or if we become scared and have to run from scary lions (more blood flow to the skeletal muscles). The same reflex happens in the fetus.
The second is amnion disruption sequence. If you'll remember what sequence meant: you have an event and from that one event, you can trace multiple things in a series. In this case the amniotic sac gets a break and fluid leaks out. The material of the sac that leaks out is relatively sharp and stiff relative to the gelatinous fetal body. The baby will put his/her hands, feet, arms and legs out into the extra amniotic space. The strands from the sac get caught around the baby extremities and cause ischemia. The strands can also be swallowed. The results can be dramatic such as a split palate or you can just lose fingers and toes.
How do you know that amniotic disruption is a sequence? In syndromes the abnormalities are bilateral and symmetrical. Sequences are asymmetric. It doesn't affect all the digits in the same way nor does it affect each side of the body in the same way.
Slide 1: Now we are going to start with autosomal recessive disorders and we've only picked a few to go over that have dental or ocular manifestations. There are a lot of these disorders, but we are only going to talk about a select few.
In autosomal recessive disorders, you need two copies of the gene in order for the abnormality to be expressed. In autosomal dominant disorders, you only need one copy of the gene. In autosomal recessive, you need two copies.
The disorder is completely penetrant ie it is readily visible that the person is affected. Everyone who has the two recessive genes has the trait. There is little variability in its expressivity as in severity or multiple sites affected. Most individuals with an autosomal recessive disorder look the same, one to one.
Both males and females can transmit the condition.
The parents are not affected because they each carry only one recessive gene. Only their children can have both of their recessive genes, one out of four times. The recurrence rate is 25%.
If there is consanguinity within the family or community, or inter-marriage, the trait will be more apparent and common.
In dominant autosomal disorders, a structural protein gets modified way down the line somewhere and autosomal recessive disorders involve enzymes. So there is no production of an enzyme or production of an enzyme that cannot do its job.
Slide 2: As I've said, there are a number of autosomal recessive disorders. We are going to cover a few of them, a couple of the lysosomal storage diseases and one of the glycogen storage diseases.
We already mentioned a variant of Ehlers-Danlos, which is autosomal recessive, which has ocular fragility. So one group of disorders won't always have the same inheritance pattern.
Slide 3: So what are lysosomal storage diseases? The lysosome is the organelle in the cell that is the breakdown factory site. Something that is supposed to be a product or that is made in the cell then has to go through a breakdown. Once that cell goes through a cell cycle or if it goes senescent, a product is taken into the lysosome and broken down, bit by bit. Its parts are recycled for use again in building of other units the cell will need.
In lysosomal storage disease there is a deficiency in hydrolases. These hydrolases are active in an acid environment, they're called acid hydrolases. They basically cleave off the -OH portion of the molecule. There are many of them and they all have specific functions.
Because this enzyme is a protein and because proteins have many parts, there are many ways its production can be interfered with. The disease can be due to a defect in any portion of the protein that affects enzyme function.
There is a natural turnover of cells, more so in the brain, in the development of the fetus. There is a high turnover of blood cells and endothelial cells and in smooth muscle cells. If a particular enzyme is missing and cannot work in the catabolic factory, then whatever the particular substrate is, it is going to pile up in the lysosome and the lysosome is going to swell.
We are going to concentrate on LSD that occur in the neurons and macrophages or the blood monocytes. Monocytes and macrophages are responsible for breaking things down, they are part of the immune system, so they have a lot of lysosomes. LSD will show up depending on which enzyme is missing and most of the time, in some form or another, in a macrophage.
4: The amount or functional integrity of the enzyme is affected. Normally the substrate is broken down bit by bit and then expelled from the cell (she is describing the top image of the slide, the one with no pile up of substrate). If there is a defect in the enzyme, then the substrate or product, depending on where is is in the cell, piles up.
5: There may be problems in the synthesis of the enzyme so where it is made or how it is packaged may be affected and they all are recessive disorders.
6: There are several LSD that affect the CNS. They affect the eyes. So whether or not you see one of these patients (they are extremely rare), there are questions on the boards about them!
All of these disorders (Tay Sachs, Niemann-Pick and Gaucher) are characterized by neuronal swelling and dysfunction. Some of them cause swelling of the liver and spleen because macrophages generally live in the liver, spleen, and lymph node. Depending on where the disorder may be, the majority of the problem will affect these cells.
