LAB 11 - Clinical Genetics

Question 1

Overall objective: integrate basic science of genetic disorders with knowledge of specific disease mechanisms for applications to clinical diagnosis, prognosis and treatment.

Answer 1

a

Question 2

Compare and contrast major categories of genetic disorders: (1) autosomal or X-linked gene mutations following the classical patterns of Mendelian inheritance, (2) chromosomal disorders with autosomal or sex chromosome abnormalities (3)Single-Gene Disorders with trinucleotide repeats, genomic imprinting (may be non-classical inheritance) or mitochondrial transmission.

Answer 2

a

Question 3

Discuss familial patterns of autosomal dominant, autosomal recessive and X-linked diseases and formulate pedigree charts.

Answer 3

a

Question 4

Identify more common clinical conditions associated with single gene point mutations, single or multiple gene deletions or gene translocations, Be able to describe mechanism for single gene disorders involving enzyme insuffiencies (e.g. lysosomal storage diseases), dysfunctions of receptor or transport proteins, abnormalities of protein structure, or abnormalities in gene regulation/expression..

Answer 4

a

Question 5

Identify common congenital or inherited diseases associated with detectable cytogenetic abnormalities and involving gene translocations, inversions, duplications or deletions such as Down’s syndrome .

Answer 5

a

Question 6

Identify rare diseases associated with non-classical inheritance, particularly trinucleotide repeat mutations.

Answer 6

a

Question 7

Autosomal Dominant Polycystic Kidney Disease (ADPCK)

Answer 7

Autosomal Dominant

S/S:

• aneurysm of the circle of Willis (saccular/Berry Aneurysm)
• Multiple benign cysts (liver, spleen, pancreas, kidney, lung)
• enlarged kidneys with multiple cysts of varying size

Genetics
Two genes with mutations have been identified: PKD1 and PKD2
- Encode for polycystin 1 and 2
- Integral membrane proteins => impaired fucntion because of nephron disruption

Question 8

Autosomal Dominant

Answer 8

NS

• Huntington Disease
• Neurofibromatosis
• Myotonic dystrophy
• Tuberous sclerosis

Urinary
-PKD

GI
- Familial polyposis coli

Skeletal

• Marfan syndrome
• Ehlers-Danlos syndrome
• Osteogenesis imperfecta
• Achondroplasia

Metabolic

• Familial hypercholesterolemia
• Acute intermittent porphyria

Question 9

Mendelian disorders

Answer 9

a

Question 10

Autosomal dominant inheritance patterns

Answer 10

Autosomal Dominant

Males ~ Females
1 biological parent typically expresses clinical manifestations
- 50% offspring chance

Question 11

Penetrance

Answer 11

Def:

Significance:

Question 12

Mucopolysaccharidosis:
IH - Hurler Syndrome
II - Hunters
III - Sanfilippo
IV - Morquio

Answer 12

Autosomal recessive

Lysosomal storage disease
deficiency of a L-iruronidase

Failure to degrade mucopolysaccharides, dermatan sulfate & heparin sulfate

• > abnormal accumulation
• > cell/organ disfunction

Mental retardation
Macroglossia - enlarged tongue
Skeletal abnormality - depositions in bone
Coarse facial features
Clouding of corneas
Joint stiffness
Hepatosplenomegaly
Cardiac valvular deformitues

General features:

1. Lysosomal storage diseases reflect an accumulation of insoluble metabolites within lysosomes. These accumulations can interfere with normal cellular functions.
2. Lysosomes contain multiple hydrolytic enzymes each of which is specific for a macromolecular substrate. In genetic disorders due to point muations, a specific enzyme may be deficient due to decreased expression or non-functional

point muations, a specific enzyme may be deficient due to decreased expression or non-functional

Histology:
Accumulation of glycosaminoglycans

1. Subendothelial arterial deposits can involve coronary and CNS vessels. Cells with accumulated glycosaminoglycans are distended . In H&E sections the cytoplasm appears clear due to displacement of the usual organelles. This material stains positive in the presence of a cationic dye Alcian blue.
2. Electron microscopy demonstrates swollen lysosomes filled with a granular substance.

Question 13

Proteins associated with AD:

Answer 13

a

Question 14

Lysosomal storage diseases
Table on pg. 229
and glycogenoses, pages 228-233

Answer 14

a

Question 15

Study table 5-6 to associated enzyme deficiency, metabolite accumulation and typical disease manifestation(s)

Answer 15

a

Question 16

Niemann=Pick Disease

Answer 16

Infant with hepatosplenomegaly, failure to thrive, vomiting, fever, lymphadenopathy and psychomotor deterioration. Liver biopsy demonstrates accumulation of sphingomyelin.

