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| Description: Types of Mutations in the H88 Gene That Cause B-Thalassemia Number of Mutations Known 22 36 38 >100 Gene Region Affected 5 I upstream region mRNA CAP site 5 I untranslated region ATG translation initiation codon Exons 1, 2, and 3 coding regions Introns 1 and 2 Polyadenylation site Throughout and surrounding the H38 gene Description Single base—pair mutations occur between -101 and -25 upstream From transcription start site, For example, a T—>A transition in the TATA sequence at -30 results in decreased gene transcription and severe disease, Single base-pair mutation (A—>C transversion) at +1 position leads to decreased levels oFmRNA. Single base-pair mutations at +20, +22, and +33 cause decreases in transcription and translation and mild disease. Single base-pair mutations alter the mRNA AUG sequence, resulting in no translation and severe disease. Single base-pair missense and nonsense mutations, and mutations that create abnormal mRNA splice sites. Disease severity varies from mild to extreme. Single base-pair transitions and transversions that reduce or abolish mRNA splicing and create abnormal splice sites that affect mRNA stability. Most cause severe disease. Single base-pair changes in the AATAAA sequence reduce the effi— ciency oFmRNA cleavage and polyadenylation, yielding long mRNAs or unstable mRNAs. Disease is mild. Short insertions, deletions, and duplications that alter coding sequences, create FrameshiFt stop codons, and alter mRNA splicing.
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