A NEW SILENT β CHAIN VARIANT: Hb HOUNSLOW [β 80(EF4)Asn→Tyr]
Préhu C1,2, Riou J2, Henthorn J3, Wajcman H1,2*
*Corresponding Author: Dr. Henri Wajcman, INSERM U841 eq 11, Hôpital Henri Mondor, 94010 Créteil, France; Tel.: +33(0)1-49-81-35-78; Fax: +31(0)148-99-33-45; E-mail: Henri.Wajcman@im3.inserm.fr
page: 29

RESULTS

An abnormal Hb was found during a neonatal screening in a child, and later in his 33-year-old father, both from Afghanistan. The father’s red blood cell parameters were: RBC 5.41 x 1012/L, Hb 14.9 g/dL, MCV 82.2 fL, MCH 27.5 pg. The variant, amounting to 37.5% of the total Hb, was observed eluting between Hb A and Hb A2 at 2.96 min. by cation exchange HPLC, using the β -Thalassemia Short Program (Bio-Rad Laboratories) (Fig. 1). It was not separated from Hb A by IEF or by any other electrophoretic methods except for globin chain electrophoresis in the presence of urea and Triton X-100 where it was slightly more hydrophobic than normal (21.5 in an arbitrary scale where normal α -globin migrates at 10.0 and β at 20.0). This increased hydrophobicity of the β chain was confirmed by analytical reversed phase HPLC [5]: its retention time was 12.0 in a scale where normal β elutes at 10.0 and normal α at 20.0 (Fig. 2). Electrospray mass spectrometric analysis of the globin showed an abnormal β chain with a mass increased by 49 ± 1Da as compared to normal, which could correspond to ValPhe, AspTyr or AsnTyr.

Globin was prepared from the total hemolysate and the chains were separated by reversed phase HPLC. Since the two b chains could not be separated at the preparative level, a mixture containing the normal and abnormal b chains was aminoethylated, digested with trypsin and analyzed by reversed phase HPLC. The elution pattern of the tryptic peptides showed that there was an abnormal βT-9 peptide that eluted after the normal one (Fig. 3). Tandem mass spectrometry analysis of this peptide and of the uncleaved βT-8,9, both of which displayed a 49 Da mass increase compared to the normal, showed that the mass difference started at ion Y3 (…Asn-Leu-Lys), for which the value was 423 Da instead of 374Da. The structural change was therefore located at position 80 where the usual asparagine was replaced by a tyrosine (Fig. 4). This variant has been named Hb Hounslow for the place where the proband lives. According to the HUGO nomenclature, this amino acid exchange should correspond, at the DNA level, to the HBB: c.241AT mutation.

Figure 1. Elution pattern of the hemolysate by cation exchange HPLC.

Figure 2. Reversed phase HPLC analysis of the globin chains.

Figure 3. Reversed phase HPLC elution pattern of the peptides resulting from digestion with trypsin of a mixture of normal and abnormal b chains.

 

 




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