Atlas de Genética

Atlas de Genética


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the three nucleotides, their location with re-
spect to the gene involved, and their clinical
features. All involve the central or the periph-
eral nervous system. Type I trinucleotide dis-
eases are characterized by CAG trinucleotide
expansion within the coding region of different
genes. The triplet CAG codes for glutamine.
About 20 CAG repeats occur normally in these
genes, so that about 20 glutamines occur in the
gene product. In the disease state the number of
glutamines is greatly increased in the protein.
Hence, they are collectively referred to as poly-
glutamine disorders.
Type II trinucleotide diseases are characterized
by expansion of CTG, GAA, GCC, or CGG trinu-
cleotides within a noncoding region of the gene
involved, either at the 5! end (GCC in fragile X
syndrome type A, FRAXA), at the 3! end (CGG in
FRAXE; CTG inmyotonic dystrophy), or in an in-
tron (GAA in Friedreich ataxia). A brief reviewof
these disorders is given on p. 394.
C. Principle of laboratory diagnosis of
unstable trinucleotide repeats
The laboratory diagnosis compares the sizes of
the trinucleotide repeats in the two alleles of
the gene examined. One can distinguish very
large expansions of repeats outside coding
sequences (50 to more than 1000 repeats) and
moderate expansion within coding sequences
(20 to 100\u2013200). The figure shows 11 lanes,
each representing one person: normal controls
in lanes 1\u20133; confirmed patients in lanes 4\u20136;
and a family with an affected father (lane 7), an
affected son (lane 10), the unaffected mother
(lane 11), and two unaffected children: a son
(lane 8) and a daughter (lane 9). Size markers
are shown at the left. Each lane represents a
polyacrylamide gel and the (CAG)n repeat of the
Huntington locus amplified by polymerase
chain reaction shown as a band of defined size.
Each person shows the two alleles. In the af-
fected persons the band representing one allele
lies above the threshold in the expanded region
(in practice the bands are somewhat blurred
because the exact repeat size varies in DNA
from different cells).
References
Strachan, T., Read, A.P.: Human Molecular
Genetics. 2nd ed. Bios Scientific Publishers,
Oxford, 1999.
Warren, S. T.: The expanding world of trinu-
cleotide repeats. Science 271: 1374\u20131375,
1996.
Rosenberg, R.N.: DNA-triplet repeats and neu-
rologic disease. NewEng. J.Med. 335: 1222\u2013
1224, 1996.
Zoghbi, H.Y.: Spinocerebellar ataxia and other
disorders of trinucleotide repeats, pp. 913\u2013
920, In: Principles of Molecular Medicine,
J.C. Jameson, ed. Humana Press, Totowa, NJ,
1998.
Fundamentals
Passarge, Color Atlas of Genetics © 2001 Thieme
All rights reserved. Usage subject to terms and conditions of license.
79Trinucleotide Repeat Expansion
A. Different types of trinucleotide repeat expression
C A G C A G ( C A G ) n C A G C A G
n = 5 \u2013 35
C A G C A G C A G ( C A G ) n C A G C A G C A G
n = 20 \u2013 >300
2 Trinucleotide expansion (CGG or CTG) in noncoding regions
1 Polyglutamine (CAG) expansion in coding regions
C G G C G G ( C G G ) n C G G C G G
n = 6 \u2013 50
C G G C G G C G G ( C G G ) n C G G C G G C G G
n = 50 \u2013 1000
B. Unstable trinucleotide repeats in different diseases
C G G G A A C A G C T G C G G
(Gln)
A U G A U G
start stop
Exon Intron Exon Intron Exon Intron
FRAXA
mental
retardation
Friedreich
ataxia
Chorea Huntington
Spinal-bulbar muscular atrophy
Spinocerebellar ataxia 1
Other neurological diseases
Myotonic
dystrophy
FRAXE
mental
retardation
others
C. Principle of laboratory diagnosis of unstable trinucleotide repeats leading to expansion
N
um
be
r o
f t
rin
uc
le
ot
id
e 
re
pe
at
s
Size markers Three normal
control persons
Three
affected persons
N
or
m
al
Ex
pa
nd
ed
 a
lle
le
1 2 3 4 5 6 7 8 9 10 11
90
75
60
45
30
15
Passarge, Color Atlas of Genetics © 2001 Thieme
All rights reserved. Usage subject to terms and conditions of license.
