Supplemental Readings and References
961
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Sac I
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Filipino-type Deletion
FIGURE VII-6
Restriction enzym e m ap o f Filipino-type deletion in a-globin gene cluster.
Bart’s is y-tetramer and does not function as a normal
hemoglobin.
Thalassemia syndromes also occur along with other mu-
tations in the a-globin gene. One example is hemoglobin
constant spring which is a result of mutation in the stop
codon and gives rise to an abnormally large a-globin chain
(Chapter 28). In rare cases, a-thalassemia is associated
with mental retardation. An X-linked form of mental re-
tardation, known as ATR-X syndrome, is a result of mu-
tations in XH2. gene. The XH2 gene is a member of a
subgroup of the family of genes that codes for proteins
within many diverse functions (e.g., DNA helicase, DNA
repair enzymes, putative global transcriptional regulatory
proteins). It is thought that XH2 mutations result in down-
regulation of a-globin gene expression.
Laboratory Diagnosis of a-Thalassemia Syndromes:
DNA-based analysis is utilized for definitive testing in
prospective parents for assessing the risk of transmis-
sion of a-thalassemia
- 1
genes and in prenatal diagnosis
to identify Hb-Bart’s hydrops fetalis. The methods may
include Southern blot analysis and PCR methodology us-
ing appropriate a-globin gene cluster probes and primers,
respectively. In Southern blot analysis, DNA obtained
from leukocytes or fetal cells is subjected to digestion
with specific restriction endonucleases, and the resulting
fragments are separated by electrophoresis. Blotting is
performed by alkaline transfer to nylon membranes and
exposed to labeled (e.g., radioactive, fluorescent) DNA
probes. The genotypes are established by using previously
defined restriction fragment length patterns. Figure VII-
5 shows the map of a-globin gene locus with restriction
sites useful for diagnosis. In the Filipino a-thalassemia-1
(a a /-Fl1), due to large deletion involving the entire
a-globin gene locus, digestion with restriction endonu-
clease, Bam HI, and Bglll is not useful in identifying the
deletion. Thus, Filipino a-thalassemia-1 is identified by
using a probe (“LO Probe”) derived from sequences of
about 4 kb 5' to the f -globin gene and digestion with the
resctriction endonuclease SacI or SstI (Figure VII-
6
).
Supplemental Readings and References
B. J. Bain, R. J. A m os, D. Bareford, et al.: The laboratory diagno-
sis o f haem oglobinopathies.
B ritish J o u rn a l o f H a em a to lo g y
101, 783
(1998).
D. K. Bowden, M . A. Vickers, and D. R. H iggs: A PC R -based strategy to
detect the com m on severe determ inates o f a-thalassaem ia.
B ritish Jo u rn a l
o f H a em a to lo g y
81, 104 (1992).
D. H. K. Chui, R. H ardison, C. Riem er, et al.: A n electronic database o f hu-
m an hem oglobin variants on the w orld w ide web.
B lo o d
9 1,2643 (1998).
N. Fischel-G hodsian, M . A. Vickers, M . Seip, et al.: C haracterization o f tw o
deletions that rem ove the entire hum an f - a globin gene com plex (—Thal
and —Fl1).
B ritish J o u rn a l o f H a em a to lo g y
70, 238 (1988).
R. J. G ibbons, D. J. Picketts, L. Villard, et al.: M utations in a putative
global transcriptional regulator cause X -linked m ental retardation w ith
«-thalassem ia (ATR-X syndrom e).
C ell
80, 845 (1995).
C. H. Joiner: Universal new born screening for hem oglobinopathies.
J o u rn a l
o f P ed ia trics
136, 145 (2000).
K. J. Skogerboe, S. F. W est, C. Sm ith, et al.: Screening for a-thalassem ia.
A rch ives o f P a th o lo g y a n d L a h o ra to iy M e d ic in e
116, 1012 (1992).
