HB-COSTA-RICA OR ALPHA(2)BETA(2)77(EF1)HIS-]ARG - THE FIRST EXAMPLE OF A SOMATIC-CELL MUTATION IN A GLOBIN GENE

We have identified a minor hemoglobin component (similar to 5%) in the blood of a healthy Costa Rican female, but not in her mother and two brothers (father not studied), that has an His-->Arg replacement at po sition beta 77 (Hb Costa Rica). No other amino acid replacements were observed and no beta- or gamma-chain-like peptides were present. Hb Co sta Rica has a normal stability. Sequence analyses of numerous polymer ase chain reaction (PCR)-amplified segments of DNA that contain exon 2 of the beta gene failed to identify a CAC-->CGC (His-->Arg) mutation. The same was the case when cDNA was sequenced, indicating that a beta -costa Rica-mRNA could not be detected with this procedure. Gene mappi ng of genomic DNA with Bg/II, BamHI, and HindIII gave normal fragments only and with the same intensity as observed for the fragments of a n ormal control. The quantities of the beta chain variants Hb J-Iran and Hb Fukuyama with related mutations at beta 77 vary between 30% and 45 % in heterozygotes, whereas that of Hb F-Kennestone with the same His- ->Arg mutation but in the (G) gamma-globin gene, is a high 40%-45% (as percentage of total (G) gamma) in a heterozygous newborn. These diffe rent observations exclude a heterozygosity of the A-->G mutation at co don beta 77, as well as a deletion comparable to that of Hbs Lepore or Kenya, or a beta-globin gene duplication, and point to a nontradition al inheritance of Hb Costa Rica. Allele-specific amplification of cDNA with appropriate primers identified the presence of a low level of mu tated mRNA in the reticulocytes of the patient, which was confirmed by dotblot analysis of the same material with P-32-labeled probes. Compa rable amplification products were not observed in genomic DNA. The A-- >G mutation apparently occurred in a somatic cell at a relatively earl y stage in the development of the hematopoietic cell system, and Hb Co sta Rica accumulated through rapid cell divisions in patchy areas in t he bone marrow (somatic mosaicism). An unequal distribution of Hb Cost a Rica over the red cells supports this possibility.