Question 17

Gaucher DIsease - Type I

Answer 17

Glucocerebroside accumulates in mononuclear phagocytes throughout body, with spleen and bone involvement predominating (no CNS involvement)

Question 18

Tay-Sachs disease

Answer 18

Relentless motor and mental deterioration leading to flaccidity, blindness, dementia, and death by age 2-3 years due to hexosaminidase-a deficiency.

Question 19

Glycogen storage disease

Answer 19

Hepatomegaly, renomegaly, hypoglycemia and cardiac disease

Question 20

Familial Hypercholesterolemia

Answer 20

Epidemiology:

1. One of most common Mendelian disorders. It results from LDLR gene mutations that alter the low density lipoprotein receptor, the form associated with 70% of total plasma cholesterol transport. Heterozygotes with one mutant gene occur one in 500 with a 2-3 fold elevation in plasma cholesterol.
2. Homozygotes may have up to 6 fold increase in plasma cholesterol and they may develop skin xanthomas, severe atherosclerosis, and have early myocardial infarctions.

Cholesterol Metabolism:
1. The liver has a central role in cholesterol metabolism since 70-75% of LDL receptors are located on hepatocytes. VLDL which is rich in triglycerides is secreted into the blood stream. In the periphery, lipoprotein lipase extracts much of the triglycerides and an intermediate density lipoprotein results (IDL). The IDL has one of two fates: 50% of IDLs are taken up by the liver via the LDL receptor and recycled into VLDL; 50% become LDL and are cleared via scavenger method or taken up by the LDL receptor in the liver.
2. Cholesterol inhibits the intercellular production of cholesterol by inhibiting HMG CoA reductase and activated acyl coenzyme A which stimulates storage of cholesterol. Cholesterol also decreases the production of the LDL receptors, thus protecting the cell from too much cholesterol.
Clinical features:
1. Clinical signs of gross hyperlipidemia include: xanthelasmata, skin and tendon xanthomata, and arcus cornealis. These xanthomata represent tumor-like accumulations of cholesterol within macrophages.
2. Secondary causes of increased cholesterol include: diet, hypothyroidism, liver disease, nephrotic syndrome, and porphyria.
Histologic features:
1. Collections of foamy macrophages are beneath the raised yellow nodules of the skin and on tendons (see below).
2. Cholesterolosis or yellow flecks consisting of foamy macrophages can be seen in the gall bladder.
Key study points:
1. Fig 5-7, below
2. Fig 5-8, below
3. Fig 5-9 Classification of LDL receptor mutations, below.

Points for discussion:

1. Familial hypercholesterolemia can be classified as which type of mendelian disorder?
2. Familial hypercholesterolemia results from mutations of which receptor”?

Question 21

Trisomy

Answer 21

Clinical and Gross External Features most frequently associated with the most common trisomies and often apparent at birth
1. Trisomy 21 - Intrauterine growth retardation, muscular hypotonia, facial features (hypoplastic maxilla, malocclusion, slanting palpebral fissures, dysplastic ears, cleft lip and palate, macroglossia, talipes equinovarus, simian crease, clinodactly of 5th fingers
2. Trisomy 18 - Prenatal growth failure, hypotonia, limited hip movement, dolichocephaly with prominent occiput, low set ears, small mouth, barrel chest, index and fifth fingers which overlap middle fingers, rocker bottom feet, umbilical hernia
3. Trisomy 13 - Intrauterine growth retardation, seizures, midface defects (proboscis, cleft lip and palate), low set and dysmorphic ears, microcephaly (with receding forehead, epicanthal folds, deep-set eyes, midline scalp defects), microphthalmia, colobomata, rocker bottom feet, talipes equinovarus or calcaneovalgus, abnormal scrotum
Key Study Points:
Review Robbins Basic pathology 9th ed, chromosomal disorders pages 236-241.
Points for Discussion
1. What is the incidence of Trisomy 21 compared with Trisomy 18, Trisomy 13, Klinefelter Syndrome, Turner Syndrome and Fragile X syndrome?

2. List the anomalies most frequently seen in Trisomy 21 patients in these organ systems
3. CNS –
4. Gastrointestinal –
5. Cardiovascular –
6. Genitourinary –
7. What is the usual outcome of infants with Trisomy 21 and what are the causes of death at different ages?

