Medical Genetics-Biochemical Genetics

Robert F. Waters, PhD

One gene-One enzyme Many proteins are different but do

not cause clinical problems Polymorphism (multiple structures)

• Multiple “normal” forms• Caused by mutations

Different types of proteins..not just enzymes

Main Categories The Hemoglobins Inborn Errors of Metabolism Genetic response to drugs

Pharmacogenetics

The Hemoglobins Defects in hemoglobin synthesis

Hemoglobinopathies• One of the first was Sickle Cell Anemia

• Neel (1949)• Pauling, et. al. (1949)

• Sickle Cell Anemia is essentially a “molecular disease”

Structure of Hemoglobin Two main parts

Globin (Polypeptide Chains)• Two alpha and two beta

Heme (Porphyrin)• Iron containing portion

Oxygen binding

Normal Hemoglobin Adult hemoglobin (HbA)

MW. Approx. 68000 Structural formula

22

• Alpha chains have 141 AAs• Beta chains 146 AAs

• Alpha and beta chains are very similar in primary and tertiary structure

Myoglobin is Different Oxygen carrying molecule of

muscles Single polypeptide chain

Different Gene Loci for Globin Part of Hb

Alpha, beta, gamma, delta, epsilon Five different structural gene loci

HbF (Fetal Hemoglobin) Formed in uterus Disappears after birth 2 2

Two different types of HbF differing by only one amino acid…glycine

HbA2

2% of adult variant 2 2

Two types of embryonic hemoglobin First 90 days

• Gower I (4) and Gower II (22)

Abnormal Hemoglobin HbS (Sickle Cell - 6:glu val) HbC (-6:glulys) Hb Hopkins-2 (- 112:hisasp) HbC Georgetown (- 6:glu val+)

+ means another substitution as well Hb Freiburg (-23 deletion)

Sickle Cell Disease Severe hemolytic anemia

Anemia Jaundice Vascular obstruction (Sickle cells)

• Painful infarcts

High incidence in equatorial Africa Separated originally by electrophoresis

Pauling, Sanger, Wells (1949)• Mixture of normal and abnormal (heterozygous)

Hereditary Persistence of HbF

Rare Sub-clinical Gene to turn on HbA formation is

blocked

Thalassemias Beta gene turned on just before

birth Alpha gene throughout gestation Derived from Greek word for sea—

thalassa Due to low production of alpha or beta

chains NOT amino acid substitution

Alpha Thalassemia Usually high expressivity

Lethal Associated with operator gene

malfunction

Beta Thalassemia Mediterranean Anemia Wide expressivity May be expressed neonatally Differential diagnosis

Different from iron-deficiency anemia

Inborn Errors of Metabolism Enzymatic defect (structure) Enzyme substrate problem Enzyme co-enzyme malfunction Enzyme co-factor (metal) malfunction Build up before—feed after Usually autosomal recessive

Otherwise would probably be lethal Some are hemizygous (XY)

Consequences-Overview Accumulation of a precursor

Accumulated precursor may be toxic• E.g., pyruvate

Alternate minor pathways may start producing other toxic metabolites

• E.g., PKU

Deficiency of product Product may be precursor to other substance.

• E.g., tyrosine and thyronines Feedback inhibition may be impaired due to lack

of product• Causes lack of feedback inhibition

Partial Listing of Lesions Glycolytic Pathway PDH Complex Albinism (Two Types) Cystinuria

Increased secretion of cystine, lysine, arginine, ornithine

Aminoaciduria AR (variable expressivity) Renal failure

Partial Listing of Lesions:Cont

Galactosemia Galactose-1-phosphate

uridyltransferase AR Hepatosplenomegaly Mental retardation Note: Galactokinase deficiency

Partial Listing of Lesions:Cont

G6PD Deficiency Many Variants Hemolytic anemia XR Oxidative stress

Sickle Cell Anemia

Partial Listing of Lesions:Cont

Glycogen storage diseases Type I (von Gierke’s)

• Hepatomegaly, mental retardation, hypoglycemia

• Glucose-6-phosphatase Type VIII (Glycogen Storage Disease)

• Hepatomegaly,mild acidosis, hypoglycemia• XR• Phorphorylase kinase

Partial Listing of Lesions:Cont

Tay-Sachs Disease Onset at 4 to 6 months Death 2 to 4 years AR Degenerative neurological changes

• Gangliosidosis• Reduced hexosaminidase

Partial Listing of Lesions:Cont

Hartnup Disease Tryptophan transport Neurological change AR

Hemoglobinopathies (See Earlier) Thalassemias

Partial Listing of Lesions:Cont

Homocystinuria Dislocated lenses AR Accumulation of methionine and

homocystine

Partial Listing of Lesions:Cont

Lesch-Nyhan Syndrome Uric Aciduria Self-mutilation Mental retardation Cerebral palsy Hypoxanthine-guanine phosphoribosyl

transferase (HGPRT) XR

Partial Listing of Lesions:Cont

Mucopolysaccharidoses I Hurler’s Syndrome “Gargoyle” Appearance Mental retardation Corneal clouding Cardiovascular degeneration Dwarfism Mucopolysaccharide accumulation

Partial Listing of Lesions:Cont Mucopolysaccharidoses II

Hunter’s Syndrome• Less severe than Hurler’s• No evidence of corneal clouding

Orotic aciduria Urinary excretion of orotic acid Precursor to pyrimidine nucleotides

Pentosuria AR “False Diabetes”

Partial Listing of Lesions:Cont

PKU Porphyria (Acute Intermittent)

Abdominal pain Neurological problems Excessive excretion of -amino levulinic

acid (ALA) Excessive hepatic ALA synthetase AR

Partial Listing of Lesions:Cont

Congenital Spherocytosis Episodes of hemolytic anemia Defect in RBC membrane AD

• Patients are heterozygous

Partial Listing of Lesions:Cont Tyrosinemia

Acute liver disease• AR• Tyrosine transaminase

Wilson’s Disease Cirrhosis of liver Neurological damage Improper copper metabolism AR Build up of stored copper Decreased serum levels of ceruloplasmin Treatment by Penicillamine removes stored Cu

Partial Listing of Lesions:Cont

Maple Syrup Urine Disease BCAA dehydrogenase

Alcaptonuria Dark Urine Disease

Other Metabolic Pathways Vitamin Pathways

E.g., holoenzyme carboxylase• Pyruvate carboxylase and biotin

Pharmacogenetics Drug reactions and interactions

associated with genetics Genetic Profiling Genetic Probes

Genetic Polymorphisms Different phenotypes G6PD Variants (Example)

BA A- BA- B- B A