* Junctional epidermolysis bullosa is a heterogeneous condition. Other subtypes of this disorder may result from mutations in the gene encoding type XVII collagen (COL17A1/BPAG2) on 10q24.3 [30].
† Junctional epidermolysis bullosa with pyloric atresia may also be caused by mutations in the gene for α6 integrin (ITGA6) on 2q31.1 [31].
‡ Some cases of epidermolysis bullosa simplex are caused by mutations in the keratin 5 gene (KRT5) on 12q13.13 [32].
§ Some cases of bullous congenital ichthyosiform erythroderma arise because of mutations in the gene encoding keratin 1 (KRT1) on 12q13.13 [33].
** Some cases of lamellar ichthyosis show linkage to different loci on 2q33–35 [34], 3p21 and 19p12–q12 [35].
Fig. 139.1 Chorionic villi sampled at 11 weeks’ gestation. These villi have had all traces of maternal decidua carefully removed so that pure fetal DNA can subsequently be extracted without the risk of maternal DNA contamination. The cleaned villi are digested using proteinase K or urea buffers, DNA is then precipitated following which previously optimized and characterized PCT protocols can be used to test for the presence or absence of mutations in a DNA-based prenatal test.
Apart from CVS, fetal DNA can also be extracted from fetal cells obtained by amniocentesis; this is usually performed at about 16 weeks’ gestation [37]. Amniotic fluid and its cells can be examined for morphological, cytogenetic, biochemical or molecular abnormalities. Amniotic fluid cells are derived from fetal epidermis, alimentary and genitourinary mucosa, and amnion. These cells have been used to assess inherited disorders with abnormal DNA synthesis and repair, as well as metabolic disorders that result in abnormal protein synthesis. However, now that the molecular basis of nearly all these conditions is known, such approaches are rarely necessary and most prenatal tests can be performed by direct analysis of genomic DNA pathology in fetal DNA following CVS or amniocentesis [38].
A number of inherited skin disorders in which specific gene mutations have been identified and used as a basis for first-trimester DNA-based prenatal diagnosis have been documented and are shown in Table 139.1, and an example of DNA analysis for one of these conditions is illustrated in Figure 139.2. To these disorders, several further conditions might be added in the near future, based on recent disclosures of pathogenic molecular events. Over the last 15 years, the most common dermatological indications for DNA-based prenatal testing have been Hallopeau–Siemens recessive dystrophic epidermolysis bullosa (EB) and Herlitz junctional EB [38]. Inherited disorders such as lamellar ichthyosis are also suitable for DNA-based prenatal testing [12], but this form of ichthyosis shows considerable genetic heterogeneity [34,35]. Therefore it is vital that specific abnormalities should first be identified in the proband, such as mutations in the keratinocyte transglutaminase gene (TGM1 on 14q11.2), before any prenatal test is planned. Clues to inherent TGM1 gene pathology in an individual case of lamellar ichthyosis can be obtained from immunohistochemical studies for the TGM1 enzyme in the proband’s skin [39,40]. Likewise, junctional EB is a heterogeneous disorder that results from autosomal recessive mutations in six different genes encoding structural components of the hemi-desmosomal anchoring filament network [41]. Therefore, determining the correct gene responsible for a specific case is crucial to the feasibility and accuracy of DNA-based prenatal testing. To that end, the development of monoclonal antibodies to candidate gene proteins and immunostaining of skin sections from affected individuals have become an integral part of the work-up for molecular studies for several recessive disorders that stem from loss-of-function mutations leading to reduced protein expression in the skin [41,42].
Fig. 139.2 DNA-based prenatal testing. Illustrated is a DNA-based prenatal test for the autosomal dominant ectrodactyly ectodermal dysplasia-clefting (EEC) syndrome. The mother had EEC syndrome, which means that the fetus had a 50% risk of also being affected. She had already had one son with EEC syndrome. (a) Sequencing of amplified DNA from the mother and the previously affected child shows a heterozygous G > A transition in exon 7 of the p63 gene, which changes an arginine residue (CGC) to histidine (CAC). The mutation is designated p.R279H. (b) Sequencing of amplified control DNA for this exon shows only the wild-type arginine codon (CGC). (c) DNA-based prenatal testing using restriction endonuclease digestion with AciI. The mutation p.R279H results in loss of a cut site for AciI. In control DNA (lanes C1 and C2), the PCR product is completely digested into fragments of 140 and 113 bp. By contrast, there is an additional undigested band of 253 bp on DNA from the affected mother and child. The fetal PCR products are digested similarly to the control DNA, indicating that the fetus has inherited two wild-type p63 alleles with respect to this mutation and is therefore predicted to be clinically unaffected with EEC syndrome.
Adapted from South et al. 2002 [18].
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Practical Aspects of DNA-Based Prenatal Diagnosis
The clinical application of DNA-based prenatal testing has had substantial benefits for families at risk for recurrence of further affected children with inherited skin disorders in subsequent pregnancies. Nevertheless, an important prerequisite to undertaking any DNA-based prenatal test procedure is the delineation of informative genetic markers. In most instances, it is important to have DNA samples from both parents and the affected individual to determine the pathogenic mutation(s) [1,2]. Availability of all these samples then permits screening for de novo
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