metabolic diseases and genetic testing-mitsouras

Question 1

What are the 2 methods of biochemical testing for metabolism errors?

Answer 1

1. measure the enzyme activity in pt cells (from biopsy or blood draw) –> enzyme assay, color change when substrate is covered to product
   - can only test one deficiency at a time
   - need the actual tissue where the enzyme is expressed
2. MS/MS: ID multiple compounds in complex biological mixture (blood sample)
   -can test several metabolites (enzyme deficiencies) at the same time.
   increases in certain metabolites from certain enzymatic deficiencies
   -heel prick of newborn
   -if show a positive, need a confirmatory test
   *screening! done on all healthy babies

Question 2

When is prenatal screening indicated?

Answer 2

family history of disorder, when patents have an affected child, when the marriage is consanguineous (between relatives–> increase change of loss of heterozygosity)

same for carrier screening –> likelihood of having an offspring with a disease

Question 3

What are the different types of Hyperphenylalanemias and what treatment is currently being used for these diseases?

Answer 3

Hyperphenylalanemias caused by defects in metabolism of Phe (essential aa)–>elevated serum Phe
Elevated serum Phe–> disruption of important cellular processes in brain (myelination & protein synthesis) –> severe mental retardation (irreversible)
Early identification of affected individuals & nutritional & therapeutic management is KEY (restrict but do not eliminate Phe from the diet)

type 1: Can be due to mutations in phenylalanine hydroxyls –> classic PKU (type I; more common) –>manifests as lethargy feeding difficulty, eczema, “mousey” odor

treatment=dietary restriction of Phe

the increases Phe/Tyr ratio is detectable by MS/MS

type 2: Can be due to defects in biosynthesis or regeneration of tetrahydrobiopterin cofactor (BH4) for phe hydroxyls –> type II & III hyperphenylalanemias (less common; 1-3% of cases)
-Type II due to mutations in dihydropteridine reductase; Type III due to mutations in dihydrobiopterin synthetase

*in type 2-3, will see other issues from the BH4 deficiency –> decrease in norepi and serotonin (tyrosine hydroxylse and tryptophan hydroxylase both need BH4 as a cofactor)

if plasma Phe is elevated and the pt does not respond to low Phe diet, it is probably a problem with BH4 biosynthesis or regeneration (type 2 or 3)

treat with low Phe diet, supplement with L-dopa, carbidopa, 5-OH-Trp and BH4

Question 4

Homocystinuria

Answer 4

Cystathione beta-synthase deficiency –> classic homocystinuria
Dramatic clinical phenotype —>dislocation of lens, mental retardation, osteoporosis, long bones & thromboembolisms of veins & arteries
Cystathione beta-synthase requires vitamin B6 as a cofactor
Can be vitamin B6-responsive (40% of patients) or vitamin B6-nonresponsive
Vitamin B6-responsive –>milder & improves with administration of large amounts of pyridoxine
Vitamin B6-nonresponsive –>restrict dietary methionine

MS/MS will detect elevated levels of methionine

Question 5

When is additional screening for chromosomal abnormalities and neural tube defects indicated?

Answer 5

1) advanced maternal age (>35 years)  increased risk for chromosomal abnormalities
2) previous child with de novo chromosomal abnormality
3) chromosomal abnormality in one of the parents
4) parents are first or second-degree relatives –> increased risk for neural tube defects

Question 6

How is triple maternal serum screen used to detect chromosomal disorders and neural tube defects?

Answer 6

o Maternal serum screening is non-invasive and can detect some chromosomal abnormalities through a blood draw on the mother
o Maternal serum screen (MSS; triple screen) at 15-22 weeks gestation for:
• Alpha-fetoprotein (AFP; neural tube defects)
• Unconjugated estriol (uE3)
• Human Chorionic Gonadotropin (hCG)
o These tests are used to test for Down syndrome, Trisomy 18 and neural tube defects.
-if AFP and uE3 are low and hGC high–> indicate down syndrome
-if all three are low–> trisomy 18
-if AFP is high and uE3 and hCG are normal–> neural tube defects

PAPP-A and beta-HCG can also be indicative of trisomies 18 and 21
o If MSS is positive → additional diagnostic testing (CV sampling/amniocentesis)

Question 7

describe non-invasive prenatal screening by fetal cell sampling

Answer 7

analysis of circulating cell-free fetal (ccff) nucleic acids in the blood sample of a pregnant woman
-can be collected at 12th week of pregnancy
-non-invasive
-no risk for miscarriage
uses next generation sequencing and determines ratio of sequences from each chromosome to show trisomy, monosomy or deletion

ex: trisomy 18, 21, 13, sex aneuploidies and some micro deletions