Codon.26.(GAG-@AAG).html

MUTATION

Codon 26 (GAG->AAG)

AMINO ACID REPLACEMENT

Hb E [beta26(B8)Glu->Lys].

NOTE

This abnormal Hb is the second or third most common beta chain variant in the world. Data about this abnormality were given in the first syllabus (1) and are reprinted here.

CONTACT	External
HEMATOLOGY	Mild microcytosis in heterozygotes; normal to mild anemia, reduced red cell survival, reduced MCV, target cells, reduced osmotic fragility in homozygotes
ELECTROPHORESIS	Hb E and Hb A can readily be separated at alkaline pH but not at acidic pH; Hb E moves much slower than Hb A, just ahead of Hb A₂ and Hb C
CHROMATOGRAPHY	Hb E and Hb A separate in cation and anion exchange chromatog-raphy; it elutes with Hb A₂ in several chromatographic systems, including cation exchange HPLC
STRUCTURE STUDIES	Tryptic digestion; fingerprinting; cation exchange chromatography; amino acid analysis; sequencing
DNA ANALYSES	A GAG->AAG mutation at codon 26
FUNCTION STUDIES	Purified Hb E has a normal oxygen affinity
STABILITY	Mildly unstable; it is sensitive to oxidative stress
OCCURRENCE	Widespread in the Far East; has been observed in combination with various Hb variants, including Hbs S, C, D, I, etc., and with different beta-thal alleles
OTHER INFORMATION	Quantity in the heterozygote with four alpha genes (alphaalpha/alphaalpha) ~30%; Hb E is one of the most frequently studied variants; references can be found in the listed monographs
REFERENCES
1.Weatherall, D.J. and Higgs, D.R., editors: The Haemoglobinopathies, Bailliere's Clinical Haematology, Vol. 6, W.B. Saunders Company, London, 1993.
2.Miller, D.R. and Baehner, R.L., editors: Blood Diseases in Infancy and Childhood, 7th edition, Mosby-Year Book, Inc., St. Louis, MO, 1995.

OTHER INFORMATION

The GAG->AAG mutation at codon 26 is associated with a beta-thal phenotype; the beta^E synthesis is less than expected because the formation of functional beta^E-mRNA is decreased due to abnormal alternative splicing of precursor beta-mRNA at a site 5' to the IVS-I. The Hb E heterozygote is mildly affected and the Hb E homozygosity is a benign disorder with a mild beta-globin chain deficit which is comparable to that seen in a beta°-thal heterozygote. However, compound heterozygotes are often severely affected as can be seen from the data for untransfused patients with Hb E-beta-thal listed in Table A. The often greatly increased production of Hb F should be noted. Conditions in which there is a considerable production of Hb A are milder than those without Hb A.

The beta^E mutation is known to occur on chromosomes with different haplotypes; three have been observed among the Southeast Asian populations and two in European families. These types are listed in Table B.

**TABLE A.** Hematological and Hemoglobin Composition Data for Patients With Different Forms of Hb E-beta-Thalassemia[1] (data are from the author's laboratory)
Beta-Thal Mutation	Origin	Hb g/dl	PCV l/l	RBC 10¹²/l	MCV fl	MCH pg	MCHC g/dl	A₂ %	F %	A %
Cds 8/9, +G	Surinam	6.5	0.338	3.67	92.0	17.7	19.2	8.0	30.4	0
Cd 19, A->G	Malaysia	10.0	0.308	5.05	61.0	19.8	32.5	?	6.8	0
Cd 35, C->A	Surinam; Malaysia	8.1	0.322	3.82	84.2	21.2	25.2	6.9	50.1	0
Cd 39, C->T	GA	8.1	0.324	4.05	80.0	20.0	25.0	6.0	57.2	0
Cds 41/42, -TTCT	China	5.8	0.238	3.47	68.5	16.7	24.4	8.0	21.4	0
Cds 71/72, +A	China	7.1	0.249	3.64	68.5	19.5	28.5	5.5	36.6	0
IVS-I-5, G->C	Malaysia	7.0	0.208	3.21	64.8	21.8	33.7	5.6	36.0	6.4
IVS-I-5, G->C	Bangladesh	7.3	0.282	3.73	75.6	19.6	25.9	6.9	64.5	6.2
Poly A, A->G	Malaysia	9.5	0.302	5.11	59.0	18.6	31.5	4.3	7.6	41.6
Lepore	Malaysia	5.8	0.143	2.04	70.0	28.4	33.6	3.4	19.5	0

[1] The MCV values are often high because of the aging of the mailed blood sample. Hb A₂ was determined as %delta (% of delta+beta+gamma) by reversed phase HPLC; Hb A by cation exchange HPLC.

**TABLE B.** Haplotypes for beta^E Chromosomes^a
Haplotype	[1]	[2]	[3]	[4]	[5]	[6]	[7]	[8]	Origin
E-a	-	+	+	-	+	+	+	-	Southeast Asian
E-b	+	-	-	-	-	-	+	-	Southeast Asian
E-c	-	+	+	-	+	+	-	+	Southeast Asian
E-d	+	-	+	+	-	+	+	-	Czech
E-e	+	n.d.	-	-	-	-	+	+	European

^aRestriction sites: [1] HincII 5' to epsilon; [2] XmnI 5' to ^Ggamma; [3] and [4] HindIII at ^Ggamma and ^Agamma; [5] and [6] HincII at psibeta and 3' to it; [7] AvaII at beta; [8] BamHI 3' to beta. The haplotypes are from Refs. 2, 3, and 4; n.d. = not determined.


REFERENCES
1.		Huisman, T.H.J., Carver, M.F.H., and Efremov, D.G.: A Syllabus of Human Hemoglobin Variants (1996), The Sickle Cell Anemia Foundation, Augusta, GA, 1996.
2.		Kazazian, H.H., Jr., Waber, P.G., Boehm, C.D., Lee, J.I., Antonarakis, S.E., and Fairbanks, V.F.: Am. J. Hum. Genet., 36:212, 1984.
3.		Nakatsuji, T., Kutlar, A., Kutlar, F., and Huisman, T.H.J.: Am. J. Hum. Genet., 38:981, 1986.
4.		Indrak, K., Fei, Y.J., Li, H-W., Baysal, E., Brabec, V., Fortova, H., Cermak, J., and Huisman, T.H.J.: Ann. Hematol., 63:42, 1991.

This material is from the book A Syllabus of Thalassemia Mutations (1997) by Titus H.J. Huisman, Marianne F.H. Carver, and Erol Baysal, published by The Sickle Cell Anemia Foundation in Augusta, GA, USA. Copyright © 1997 by Titus H.J. Huisman. All rights reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, microfilming and recording, or by any information storage and retrieval systems, without permission in writing from the Author.