Fig. 8.1
Clinical presentation of the probands with JEB-PA. (a) A hypoplastic ear with atrophic surrounding skin and (b) loss of epidermis with exposure of reddened subcutaneous tissue. (c) Markedly atrophic ears but the external auditory meatuses were apparent
8.4 Pathogenesis
In lethal JEB-PA, mutations usually consist of premature termination codons (PTC) affecting both ITGB4 alleles which result in the complete absence of α[alpha]6β[beta]4 integrin; missense or splice site mutations are more prevalent in nonlethal forms [16, 17, 22]. However, it is not only the nature but also the position of mutations reflected in the protein functional domains of β[beta]4 integrin that affect the phenotype of JEB-PA [23]. There are unknown factors such as epigenetic or environmental factors that may influence whether pyloric atresia occurs or not, as has been described in variability of pseudosyndactyly in recessive dystrophic EB with metalloproteinase 1 isoforms [24]. Why the skin splits in the lower basal layer in some of these cases, causing EBS, rather than the lamina lucida, causing JEB, may relate to the location of the mutations.
8.5 Prevalence of PTC/PTC Mutations in Lethal JEB-PA
Nonsense, small out-of-frame insertion or deletion mutations in ITGB4 predicted synthesis of a truncated polypeptide and/or downregulation of the ITGB4 mRNA levels by nonsense-mediated mRNA decay [25, 26]. Thus, no functional integrin β[beta]4 polypeptides are synthesized, resulting in the JEB-PA phenotype. The presence of PTC mutations in both alleles, either in a homozygous or compound heterozygous state, would result in a lethal phenotype [16]. For example, p.C738X/c.4791delCA combined two PTC mutations. The p.C738X mutation within the large cytoplasmic domain was adjacent to the transmembrane segment and is predicted to cause deletions of the entire intracellular domain of the integrin β[beta]4 polypeptide, which could affect HD assembly, but ligand binding is preserved [27]. The c.4791delCA mutation is predicted to delete the region spanning the last 278 amino acids, which have been identified to interact with the 180-kD bullous pemphigoid antigen (BP180) [28].
However, some nonlethal cases were homozygous for PTC/PTC mutations like 4790delTC/4790delTC, 4580del2/4580del2 and 5046delC/5046delC, respectively [17, 29]. IFM was only reported for the first one, showing reduced but positive staining of both integrins α[alpha]6β[beta]4. One case was heterozygous for a PTC mutation (3793+1G-A/W1478X) with positive but reduced staining for integrin β[beta]4 and normal integrin α[alpha]6 staining [15]. All of these mutations predicted truncation of integrin β[beta]4 polypeptides close to the carboxyl-terminal end and might have minor effects.
8.6 Missense/Missense Mutations Resulted Different Clinical Variants in a Position-Dependent Pattern
Lethal and nonlethal JEB-PA cases with ITGB4 [16, 17, 29, 30] mutations had missense/missense combinations, which suggested that the position of these mutations influenced the phenotype. The mutations in three lethal cases were located in the extracellular domain of the integrin β[beta]4 protein [16, 17, 29]. The mutations p.C61Y/p.C61Y and p.C245Y/p.C61Y changed cysteine residues to lysine, which may interfere with the formation of intra- or inter-chain disulfide bonds and subsequently change the conformation and/or ligand-binding affinity of integrin β[beta]4 [16]. Some of the missense mutations in ITGB4, including these ones, resulted in substitution of highly conserved cysteine residues, most of which were associated with a severe phenotype [22]. Alternatively, missense mutations that affect highly conserved residues may have significant effects. For example, p.D131Y/p.G273D, which was lethal at only 2 months of age, and aspartic-131 and glycine-273 are located in a highly conserved region, so these mutations may abolish important ligand binding sites of integrin β[beta]4 [17].
Previous work has shown that the recruitment of plectin into HD was dependent on a region of the integrin β[beta]4 cytoplasmic domain containing the first two FNIII repeats and a short region of the CS [5, 9]. Two missense mutations, p.R1281W and p.R1225H, in the nonlethal form of JEB-PA were located in the second FNIII repeat [16, 17]. R1281W was located in a loop region that connected two β[beta] strands, whereas R1225H is located at the N-terminal end of the second FNIII repeat [31]. Both mutations caused a disruption of the interactions with plectin. Thus, the linkage of the intermediate filaments to HD was likely to be compromised because of an inability of integrin β[beta]4 to recruit plectin into HD. This helps to explain why these mutations caused a nonlethal type of JEB-PA.
8.7 Missense/PTC Mutations Associated with Lethal and Nonlethal Phenotype
The presence of a missense mutation in one allele combining with a PTC mutation could predict a more variable phenotype. Six lethal cases in a previous study [17, 21, 32, 33] and four nonlethal cases [15, 18, 29, 34] were compound heterozygotes for PTC and missense mutations. PTC could cause mRNA decay or synthesize truncated non-functional integrin β[beta]4 polypeptides. Therefore, the missense mutation would direct the phenotype of patients. For example, p.C245G along with p.R252C highly conserved amino acids located in a putative ligand-binding region in integrin β[beta]4 polypeptides in human, rat and mouse [17, 21]; these mutations created or abolished cysteine residues changing disulfide bonding and the secondary structure of the integrin β[beta]4 polypeptides. Mutations p.V325D and p.G273D also occurring in a conserved position substitute a nonpolar for an acidic residue [17]. Therefore, these phenotypes were lethal when PTCs were combined with missense mutations, including c.120delTG/p.C245G, c.658delC/p.R252C, c.1874delTCTinsC/p.V325D, c.4298–4299ins4/p.R252C and c.3903dupC/p.G273D [17, 21, 32].
In nonlethal cases, such as p.C38R /c.4776delG, c.4776delG resulting in a downstream PTC also predicted an unstable mRNA transcript. The missense mutation, p.C38R, arose in the part of the extracellular amino-terminal domain, a position in a highly conserved region in terms of different integrin β[beta]4 polypeptides and different species. So the mutation might disrupt heterodimer formation with the integrin α[alpha]6 subunit or interaction with ligands within the lamina lucida. Perturbation rather than abolition of β[beta]4 subunit function by p.C38R might explain the mild phenotype with only minimal cutaneous involvement and no evidence of other manifestation [15].
8.8 Conclusion
In summary, the results indicated that PTC mutations in both alleles either in a homozygous or in a compound heterozygous state would more likely result in a lethal phenotype. The missense mutation, either in combination with a PTC mutation or in both alleles, could predict lethal and nonlethal varieties. Some of the missense mutations involving substitutions of highly conserved amino acids might be associated with lethal phenotype and affecting less conserved amino acids are associated with a milder phenotype. However, it was difficult to predict the precise genotype-phenotype correction, as it depends on the nature of the mutations in the ITGA6 and ITGB4 genes as well as the patient’s other genetic background risks.
References
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Hogervorst F, Kuikman I, von dem Borne AE, Sonnenberg A. Cloning and sequence analysis of beta-4 cDNA: an integrin subunit that contains a unique 118 kd cytoplasmic domain. EMBO J. 1990;9(3):765–70.PubMedCentralPubMed
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