Phenotypes of Hb Okayama Mutation
Authors: Ray Rudra , Biswas Ankita , Bhattacharjee Sunistha , Bhattacharyya Maitreyee 
Published: October 2019

Introduction:

Since its first detection in the year of 1983 very little has been reported about Hb Okayama mutation. Hb Okayama, reported as a silent mutation (Globin Gene Server Hb Var ID: 220, dbSNP rs713040), happened to be detected in the process of HPLC analysis for the measurement of HbA1c in Japanese diabetic patient [1,2,3]. Till date only few Hb Okayama has been reported from Japanese and Austrian ethnicity [2,3,4]. Its phenotypes and co-inheritance with other beta globin gene mutation is not yet known. In this study we report phenotypes of Hb Okayama in heterozygous as well as compound heterozygous state when it is co-existing with a common beta globin gene mutation. To the best of our knowledge no such study has been reported in world literature.

Methodology:

Two groups of patients- beta carriers requiring blood transfusion; and patients with low MCV, MCH but normal in HPLC were further investigated by molecular analysis. ARMS PCR analysis for beta globin gene mutation detection, GAP PCR for alpha deletion and triplication analysis and DNA sequencing (ABI 3500 Genetic Analyzer) to identify rare beta mutations were carried out.

Result:

Among around 300 patients subjected to molecular investigation over last three years there were 118 cases of thalassaemia trait (by HPLC and confirmed to carry a heterozygous common beta mutations by ARMS PCR analysis), but were requiring blood transfusion or behaving like intermedia. They were subjected to alpha globin gene triplication analysis by GAP PCR. There were 84 cases found to carry absence of alpha triplication with heterozygous beta mutation. Beta globin gene sequencing analysis of these 84 patients revealed that there were 4 patients carrying Okayama heterozygous mutation [ Figure 1, Figure 2 ] along with a common heterozygous beta gene mutation ( IVS 1-5 G>C mutation). HPLC report of these patients carrying Hb Okayama along with IVS 1-5 mutations in compound heterozygous state showed increased HbA2 values like that of beta trait [Table 1 ]. Another group of patients with low MCV, MCH but Normal in HPLC were subjected to alpha deletion analysis after ruling out low Ferritin level. There were 72 patients showing absence of alpha deletion by GAP PCR analysis and carried low MCV, MCH value with Normal HPLC parameters were also subjected to beta globin gene sequencing which revealed the presence of Hb Okayama mutation among two patients in heterozygous state [Figure- 1, Figure 2], who had HbA2 values in normal or border line range with low MCV, MCH levels [Table-2].

Discussion:

HbA2 measurement is used as the marker for screening of beta trait. Silent beta-thalassaemia carriers represent normal HbA2 level which makes their identification difficult. Okayama mutation or Hb Okayama is known to be a silent mutation [1-4] with normal HPLC. Hb Okayama is structural beta variant with a change of amino acid ( His > Gln) at Codon 2 (CD 2). Change in the nucleotide sequence at 70603 position from T to A or C (CAT>CAA or CAG) of beta globin gene (NG_000007.3) causes this mutation. The phenotypic associations of Hb Okayama in thalassaemia have very little been known till date. There has been report of very high expression of HbF(70%) value in HPLC resulting from compound heterozygous mutations one of which being silent (Cap+1) [ 5] ; where as there are also reports where the phenotypes of compound heterozygous including a silent mutation showing normal HPLC parameters [ 6 ]. However, in those studies no information about the clinical history and blood transfusion is described. In this study Hb Okayama heterozygous co-inheriting with IVS1-5(G>C) heterozygous mutation showed beta trait like HPLC parameters though all the patients carrying these compound heterozygous mutations required blood transfusion. The silent feature of Hb Okayama was evident in the case of the patients carrying only Hb Okayama mutation who showed absolutely normal HPLC parameters with low MCV, MCH and none of them requiring blood transfusion.

Conclusion:

Beta globin gene expression analysis to understand the association of Hb Okayama mutation in heterozygous and compound heterozygous states will enable to explain the mechanism of its phenotypes. How this mutation interferes with the expression of HbF or switching of delta globin gene is also to be understood as the HbF levels in HPLC was found to be like beta traits in this study.

Disclosures

No relevant conflicts of interest to declare.