In Tay Sachs, the biggest problem is neuronal swelling and dysfunction. In Niemann-Pick, it's both the CNS and the liver and spleen. In Gaucher, it's less the CNS and mostly the liver, spleen, bone marrow and lymph nodes that are affected. (that's what the little asterisks mean)
Most of these LSD, particularly Tay Sachs and Niemann-Pick, have a higher carrier rate in Easter European Jews. In New York, there is a large cluster of this population and they are routinely screened for these disorders. In depends on where you will practice whether or not you will see these.
7: In Tay Sachs, the enzyme that is affected is the alpha subunit of hexosaminidase A and what results is the buildup of GM2 gangliosides. Gangliosides are neuronal components in the membranes. In Niemann-Pick, there is a deficiency in sphingomyelinase. Sphingomyelin is what is wrapped around nerves for increased conductivity. The deficiency results in a pileup of sphingomyelin. In Gaucher's, there are three different types and they are covered in your lab.
8: This was pulled from the paper a few years ago. It is about a woman who adopted a child, that was normal at birth, but at a few months of age, started to deteriorate rapidly. He started to lose posturing and muscle tone. He wasn't able to speak and had trouble swallowing. He died within a couple of months of the picture being taken.
9+10: The onset for Tay Sachs is three to six moths of age and death occurs between ages 2 and 5. It's a rapidly progressing disorder, all due to the buildup of GM2 gangliosides.
There is blindness and hypotonia because innervation to the muscles is affected. There is also mental retardation because it affects brain cells. They also have a retinal cherry red spot.
11+12: GM2 gangliosides pileup in all nerve cells , including ones in the GI tract and optic nerve and retina. The pileup results in a pale, whitish appearance of the retina and the cherry red spot is the ophthalmic artery.
13: You can also have cherry red spot in Niemann-Pick because it also affects neurons. So cherry red spots can be seen with both Tay Sachs and Niemann-Pick because it is the same process,but the enzymatic problem is different.
With Neimann-Pick, there is also early onset. It also affects the macrophages, which reside in liver, spleen and lymph nodes. With N-P, you have psychomotor deterioration and other similar problems as Tay Sachs. It is also found in the same population of EE Jews, also called Ashkenazi Jews.
14+15: This disorder is very rare. It is an autosomal trait that tends to be lethal very early on. They are not passed down from affected individuals. Early onset of psychomotor deterioration: they can't sit up, they can't swallow and their GI tract doesn't work very well. They have seizures. These globules in this macrophage are filled with sphingomyelin, which give ganglion cells a bubbled appearance (Slide 15). Ganglion cells tell the autonomic nervous system and our spinal cord what to do.
16: If you look electron-microscopically, you can see whorls in cells of builtup membrane material. You don't have to know what an EM picture looks like, but you should know what a cherry red spot is.
17: The liver has many functions, one of which is breaking down glycogen. Glycogen is essentially animal starch. We don't make cellulose and starch like plants do, we make glycogen. We take glucose molecules and hook them together in an elaborate pattern and we store it in our liver. When we need more sugar in our bloodstream, the liver responds by taking off glucose molecules, one at a time.For people who have glycogenoses, they lack one of the many enzymes necessary for breaking down glycogen.
18: We are going to talk about just a few of these disorders you may encounter. They don't have ocular or dental manifestations, but they may become hypoglycemic in your office, they may have muscle pain or tired muscles. They will be in your patient population, but they aren't going to have ocular or dental features. However, because they are autosomal recessive disorders, and because there are board questions on them, I will talk about them.
The first one, Type I, is more severe and it is due to a defect in one of the very first enzymes responsible for cleaving off a glucose molecule. The second one called Type 5, also called McArdle's, is due to the muscle's ability to break down glycogen. Lactic acid accumulation in the muscles is due to glycogen being broken down.
In Type I, also called VonGierke, the substrate that builds up is glycogen. Usually these individuals are newborns. They have severe hypoglycemia. While in utero, the mom is supplying the glucose, but as soon as they are born, they immediately crash with sever hypogylcemia and they end up seizing. They are treated with enzyme supplementation, but this is a dramatic, severe disease you may not see.
McArdle's disease you might see. This is the one with skeletal muscle phosphorylase, they are unable to breakdown the glycogen in the skeletal muscle.
19: There is a third type of LSD that affects connective tissue. Connective tissue cells have to make ECM and they have to process proteins within their cells to make the ECM tissue. The cells that do this are fibroblasts and/or endothelial cells.
