Genetic & Metabolic Disease

Question 1

Typical inheritance pattern of enzymopathies?

Answer 1

As nearly all enzymes produced in excess, most heterozygotes are phenotypically normally, thus most enzymopathies are autosomal recessive, resu;ting in nearly no enzyme activity. NB X-linked enzymopathies

Question 2

X-linked enzymopathies:

Answer 2

Hemophilia A - Factor VIII
Hemophilia B - Factor IX
G6PD deficiency
Lesch-Nyhan Syndrome - HGPRT deficiency

Question 3

How can lyonization cause manifestations of X-linked disease in females?

Answer 3

Random inactivation of one X chromosome ( => Barr body); some may have unusually high expression of the deficient allele. These individuals are called mosaics or manifesting heterozygotes

Question 4

Why do some diseases show autosomal dominant inheritance? Particularly non-enzymatic structural proteins or membrane receptors.

Answer 4

1) More than 50% of normal gene product is needed for normal function
2) Defective protein adversely affects normal protein function
3) Defective protein has novel, detrimental property

Question 5

PKU

Answer 5

Phenylalanine hydroxylase (PAH gene), AR. Defective conversion of phenylalanine to tyrosine => buildup of phenylalanine. Alternate cause may be BH4 (tetrahydrobiopterate, a cofactor for PAH and tyrosine => dopa) deficiency, in which reduced dopamine => raised prolactin.

Accumulation of phenylalanine in neurons causes neuronal damage, mental retardation, growth retardation, and motor dysfunction. Reduced dopamine, epinephrine, & norepinpehrine due to lack of tyrosine precursor. Musty odor (phenylketones in sweat), increased risk of eczema, fair skin (tyrosine is precursor to melanin).

Treat with phenylalanine-restricted diet (NO aspartame).

Screened by Guthrie test

Abandoning restricted diet in later life as tolerance increases can cause irreparable CNS damage and mental retardation in the developing heterozygote fetus due to high diffusion across palcenta - “maternal PKU”

Question 6

Galactosaemia

Answer 6

Galactose-1-phosphate uridyltransferase, AR

Question 7

MCADD

Answer 7

Medium chain acyl-CoA dehydrogenase, AR

Question 8

Thalassemias

Answer 8

a- or b- haemoglobin, AR

Question 9

Cystic fibrosis

Answer 9

CFTR (cystic fibrosis transmembrane regulator), AR

Heterogeneity in disease due to various mutations & environmental variables.

Question 10

Tay-Sachs

Answer 10

Hexosaminidase A, AR

Question 11

Osteogenesis imperfecta

Answer 11

Type I and II collagen, AD

Question 12

Marfan Syndrome

Answer 12

Fibrillin, AD

Question 13

Hereditary spherocytosis

Answer 13

Spectrin (within RBC membrane), AD

Question 14

Achondroplasia

Answer 14

FGFR3, AD

Question 15

Familial hypercholesterolaemia

Answer 15

LDL receptor, AD

Question 16

Southern blot

Answer 16

Specific DNA sequence identified by radioactively-labelled complementary nucleic acid (DNA or RNA) probe. Used to look for gene mutations

Question 17

Northern blot

Answer 17

Specific RNA sequence identified by complementary radiolabelled probe. Used to assess gene expression as a function of mRNA levels

Question 18

PCR

Answer 18

DNA sequence amplification using end-primers, by denaturation, annealing (primer hybridization), & extension by heat-insensitive DNA polymerase

Question 19

Western blot

Answer 19

Assess protein levels by denaturation, size separation by gel electrophoresis, primary antibodies to the sequences, and labelled secondary antibodies

Question 20

Newborn screening for:

Answer 20

PKU, congenital hypothyroidism (cretinism), congenital adrenal hyperplasia, galactosemia, MCADD

Question 21

Genetic screens before birth

Answer 21

1) Pre-implantation diagnosis via IVF
2) Amniocentesis (20-30mL of aminotic fluid at 15-17 weeks
3) Chorionic villous sampling (a few mg of villous tissue at 10-11 weeks)

Question 22

Hardy-Weinberg equilibrium

Answer 22

(p+q)^2 = 1
p^2 + 2pq + q^2 = 1

Only applies in “genetic equilibrium”