Haemoglobinopathy Diagnosis (uk thal standards 2016, NHS thal screening, GTG 2014)

N.B. Limited text functions on this site have made this page a challenge, all Greek characters have been swapped back into the Latin.

 

Intro

 

Hbpathies are the commonest recessive monogenic disorders worldwide

 

Thalassaemias  = Reduced production of haemoglobin E.g. a/b/d/e/g-thal

Haemoglobin variant = Altered structure of haemoglobin. E.g. HbS/D/O (b-chain), HbG (a-chain)

Thalassaemic haemoglobin variant = E.g.  Hb Constant Spring (a-chain), Hb Lepore, Hb E (b-chain)

 

Chromosome 16 - a1, a2, z genes

Chromsome 11 - b, d, e, g genes

hb 1.png

 

typical Geographic Distributions

 

Homozygous a0 - China, Taiwan, SE Asia, Cyprus, Greece, Turkey, Sardinia

 

b-Thal - Anywhere other than North Europe

 

HbS - Africa, Greece, Southern Italy, Turkey, Middle East, India

 

HbE - SE Asia

 

HbC - West Africa

 

alpha-Thalassamaemia

 

There are four alpha chain alleles (a1 and a2 genes on each copy of ch. 16). These are written as aa/aa when all four alleles are present and functioning. The number of absent or dysfunctional alleles correlates with the clinical picture as below:

-a/aa, a+ Trait

Hb normal, MCV low, MCH low. Normal electrophoresis

 

-a/-a, Homozygous a+ Trait

Hb normal, MCV low, MCH low (<25). Normal electrophoresis

 

--/aa, a0 Trait

Hb normal, MCV low, MCH low (<25). Normal electrophoresis

 

--/-a, HbH Disease

Hb 70-110, microcytic, hypochromic

Results in production of HbH from 4 b chains. Detectable by electrophoresis, unlike the traits.

Spectrum of clinical phenotype from asymptomatic to severe anaemia and even hydrops.

 

--/--, Hydrops Fetalis

Incompatible with fetal life due to absence of a chain synthesis.

Results in production of Hb Barts from 4 g chains.

 

aTa/aTa, Non-deletional a-thal

E.g. Saudi T

In ‘standard’ deletional a+ thal the remaining genes are upregulated to compensate.

In homozygous aTa/aTa, this does not happen --> as a result HbH disease can occur even with two functioning a genes.

 

b-Thalassaemia

 

-/b, b-Thal Trait

Hypochromic, microcytic anaemia with a raised HbA2

 

-/- (b0) or  -/partial (b+), b-Thal Major

1 in 4 births where both parents are -/b

Mutations --> deletion of b gene, d&b genes or even b, d & g genes

Frequently the result of inheritance of two different mutations, i.e. compound heterozygotes.

a chains still produced in normal quantities and so become present in excess --> pathology of b-Thal

 

Clinical Features

Severe anaemia at 3-6 months when the switch from HbF to HbA occurs

Hepatosplenomegaly, Bone expansion, Iron Overload, Infection, Osteoporosis

 

Transfusion

?When to start – case by case decision

Vol/Freq – aim pre-Tx Hb 90-100, often equates to 2-3 units per month in adults.

 

d-Thalassaemia

 

Classified as d0 and d+, similar to b-Thal.

No clinical significance other lowering the HbA2 and so compromising the diagnosis of a co-inherited b-Thal.

 

d&b-Thalassameia

 

Classified as db0 and db+, depending on residual output of chains from the affected chromosome.

Includes Hb Lepore – a fusion mutation of the d&b genes.

 

egdb- Thalassaemia

 

Rare, results from large deletions

Severe haemolytic, hypochromic anaemia at birth. Improves after age of 3-6 months.

Only reported in heterozygotes. Homozygous state thought to be lethal at early fetal stage

 

Screening

 

Pre-conception Screening

Recommended to offer to at risk groups, i.e. most non-Northern European ethnicities.

If abnormality detected, partner should be tested

 

Antenatal Screening

Purpose is to Detect:

1.     Significant maternal hbpathy - SS, SC, S/b, HbH, b-thal intermedia

2.     Maternal carrier states - AS, AC, AD, AE, AO, ALepore, b-trait, db-trait, HPFH, homozygous a0 trait

 

High prevalence areas = ≥1.5 SCD per 10,000 pregnancies --> universal lab screening

Low prevalence areas = <1.5 SCD per 10,000 pregnancies --> FOQ to direct lab screening

Screening reports must be available within 3 working days

 

Newborn Screening

All newborns screened for SCD (and any <1 year old arriving in the country)

Aimed at providing early diagnosis to prevent long term morbidity

May or may not detect other hbpathies.