Question 22

Klinefelter Syndrome

Answer 22

Cytogenetic disorders of sex chromosomes

Clinical features:
1. Many patients are tall with a distinctive increase in length between the soles and the pubic bone. Patients often have small atrophic testes, small penis, and lack of secondary male sex characteristics such as a deep voice and male distribution of pubic hair. Gynecomastia may be present.
Gross features:
1. Testes are small and atrophic without masses.
Histologic features:
1.Variable.
2.Normal and atrophic seminiferous tubules intermixed throughout the parenchyma. Increased prominence of Leydig cells.
3.Atrophic tubules replaced by thick hyalinized collagenous bundles. Increased prominence of Leydig cells.
Key study points:
1. This patient would be classified clinically as a young man with hypogonadism and infertility. Infertility in men can be genetic or secondary to other diseases affecting the testes (infections, the use of drugs, exposure to heat, trauma and cryptorchidism) or due to endocrine dysfunction,( hypopituitarism).
2.In the workup of an infertile couple, the male partner’s ability to produce sperm is evaluated through seminal fluid analysis. An adequate specimen is obtained by masturbation after 2-3 days of abstinence. Parameters evaluated include volume, viscosity, gross and microscopic appearance, sperm count (n= >20 million/mL), sperm motility and morphology.
3.Klinefelter syndrome occurs in about 1 in every 660 live male births. It is one of the most common causes of hypogonadism in the male. Signs and symptoms appear at puberty leading to a diagnosis in boys at age 15 and 16.
4.Klinefelter syndrome can be defined as male hypogonadism which occurs when there are two or more X chromosomes, and one or more Y chromosomes in an individual. Ninety percent of cases show a 47, XXY karyotype. Maternal and paternal nondisjunction at the first meiotic division are roughly equal.
5.Follicle stimulating hormone (FSH) is consistently elevated, while testosterone levels are variably reduced
6.Mental retardation may occur. It is more common in patients with increased copies of X chromosome.
7.There is an increased risk of non-seminomatous germ cell tumors of the mediastinum, SLE, type 2 diabetes, mitral valve prolapsed and a 20 times higher risk of breast cancer.

Points for discussion:

1. Would a patient with Klinefelter syndrome be able to have children
2. What abnormality seen on PE is associated with infertility?

Question 23

Thalassemia

Answer 23

Autosomal recessive disorders

Clinical features:
1. Autosomal recessive disorders represent a large proportion of the Mendelian disorders. They result when bBoth alleles from a given gene locus need to be mutated for the disease to manifest. The parents of the proband are usually not affected clinically. If the disease appears more than expected within a given population suspect consanguinity.
.
2. The expression of the disease tends to be uniform with frequent complete penetrance, onset of the disease is usually early in life (unlike some AD disorders), and new onset mutations are generally not seen clinically for several generations.

Clinical Gross features of Thalassemia:
1. This AR disorder will be discussed in detail in Chapter 11: Hematopoietic and lymphoid systems. You are now help responsible for understanding AR disorders and their inheritance patterns.

2. Thalassemias are a heterogeneous group of disorders which may have mutations of the alpha or beta globin genes causing defects in globin synthesis.
3. The disease spectrum varies from carrier states to fatal outcomes, depending on how many mutated genes are inherited.
   Key study points:
4. Review Mendelian disorders
5. Review Autosomal Recessive disorders and their characteristics.
   Points for discussion:
6. The patient (mother) has four children. What is the likelihood that each child will inherit the disease?
7. Compare AD and AR disorders.
8. How do X-linked disorders differ from AR disorders? Can you give some examples?

Question 24

Alkaptonuria (Ochronosis)

Answer 24

General storage disease features:
1. Ochronosis was the classic “inborn error of metabolism” and is an autosomal recessive disorder. It is dramatic due to production of black urine. Pigment also deposits in connective tissues of the sclera, cardiac valves, cartilage and joints.
2. Phenylalanine and tyrosine can not be further metabolized and therefore accumulate.
Genetic features:
1.There is lack of homogentisic acid oxidase leading to homogentisic acid accumulation in the body. This is excreted imparting a black color to the urine.
Histologic features:
1. Excess homogentisic acid binds to collagen imparting a blue-black color (ochronosis).
2. Pigment accumulation in the articular surface of joints leads to decreased resiliency and the cartilage becomes brittle and denuded.
Key study points:
There are multiple storage disease. You should be familiar with glycogen storage diseases as well as ochronosis.
There are three major subtypes discussed in Robbins: hepatic (Von Gierke’s), myopathic (McCardle’s) and systemic Pompe’s). All are relatively rare. You should be familiar with the general features.
Points for Discussion:
Discuss other disorders of metabolism and glycogenosess.
Review tables 6-4 and 6-5 in Robbins Basic Pathology 9th ed.