80
DNA Repair
Life would not be possible without the ability to
repair damaged DNA. Since replication errors,
including mismatch, and harmful exogenous
factors are everyday problems for a living or-
ganism, a broad repertoire of repair genes has
evolved in prokaryotes and eukaryotes. The fol-
lowing types of DNA repair can be distinguished
by their basic mechanisms: (1) excision repair
to remove a damaged DNA site, such as a strand
with a thymine dimer; (2) mismatch repair to
correct errors of replication by excising a
stretch of single-stranded DNA containing the
wrong base; (3) repair of UV-damaged DNA
during replication; and (4) transcription-
coupled repair in active genes.
A. Excision repair
The damaged strand of DNA is distorted and can
be recognized by a set of three proteins, the
UvrA, UvrB, and UvrC endonucleases in pro-
karyotes and XPA, XPB, and XPC in human cells.
This DNA strand is cleaved on both sides of the
damage by an exonuclease protein complex,
and a stretch of about 12 or 13 nucleotides in
prokaryotes and 27 to 29 nucleotides in eukary-
otes is removed. DNA repair synthesis restores
the missing stretch and a DNA ligase closes the
gap.
B. Mismatch repair
Mismatch repair corrects errors of replication.
However, the newly synthesized DNA strand
containing the wrong base must be distin-
guished from the parent strand, and the site of a
mismatch identified. The former is based on a
difference in methylation in prokaryotes. The
daughter strand is undermethylated at this
stage. E. coli has threemismatch repair systems:
long patch, short patch, and very short patch.
The long patch system can replace 1 kbDNA and
more. It requires three repair proteins, MutH,
MutL, and MutS, which have the human homo-
logues hMSH1, hMLH1, and hMSH2. Mutations
in their respective genes lead to cancer due to
defective mismatch repair.
C. Replication repair of UV-damaged
DNA
DNA damage interferes with replication, es-
pecially in the leading strand. Large stretches
remain unreplicated beyond the damaged site
(in the 3! direction of the new strand) unless
swiftly repaired. The lagging strand is not af-
fected as much because Okazaki fragments
(about 100 nucleotides in length) of newly syn-
thesized DNA are also formed beyond the dam-
aged site. This leads to an asymmetric replica-
tion fork and single-stranded regions of the
leading strand. Aside from repair by recombina-
tion, the damaged site can be bypassed.
D. Double-strand repair by
homologous recombination
Double-strand damage is a common con-
sequence of \u3b3 radiation. An important human
pathway for mediating repair requires three
proteins, encoded by the genes ATM, BRCA1, and
BRCA2. Their names are derived from important
diseases that result from mutations in these
genes: ataxia telangiectasia (see p. 334) and
hereditary predisposition to breast cancer
(BRCA1 and BRCA2, see p. 328. ATM, a member
of a protein kinase family, is activated in re-
sponse to DNA damage (1). Its active form
phosphorylates BRCA1at specific sites (2).
Phosphorylated BRCA1 induces homologous re-
combination in cooperation with BRCA2 and
mRAD5, the mammalian homologue of E. coli
RecA repair protein (3). This is required for effi-
cient DNA double-break repair. Phosphorylated
BRCA1 may also be involved in transcription
and transcription-coupled DNA repair (4).
(Figure redrawn from Ventikaraman, 1999).
References
Buermeyer, A.B. et al.: Mammalian DNA mis-
match repair. Ann. Rev. Genet. 33:533\u2013564,
1999.
Cleaver, J.E.: Stopping DNA replication in its
tracks. Science 285:212\u2013213, 1999.
Cortez D., et al.: Requirement of ATM-depend-
ent phosphorylation of Brca1 in the DNA
damage response to double-strand breaks.
Science 286:1162\u20131166, 1999.
Masutani, C., et al.: The XPV (xeroderma pig-
mentosum variant) gene encodes human
DNA polymerase. Nature