 

Low Prevalence Area Screening Algorithm Simplified

FOQ = Family Origins Questionnaire given to all pregnant women in low prevalence areas   Full algorithm here

FOQ = Family Origins Questionnaire given to all pregnant women in low prevalence areas

Full algorithm here

Haemoglobin Electrophoresis

 

Alkaline Electrophoresis

Cellulose Acetate or Agarose Gel (pH 8.2-8.6)

Hb negatively charged and will move towards the positive anode

A split A2 band is suggestive of an a chain variant

 

Acid Electrophoresis

Citrate Agar or Agarose Gel (pH 6.0-6.2)

Hb complexes with agaroprotein and moves towards the positive anode

Non-complexed Hb will move toward the negative cathode.

Used to distinguish S from D, and C from E & O-Arab

Different haemoglobins form bands on the electrophoresis strip at typical locations, as shown here:

hb 4.png

Sickle Solubility Test

 

Purchased as a commercial kit

Detects HbS down to a concentration of approx. 20%

 

False negatives – Anaemia, Infancy, Recent Transfusion

False positives – Paraprotein, Hyperlipidaemia, Heinz Bodies, Leukocytosis

 

HbH Preparation

 

HbH precipitates with brilliant cresyl blue (a ‘supravital’ stain)

Results in golf ball cells and Heinz bodies (if hyposplenic)

 

high performance liquid chromatography (HPLC)

 

  1. Mixture of molecules (normal and variant Hb) with a net positive charge are adsorbed onto a negatively charged column.

  2. These are then eluted off in a mobile phase – a liquid containing increasing concentrations of cations flows through the column, competing for the anionic binding sites.

  3. As the positive Hb molecules are eluted off they are detected optically and the retention time is recorded. The area under the peak can quantify each Hb.

hb 5.png

 

Advantages over Electrophoresis

  • Less labour intensive

  • Very small sample required

  • Quantification possible

  • Greater range of Hb’s identified

  • A2 detected and quantified à easier to diagnosis b-thal

 

Disadvantages compared to Electrophoresis

  • High capital and reagent costs

 

Glycated haemoglobins

  • Each haemoglobin present can undergo ‘post-translational modification’, usually glycation, which affects its charge and so appears as a separate peak to the left of the original Hb.

  • Commonest example – the glycated A peak will increase in diabetes (HbA1c)

  • In SCD, there is a factitious rise in A2 as the glycated S sits in the A2 window.

 

interpreting HPLC Results

 

Low A2, 0-2%, Normal FBC

d-thal trait

d-variant (e.g. A2 prime) will produce to 2 equal peaks, in the A2 and S windows (‘Split A2’)

a-variant (e.g. G-Philidelphia) will produce to 2 unequal peaks (‘Split A2’)

 

Normal A2, 2-3.5%, Normal FBC

Normal.

Pitfall: Silent b-thal mutations (e.g. +1480(CàG))can have a normal HbA2

Pitfall: Delta variants (e.g. A2 prime) give a peak in the S window. Need to add the normal and variant A2 together to get total, otherwise might miss a b-thal.

Pitfall: The HPLC trace baseline can wander up, giving an underestimated A2.

 

Normal A2, 2-3.5%, low MCH

a-thal trait - Indistinguishable from iron deficiency

a0 and a+ - Indistinguishable on HPLC

egdb-Thal - Once an adult, indistinguishable from a and iron def.

Therefore, if MCH <25 and a0 possible based on ethnicity —> DNA Analysis

 

Raised A2, >3.2% (b chain disorders) 

≥3.5% + MCH <27 - Heterozygous b-thal

>4% + MCH normal - Mild b-thal

>10% - Hb Lepore heterozygous

>15% - Hb E

 

In b-thals, co-existence of a-thal will reduce A2 by 5% for each missing a gene.

E.g. HbE typically shows HbA of 30%. HbE/a+ homozygous will have 20% HbA2

Pitfall: Normal individuals with low MCH due to iron deficiency or a-thal trait, may have a HbA2 of 3.5-4% due to hyperthyroidism or drugs for HIV.

 

Raised HbF, >0.8%

>5%  + MCH <27 - Could be heterozygous db-thal

>5%  + MCH normal - Hereditary Persistance of Fetal Haemoglobin (HPFH)

 

HPFH

  • Deletional – results from db chain deletions

  • Non-deletional – may be a beneficial modifier

 

Pitfall: HPFH often associated with a co-inherited a-thal and so MCH not a reliable means to differentiate between HPFH and db-thal.

 

HbS

80-95% - HbSS

40% - S trait (40% not 50% as a chains prefer the normal b chains)

 

HbS% will rise if S trait combined with b-thal as alpha chains can no longer preferentially bind with the normal beta chains. Recently transfused patients will have a lower HbS%

 

Pitfall: S, C and E can carry over from previous sample and appear in the next patient’s trace.

 

Extra Notes on Variants

b-variants (S, C, E, D, O) will make up roughly 50% of Hb (2 genes)

a-variants (G) will make up roughly 25% of Hb (4 genes)

 

If an a-variant and b-variant co-exist three peaks will form – the a, b and hybrid.

 

E.g.

hb 6.png