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732 Part VI: The Erythrocyte Chapter 48: The Thalassemias: Disorders of Globin Synthesis 733

Figure 48–6. Activation of cryptic splice sites in
exon 1 as the cause of β -thalassemia, hemoglo-
+
bin E, and hemoglobin Knossos. The similarities
between the 5′ splice region of intron 1 and the
cryptic splice region in exon 1 are shown in capitals.

The boundaries of exons and introns are marked by invari- mRNA. 79–81 For example, a T→C substitution in this sequence leads to
ant dinucleotides, GT at the 5′ (donor) and AG at the 3′ (receptor) only one-tenth the normal amount of β-globin mRNA and hence the
sites. Single-base changes that involve either of these splice junctions severe β -thalassemia phenotype. 79
+
0
totally abolish normal RNA splicing and result in the β -thalassemia
phenotype. 7,62–66 Mutations Causing Abnormal Translation of Messenger RNA
Highly conserved sequences involved in mRNA processing sur- Base substitutions that change an amino acid codon into a chain ter-
round the invariant dinucleotides at the splice junctions. Different mination codon, that is, nonsense mutations, prevent translation of the
varieties of β-thalassemia involve single-base substitutions within the mRNA and result in β -thalassemia. Many substitutions of this type
0
consensus sequence of the IVS-1 donor site. 55,58,63–69 These mutations are have been described. For example, a codon 17 mutation is common
7,44
particularly interesting because of the remarkable variability in their in Southeast Asia, 82,83 and a codon 39 mutation occurs at a high fre-
associated phenotypes. For example, substitution of the G in position quency in the Mediterranean region. 84,85
+
5 of IVS-1 by C or T results in severe β -thalassemia. On the other The insertion or deletion of one, two, or four nucleotides in the
55
hand, a T→C change at position 6, found commonly in the Mediterra- coding region of the β-globin gene disrupts the normal reading frame
nean region, results in a very mild form of β -thalassemia. The G→C and results, upon translation of the mRNA, in the addition of anomalous
70
+
change at position 5 has also been found in Melanesia and appears to amino acids until a termination codon is reached in the new reading
be the most common cause of β-thalassemia in Papua New Guinea. 71 frame. Several frameshift mutations of this type have been described.
7,44
RNA processing is affected by mutations that create new splice sites Two mutations—the insertion of one nucleotide between codons 8 and
within either introns or exons. Again, these lesions are remarkably vari- 9 and a deletion of four nucleotides in codons 41 and 42—are common
able in their phenotypic effect, depending on the degree to which the in Asian Indians. The latter deletions are found frequently in different
63
new site is utilized compared with the normal splice site. For example, populations in Southeast Asia. 83
the G→A substitution at position 110 of IVS-1, which is one of the most An unusual β -thalassemia was described in a patient from the
+
common forms of β-thalassemia in the Mediterranean region, leads to Czech Republic in whom a full-length L1 transposon was inserted into
only approximately 10 percent splicing at the normal site and hence the second intron of β-globin, creating a β -thalassemia phenotype by
+
results in a severe β -thalassemia phenotype. 72,73 Similarly, a mutation an undefined molecular mechanism. 86
+
that produces a new acceptor site at position 116 in IVS-1 results in
little or no β-globin mRNA production and the β -thalassemia pheno- Dominantly Inherited β-Thalassemia
0
type. Several mutations that generate new donor sites within IVS-2 of Families in which a picture indistinguishable from moderately severe
74
the β-globin gene have been described. 55,68 β-thalassemia has segregated in mendelian dominant fashion have been
Another mechanism for abnormal splicing is activation of donor reported sporadically. 87,88 Because this condition often is characterized
sites within exons (Fig. 48–6). For example, within exon 1 is a cryptic by the presence of inclusion bodies in the red cell precursors, it has
donor site in the region of codons 24 through 27. This site contains a GT been called inclusion body β-thalassemia. However, because all severe
dinucleotide. An adjacent substitution that alters the site so that it more forms of β-thalassemia have inclusions in the red cell precursors, the
closely resembles the consensus donor splice site results in its activation, term dominantly inherited β-thalassemia is preferred. Sequence anal-
7,89
even though the normal site is active. Several mutations in this region ysis has shown that these conditions are heterogeneous at the molecular
can activate this site so that it is utilized during RNA processing, with level, but that many involve mutations of exon 3 of the β-globin gene.
the production of abnormal mRNAs. 75–78 Three of the substitutions— The mutations include frameshifts, premature chain termination muta-
A→G in codon 19, G→A in codon 26, and G→T in codon 27—result in tions, and complex rearrangements that lead to synthesis of truncated
reduced production of β-globin mRNA and an amino acid substitution or elongated and highly unstable β-globin gene products. 7,89–93 The most
so that the mRNA that is spliced normally is translated into protein. common mutation of this type is a GAA→TAA change at codon 121 that
The abnormal hemoglobins produced are hemoglobins Malay, E, and leads to synthesis of a truncated β-globin chain. Although an abnormal
94
Knossos, respectively, all of which are associated with a β-thalassemia β-chain product from loci affected by mutations of this type is unusual,
phenotype, presumably as a result of reduced overall output of normal many of these conditions are designated as hemoglobin variants.
mRNA (Fig. 48–6). A variety of other cryptic splice mutations within The reason why mutations occurring in exons 1 and 2 produce the
introns and exons have been described. 44 classic form of recessive β-thalassemia whereas the bulk of the domi-
Another class of processing mutations involves the polyadeny- nant thalassemias result from mutations in exon 3 has become clearer.
lation signal site AAUAAA in the 3′ untranslated region of β-globin In the former case, very little abnormal β-globin mRNA is found in the

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