Fetal hemoglobin regulating genetic variants identified in homozygous (HbSS) and heterozygous (HbSA) subjects from South Mexico

Hemoglobin S is caused by a nucleotide change in HBB gene (HBB:c.20A>T, p.Glu6Val), is presented in diverse forms: simple carriers (HbSA), homozygotes (HbSS) also known as sickle cell anemia, and compound heterozygotes with other β-hemoglobinopathies. It is worldwide distributed, in Mexico, is frequently observed in the southern states Guerrero, Oaxaca and Chiapas. Elevated fetal hemoglobin (HbF) is associated with mild phenotype; single-nucleotide variants (SNVs) in modifier genes, such as BCL11A, HBG2, HBBP1 pseudogene and HBS1L-MYB intergenic region, upregulate HbF synthesis. The aim of this study was to identify HbF regulating genetic variants in HbSS and HbSA Mexican subjects. We studied 39 individuals (HbSS = 24, 61%, HbSA = 15, 39%) from Chiapas (67%) and Guerrero (33%), peripheral blood was collected in ethylenediamine tetraacetic acid (EDTA) for molecular and hematological studies, DNA was isolated by salting-out technic and genotyping was performed through allelic discrimination by real time polymerase chain reaction (RT-PCR) using Taqman® probes for 15 SNV (in BCL11A: rs6706648, rs7557939, rs4671393, rs11886868, rs766432, rs7599488, rs1427407; HBS1L-MYB: rs28384513, rs7776054, rs9399137, rs4895441, rs9402686, rs1320963; HBG2: rs7482144; and HBBP1: rs10128556). The obtained data were analyzed using IMB SPSS v.22.0 software. All minor alleles were observed in frequencies over 0.05, the most frequent was rs9402686 (0.82), while the less frequent was rs101028556 (0.08). In HbSS group, the mean fetal hemoglobin was 11.9 ± 5.9% and was significantly elevated in BCL11A rs11886868 wildtype homozygotes and in carriers of HBS1L-MYB intergenic region rs7776054 (p = 0.04 and p = 0.03, respectively). In conclusion, in HbSS Mexican patients, two SNVs were observed related to increased HbF; BCL11A rs11886868 and HBS1L-MYB rs7776054.

Three novel HBB mutations, c.-140C>G (-90 C>G), c.237_256delGGACAACCTCAAGGGCACCT (FS Cd 78/85 -20 bp), and c.315+2T>G (IVS2:2 T>G). Update of the mutational spectrum of β-Thalassemia in Mexican mestizo patients

INTRODUCTION

Beta-thalassemia (β-thal) is frequent in Mexican patients with microcytosis and hypochromia. We report three novel mutations and analyze the actual mutational spectrum in Mexican population.

METHODS

One hundred and forty-nine β-thal Mexican mestizo patients were studied (154 alleles). ARMS-PCR was performed to identify Cd39C>T, IVS1:1G>A, IVS1:110G>A, -28A>C, initiation codonA>G and IVS1:5G>A mutations, and gap-PCR for δβ-thal Spanish type. DNA sequencing of HBB gene was carried out in negative samples for the initial screening.

RESULTS

Fifteen different HBB gene mutations were observed in 148 alleles; three of them are novel: -90C>G, 20 bp deletion (at codons 78/85), and IVS2:2T>G; the mutation IVS1:6T>C that was observed for first time in our population; and eleven previously described mutations. Six alleles showed normal HBB sequence. To date, a total of 21 different mutations have been observed in Mexican patients; the four most frequent mutations are of Mediterranean origin: Cd39C>T (37.2%), IVS1:1G>A (17.3%), IVS1:110G>A (13.9%), and δβ-thal Spanish type (9.0%), which represent 77.4% of the total studied alleles.

CONCLUSION

Considering the novel mutations -90C>G, -20 bp Cd78/85, IVS2:2T>G and the first observation of IVS1:6T>C, the molecular spectrum of β-thal in Mexicans comprises 21 different mutations, confirming the high allelic heterogeneity in Mexicans.