Mucopolysaccharides are part of a larger family called glycosoaminoglycans. They are in the cellular matrix and provide tensile strength. You need collagen plus glycosoaminoglycans (GAGs) to hold everything together. If there is an abnormality in the breakdown of these material, the cells that make them will swell. The easiest place to see the swelling is in the face and in the extremities, particularly on babies. So that's what is means: progressive connective tissue increases with coarse facies. Coarse facies means the face is swollen and unusual looking, dysmorphic.
You may have joint stiffness because GAGs are in the bursa sacs of joint to provide fluidity. If stuff builds up, it doesn't allow for joint mobility. Another place they build up is the cornea.
Hurler syndrome or MPS I has to do with an in ability to breakdown heparan sulfate, a component of the cornea. A buildup of heparan sulfate in the cornea results in corneal clouding.
Another place these molecules are important are the interstitium or connective tissue of the heart. Your heart becomes big if you are unable to metabolize the molecules. It's not the muscles that are big, it's the connective tissue enlarging and not allowing muscle contraction in a normal way. So these patients are at risk for cardiac failure.
20: Virtually all of these disorders, with the exception of Hunter's disease, are autosomal recessive disorders. There is an X-linked form of mucopolysaccharidoses called Hunter's disease. You only have to have one X chromosome to have the disease. Females are not likely to have Hunter's because they have two X's. Males are at risk if the mom passes one of those bad X's. These patients have mental retardation and have the coarse facies look as well.
21: Here's an example of corneal clouding. It is found in Hurler's disease (MPS I). Corneal clouding is NOT seen in Hunter's (MPS II).
22: At birth, the baby looks normal (the mark on the face is due to injury induced by forceps delivery). But by the time she is 2 months, she looks like she is 6 months old. She has prominent cheeks.
23: Here her face and arms look bulky relative to a normal 5 yr old. The inability to break things down can affect multiple systems depending on where the enzymopathy is.
24: You can't look at an individual and tell they have Hunter's. But if your are given a choice of this person having coarse facies, cardiomegaly, abnormalities of joint movement, and his mother and father are unaffected, the person is male and there is no corneal clouding, you should know it is likely Hunter's. You'd have to be told there is NO corneal clouding and it is a male.
25: There are different kinds of MPS and they show different effects. This baby looks abnormal due to bulkiness/puffiness of the body, extremities and eyelids.
26: There are many types of X-linked recessive disorders, Hunter's is one of them. There are other ones such as hemophilias (defects in the clotting factors of the blood), some disorders of immunities and some forms of diabetes (not diabetes mellitus but diabetes insipidous which is characterized by frequency of urination due to the hypothalamus not producing a hormone).
What we are going to concentrate on is Fragile-X syndrome. It is a segway into a different concept. This is where genetics and medicine have made the most progress in new information in the last five years.
27: You get the same numbers of chromosomes from your mom and dad. But a particular number of chromosome can be inactivated differently depending on who you inherent it from, the mom or dad. For example, the number chromosome 7 has different parts inactivated depended on which parent you got it from. Even if you Geimsa stain the chromosomes and line them up, you won't see a difference. There are parts on the genome of each one of them that are selectively turned off. The concept is called genomic imprinting.
28: These features are non-Mendelian. That means non-Mendelian genes: you pick a tall plant a short plant and get a medium plant, you put together too short plants and get a short, two half-short tall and one tall, etc. This is different this is a superimposed process on top of the material you already get. This was discovered on a tiny deletion on the #15 chromosome, the long arm of 15 chromosome, 15q. In two types of individuals, they detected a deficiency in the 15q chromosome. Initially, it was thought to be in the same place. But is has been shown that even though they are next to each other they are not overlapping. If you take the gene out, there is more than one part in the gene that is missing.
This confused people because it was a mystery as to why two people with different phenotypes appeared to be affected by one gene (they should have similar phenotypes). It occurred to people then that the deletion could have either came from their mom or dad (the genes being imprinted differently from each parent), producing different phenotypes.
29: Genomic imprinting is epi-genetic, which means it is on top of the genome that you start with.
30+31: Remember we talked about the germ line, those cells that go on to make your ova or sperm. When you are an embryo, you inherit your parent's information, you get your imprinted info from you mom and imprinted material from your dad and it goes to all of your somatic cells. The cells destined to become your ova or sperm, however, release all the imprinting. The process of imprinting has to do with sticking a methyl group on parts of the chromosome. As an embryo, you re-imprint all your genetic material (how you imprint has a lot to do with your sex ie a male will imprint his genome differently than a female).