Detection of the Sickle Mutation
Sickle cell disease results from a specific point mutation ( A > T) causing the substitution of
glutamic acid by valine at amino acid six in the beta-globin gene. This disease is inherited as an
autosomal recessive disorder and has a high carrier frequency of about 8% among Americans of
African origin. Carriers are said to have sickle cell trait and are usually asymptomatic while
individuals affected with sickle cell disease usually have serious medical complications. As shown
in the figure below, the mutation can be detected by a DNA-based test with 100% accuracy due to
the loss of a specific recognition site for the restriction endonuclease Dde I (or Mst II).
In the prenatal testing example shown below, lane 1 shows DNA size markers, lane 2 a no DNA
control, and lane 3 a full length PCR product of the beta-globin gene. Lanes 4 - 8 show beta-
globin PCR products digested with the restriction endonuclease Dde I. Lane 4 shows a no DNA
control, lane 5 shows a normal individual (HbB/HbB), lane 6 a carrier mother (HbB/HbS), lane 7
her carrier fetus (HbB/HbS), and lane 8 the carrier father (HbB/HbS). HbS carriers are detected by
the presence of the 381 bp DNA fragment. An affected individual (HbS/HbS) would not have any
201 bp and 180 bp DNA fragments.