Lecture 19 - Prenatal Diagnosis In Thalassemia

Question 1

Haemoglobinopathies = inherited disorders of globin (a2b2)

Answer 1

1) thalassaemia syndromes - ratio of a/b proteins

2) Variant haemoglobins - abnormal a or b proteins

Question 2

Facts about haemogloninopathies

Answer 2

• inherited ones have been positively selected for by malaria - found in regions where malaria is/was endemic
• important ones are a thalassemia, b thalassemia, HbS, HbE
• risk populations: SEA, southern chinese, mediterranean, middle eastern, black africans

Question 3

Difference between thalassemias and variant haemoglobins

Answer 3

• Thalassemia: positive family history, almost always inherited, MCV/MCH low
• Variant Hb: Spontaneous or inherited, MCV/MCH normal (HbS) or low (HbE)

Question 4

Inheritance patterns in thalassemia

Answer 4

• autosomal recessive
• autosomal co-dominant if carrier has low MCV and in large penetrance
• autosomal dominant b thalassemia: rare
• compound states: HbH
• gene-gene interactions (a and b thalassemia: inheritance of a thalassemia reduces severity of phenotype of b thalassemia due to reduced a chain)

Question 5

Pathogenesis - thalassemia syndromes

Answer 5

• a/b globin chain imbalance
• iron overload
• hypersplenism
• complications blood transfusions
• gene/gene interactions

Question 6

Diagnosis of b thalassemia

Answer 6

• non deletional gene disorder, usually single nucleotide polymorophism
• microcytic hypochromic anemia
• blood film
• increase HbA2 in heterozygous or intermedia
• b/a biosynthesis ratio (0.5/0.7)
• DNA analysis: point mutation
• Family studies

Question 7

A thalassemia

Answer 7

• mostly a deletional disorder
• a+ deletions: 3.7 or 4.2 kbp deletion
• a0 deletions: specific for some regions of the world

Question 8

A thalassemia genotype and phenotype

Answer 8

• aa/aa: normal
• a-/aa: a+ normal
• a-/a-: a+, decreased MCV/MCH
• –/aa: a0: decreased MCV/MCH
• a-/–: HbH
• –/–: HbBarts hydrops fetalis

Question 9

Diagnosis of a thalassemia

Answer 9

• normal haematology
• microcytic, hypochromic RBC - anemia
• blood film
• HbH inclusions
• a/b biosynthesis ratio
• DNA analysis for deletions
• Family studies

Question 10

Pathophysiologic consequences of switch from HbF to HbA

Answer 10

• predominant circulating hemoglobin at the moment of birth is hbF
• infants do not depend on normal amounts and function of HbA until they are 4-6mo
• a-chain hemolobinopathies tend to be symptomatic in utero and at birth
• b chain abnormalities are asymptomatic until 4-6mo
• a chains are needed to form HbF and HbA, but b-chains are required only for HbA

Question 11

Options for prenatal diagnosis

Answer 11

• ultrasound: not very sensitive
• chromosome analysis
• biochemical screen
• fetal sampling
• DNA testing

Question 12

Prenatal diagnosis of b-thal

Answer 12

• routinely and reliably diagnosed using fetal DNA obtained between 8 and 18 weeks of gestation
• most reliable methods are based on direct DNA analysis: amniocentesis and CVS. Preferable to obtain DNA ealry on
• new techniques: DNA directly from maternal plasma or isolate fetal nucleated RBC from maternal peripheral blood

Question 13

Sources of Fetal DNA

Answer 13

• CVS: 11 weeks, aim is in 1st semester
• amniocentesis: 15 weeks, 2nd trimester
• Fetal blood: 18 weeks - not ideal

Question 14

Heterogeneity of b-thalassemia mutations complicates the approach to prenatal diagnosis BUT

Answer 14

• 15 b-thal mutations account for more than 90% of individuals affected worldwide
• 3-6 mutations usually account for the vast majority of severe cases in any ethnic group

Question 15

Prenatal diagnosis of a-thalassemia

Answer 15

• amniocentesis in pregnancies at risk of homozygous a-thal and the hydrops fetalis syndrome
• molecular hybridization
• detect complete absence of a globin genes in fetal fibroblasts
• ultrasonography detects hydrobs fetalis
• DNA studies or globin synthesis evaluation may be used to confirm the diagnosis in utero

Question 16

Options after prenatal diagnosis

Answer 16

• termination of pregnancy up to 20 weeks
• nothing
• intrauterine blood transfusion, bone/SCT

Question 17

Homozygous a thalassemia

Answer 17

• HbH, hydrop fetalis
• usually die in utero
• some infants have had successful blood exchange transfusion immediately after birth
• also possible to salvage affected fetuses by in utero blood transfusions
• limb and urogenital defects are present in a substantial portion of infants
• developmental delay or other neurologic abnormalities

Question 18

Infications for PND

Answer 18

• HbBarts hydrops fetalis: a0 in each partnet
• b thalassemia major: trait in each patner
• HbH disease: a0 in one and a+ in the other
• various Hb variant combination: HbSS or HbS, HbE with b thalassemia

Question 19

Preimplantation genetic diagnosis

Answer 19

• involves IVF
• biopsy taken of dividing blastomere
• in theory, useful for a wide number of genetic disorder
• limited by technology
• required confirmation by amniocentesis or CVS
• sexing and simple genetic tests feasible