Abstract
Newborns with trisomy 13 have elevated hemoglobin (HB) F; those with trisomy 21 have increased HB A. To examine this “switch”, blood was obtained from neonates, incubated with 3H-leucine, and globin chains separated on carboxymethylcellulose columns in urea: β/α ratios were in the range of β-thal trait in trisomy 13, and in 4/6 with trisomy 18. (β+γ)/α was also low. β/α was above normal in 2/6 with trisomy 18, and in trisomy 21. (β+γ)/α was high. Thus the high HB F in trisomy 13 results from normal γ/α synthesis, but a delay in turning on β chain. High HB A in trisomy 21 is due to an early and coordinated switch from γ to β synthesis. Trisomy 18 infants represent both types. Finally these data emphasize that abnormalities other than thal genes may lead to aberrant β/α ratios in neonates and also may affect the β/γ ratios in utero that are used to define hemoglobin phenotypes in prenatal diagnoses.
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