Hb St. Jozef, A Val→Leu N-Terminal Mutation Leading to Retention of the Methionine, and Partial Acetylation Found in the Globin Gene in Cis with a −α^3.7 Thalassemia Deletion

Abstract

We report a new hemoglobin (Hb) variant found in a 6-year-old girl of Moroccan origin, living in the Dutch city of Gouda. The child was referred because of microcytic and hypochromic parameters. A normal zinc protoporphyirin (ZPP) value excluded iron deficiency and gap-polymerase chain reaction (gap-PCR) revealed a heterozygosity for the common −α^3.7 thalassemia deletion, partially justifying the hematological picture. The Hb pattern on alkaline electrophoresis and capillary electrophoresis was normal, while a fraction of 9% preceding the Hb A peak, remained visible on different high performance liquid chromatography (HPLC) devices. This fraction, located in front of the Hb A peak, is usually considered as a Hb A derivate that becomes more expressed in older samples. However, the sample was freshly collected and the peak unusually evident. Therefore, direct sequencing of the α-globin genes was performed revealing a GTG→CTG transversion at codon 1 of the α1-globin gene or of the hybrid gene. This point mutation induces a single amino acid substitution from valine to leucine. Electrospray-mass spectrometry (ES-MS) analysis revealed, in addition to this substitution, that the N-terminal methionine was retained and that about 20% of the variant was acetylated. As expected for an association with a −α^3.7-thalassemia (thal) deletion, the non acetylated and acetylated abnormal α chain amounted to 32% of the total α chains. Family studies revealed that the mutated codon was located in cis of the deletion.

Keywords:

• α Chain
• Abnormal hemoglobin (Hb)
• N-terminal processing
• Thalassemia