Saturday, November 10, 2012

Other Hemoglobins like HbC, HbD Punjab, Hb Lepore and HbE

Hemoglobin C: 

Hb C arises from substitution of lysine for glutamic acid at position 6 of β-globin chain. This may be homozygous or heterozygous state. It is the second most commonly studies after HbS of all Hb variants. There is large band in C position. HbF is variable.

Hemoglobin D Punjab: 

It is an Hb variant in which glutamic acid at position 121 of β-globin chain is replaced with glutamine. There is normal or marginally raised HbF and HbA2 with large band in HbD. There is a band in S position which migrates to A position in acid electrophoresis. There is mild decrease in Hb levels, MCV and MCH with target cell observed.

Hemoglobin Lepore: 

Hb lepore is classified as δβ hybrid Hb variant on the basis that the non-α-chain is a hybrid of δ and β globin chains. It is unique in that it is the only hemoglobinopathy named after the family name of the index case. It arise because there are deletions of part of 3’ portion of the δ globin gene are in 5’ portion of the β-globin chain with resultant formation of a δβ-fusion gene. There is greatly raised HbA2 with marginally reduced HbA. The HbA2 concentration is usually >10% of total Hb. 

There is band in S position or between A and S position. At acid pH there is a single band in A position for all Hb lepore variants. There is greatly reduced level of Hb, MCV, MCH. The similarity of the hematology in Hb Lepore and β-thalassemia makes the careful review between these two conditions. The two key pieces of evidence to distinguish between two are the finding of greatly raised HbA2 and small band in S position on electrophoresis at alkaline pH in Hb Lepore.

Hemoglobin E (β26 Glu → Lys):

It is also the most prevalent hemoglobin variant, and mainly found in south-east Asia. HbE has electrophoretic mobility similar to HbA2 or C at pH 8.6 but can be differentiated by citrate agar electrophoresis at acid pH as it migrates to HbA position. Such Hb are unstable.

Elongation Hemoglobins: 

These results from lengthening of either C or N terminus of either globin chain. E.g. Hb Constant Spring. In this variant the C-terminal TAA codon in α-chain gene is changed to CAA and there is an addition of 31 amino acid sequences at the C terminal end to give an α-globin chain of 173 amino acid residues rather than normal 142 residue. This increase in length results in instability of Hb variant, and synthesis of this elongated globin chain is reduced. In electrophoresis this variant migrates cathodally to the application point may be seen. This electrophoretic mobility is unique is that it is the only Hb variant that moves towards the cathode rather than anode.

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