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Technical Briefs |
1
Department of Biopathology and Diagnostic Imaging, Tor Vergata University of Rome, Via Di Tor Vergata 135, 00133 Rome, Italy;
2
CSS-Mendel, Piazza Galeno 3, 00161 Rome, Italy;
3
Istituto di Ricovero e Cura a Carattere Scientifico, Fatebenefratelli, Via Pilastroni 4, 25125 Brescia, Italy;
a author for correspondence: fax 39-06-20427313, e-mail novelli{at}med.uniroma2.it
Myotonic dystrophy (DM; MIM 160900) is an autosomal dominant disorder associated with expansion of an unstable CTG trinucleotide repeat in the 3' untranslated region of the DM kinase gene (DMPK) on chromosome 19q13 (1). Patients are heterozygous for expanded alleles in the range of 504000 repeats (1). The molecular diagnosis of DM routinely is performed by analyzing the CTG number on genomic DNA extracted from various biological sources, including trophoblast cells sampled at 1011 weeks of amenorrhea during the first trimester of pregnancy (2)(3). We evaluated the possibility of using maternal plasma for prenatal diagnosis of DM, by monitoring the pregnancy of an unaffected woman whose husband was affected by DM (70 CTG repeats).
All participants gave oral and written informed consent.
A blood sample (~10 mL) was collected at 10 weeks of gestation before chorionic villus sampling (CVS) and was centrifuged at 3000g for 10 min. Plasma was carefully removed from EDTA-containing tube and centrifuged again at 3000g for 10 min. DNA was then extracted from 2 mL of the centrifuged plasma with a QIAamp Blood Kit (Qiagen). The elution volume of the final step was 300 µL. Genomic DNA was also extracted from chorionic villi and peripheral blood lymphocytes of both parents.
To check for the presence of fetal DNA in maternal plasma, we performed
microsatellite DNA analysis (CSF1PO) and Y-specific PCR (amelogenin)
amplification after having ascertained that the fetus was a male (Fig. 1
, A and B). DMPK CTG repeat amplification was carried
out as reported previously (2) with a slight modification. A
first round of PCR consisting of 15 cycles (30 s at 94 °C, 1 min at
62 °C, 5 min at 68 °C, and a final elongation of 5 min at
68 °C), was performed in 30 µL of reaction mixture, using 25 pmol
each of forward and reverse primers DMK9003
(5'-CACAGGCTGAAGTGGCAGTTCCA-3') and DMK11111
(5'-TGTCGGGGTCTCAGTGCATCCA-3') (2), and 510 µL of the
extracted DNA. We reamplified 1 µL of this first-round reaction,
using 25 pmol each of forward and reverse primers MDY-1D
(5'-GCTCGAAGGGTCCTTGTAGCCG-3') and MDY-Z2A (5'-TTCCCGAGTAAGCAGGCAGA-3')
(3) for 40 additional cycles, using the same cycling and
reaction conditions. Amplicons were separated by 1% agarose gel
electrophoresis and blotted onto a nylon membrane. Filters were
hybridized with (CTG)5
32P-labeled oligonucleotide as described
(3). The same protocol was used for genomic DNA extracted
from CVS and peripheral blood lymphocytes.
|
CTG-expanded alleles were detected in paternal DNA (70 CTG repeats),
maternal plasma DNA (150 CTG repeats), and trophoblast DNA (150 CTG
repeats). A single wild-type allele of approximately five CTG repeats
was found in the maternal genomic DNA (Fig. 1C
).
To demonstrate that large, CTG-expanded DMPK alleles (up to
2000 CTGs) can be detected in maternal plasma, we performed a long-PCR
to amplify an 8-kilobase DNA fragment of the basic protein Y2
(BPY2) gene mapping to the Y-chromosome (Fig. 1D
). PCR
consisting of 35 cycles (2 min at 94 °C, 30 s at 65 °C, 6
min at 68 °C, and a final elongation of 5 min at 68 °C) was
performed in 30 µL of reaction mixture using 25 pmol each of forward
and reverse primers 7R (5'-GGTATCTGAAGCTGGGTATATGAC-3') and 7F
(5'-AGATAACATCCATCGTGGCTCTG-3'; A. Pizzuti, unpublished data), and
510 µL of plasma extracted DNA.
These results support the possibility of performing prenatal diagnosis of DM with maternal plasma. At present, this test seems appropriate only for monitoring paternally inherited expanded alleles. Noninvasive DM prenatal diagnosis was reported previously by our group on trophoblast cells retrieved from the lower part of the uterine cavity (4). However, the amount of fetal DNA recovered with that procedure is low compared with the amount of fetal DNA recovered from maternal plasma (4)(5)(6). We conclude that this noninvasive method, which allows first-trimester DM prenatal diagnosis using maternal plasma, has the potential to become an alternative procedure in selected cases.
Acknowledgments
This work was supported by grants from Italian Telethon (Project 1061) and the Italian Ministry of Health.
References
The following articles in journals at HighWire Press have cited this article:
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