Genetic heterogeneity of cutis laxa: a heterozygous tandem duplication within the fibulin-5 (FBLN5) gene

Abstract

Inherited cutis laxa is a connective tissue disorder characterized by loose skin and variable internal organ involvement, resulting from paucity of elastic fibers. Elsewhere, frameshift mutations in the elastin gene have been reported in three families with autosomal dominant inheritance, and a family with autosomal recessive cutis laxa was recently reported to have a homozygous missense mutation in the fibulin-5 gene. In the present study, we analyzed the gene expression of elastin and fibulins 1-5 in fibroblasts from five patients with cutis laxa. One patient was found to express both normal (2.2 kb) and mutant (2.7 kb) fibulin-5 mRNA transcripts. The larger transcript contains an internal duplication of 483 nucleotides, which resulted in the synthesis and secretion of a mutant fibulin-5 protein with four additional tandem calcium-binding epidermal growth factor-like motifs. The mutation arose from a 22-kb tandem gene duplication, encompassing the sequence from intron 4 to exon 9. No fibulin-5 or elastin mutations were detected in the other patients. The results demonstrate that a heterozygous mutation in fibulin-5 can cause cutis laxa and also suggest that fibulin-5 and elastin gene mutations are not the exclusive cause of the disease.

Figure 1

Northern blot analysis of gene expression in fibroblasts from patients with cutis laxa. All fibroblast cultures were derived from skin tissue except for the culture from patient CL-2, which was established from lung tissue. Fibroblasts from patient CL-7 (GM02768) were obtained from Coriell Cell Repositories. Control dermal and lung fibroblasts were cultured from clinically normal individuals. Fibroblasts were grown in Dulbecco’s modified Eagle medium with 10% fetal bovine serum at 37 °C in 5% CO₂. The fibroblast cultures were examined at passage 4–7 except for CL-7, which was examined at passage 11–13. Total RNA (10 μg) from fibroblasts of the control subjects and patients CL-2–CL-8 was transferred to Duralon-UV membrane (Stratagene) and hybridized sequentially with α-[³²P]dCTP–labeled cDNA probes for fibulin-5, fibulin-4, elastin, fibulin-3, fibulin-2, fibulin-1, and β-actin, in that order. The cDNA probes were generated by RT-PCR. The sizes of the transcripts are indicated on the right. Arrows highlight qualitative and quantitative changes of gene expression (see text).

Figure 2

Schematic diagram of normal and mutant fibulin-5 proteins and genes. A, top, Human fibulin-5 protein consists of 448 amino acids, including a signal peptide (SP) of 23 residues at the N-terminus, six tandem cbEGF motifs (domain II, a–f), and a C-terminal globular domain (domain III) of 134 amino acids. The first cbEGF motif (IIa) is atypical, as it contains an unusually long linker sequence of 28 amino acids between the 4th and 5th cysteine residues. An RGD sequence is present in the first cbEGF domain. The locations of three primer pairs (1F/1R, 2F/2R, and 3F/3R) used for RT-PCR amplification of the fibulin-5 transcript are indicated. Bottom, The fibulin-5 gene, consisting of 11 exons spanning 80 kb of DNA on human chromosome 14. The six cbEGF motifs are each encoded by a single exon (exons 4–9). Exons 1 and 2 encode the signal peptide, exon 3 encodes a partial cbEGF motif, and exons 10 and 11 encode the C-terminal globular domain. A large intron of 42 kb separates exons 4 and 5, which encode the first and second cbEGF motifs. B, top, The mutant fibulin-5 protein with a tandem duplication of four cbEGF motifs, IIb–IIe (shaded). Bottom, The mutant allele from patient CL-5 containing a tandem gene duplication of 22 kb, extending from intron 4 to exon 9.

Figure 3

DNA sequence analysis of the fibulin-5 mutation in patient CL-5. A, Sequence of the duplication boundary in the fibulin-5 mRNA. RT-PCR amplification of total RNA from patient CL-5, with primers 3F and 660R (see fig. 2B), generated a 333-bp mutant-specific product, which was sequenced with the primer 3F. Sequencing showed that nucleotide 852 in the fibulin-5 transcript was followed by nucleotide 369, corresponding to the joining exon 8 and exon 5. DNA and amino acid sequences of the mutant fibulin-5 and the corresponding regions of the normal fibulin-5 are shown. Arrow and arrowheads mark the exon boarders. B, Sequence of the duplication boundary in the fibulin-5 gene. PCR amplification of genomic DNA from patient CL-5 with primers 3F and 660R (fig. 2B) yielded an 11-kb product, which was cloned and sequenced with primer I8F3 (TCCGTAGTAGTGCCAGGCAA). Sequencing showed that, following nine nucleotides from the beginning of exon 9, there is the sequence of intron 4 at 9,063 nucleotides upstream of exon 5. Sequences of the mutant allele, and the corresponding regions of the normal allele in intron 8/exon 9 and intron 4, are aligned (E9 = exon 9; I4 = intron 4; I8 = intron 8). Exon sequences are shown in uppercase letters, and intron sequences are shown in lowercase letters. The box depicts the 3-bp sequence that is identical between exon 9 and intron 4, where the recombination occurred. The box with dotted lines shows that the nucleotide identity extends an additional 4 bp if a single-nucleotide gap in intron 4 is introduced.

Figure 4

Analysis of the mutant fibulin-5 protein in patient CL-5. A, In vitro transcription/translation. The full-length normal and mutant fibulin-5 cDNAs were RT-PCR amplified, using Turbo pfu polymerase (Stratagene), with 1F/3R primers, and cloned into the pcDNA3 expression vector downstream of the T7 promoter. The expression vectors were subjected to coupled in vitro transcription/translation using the TNT Coupled Reticulocyte Lysate System (Promega) in the presence of fluorescent lysyl-tRNA (FluoroTect GreenLys, Promega). The resulting products were separated on a 7.5% polyacrylamide gel and detected by a FluorImager. Translation of the normal (lane 1) and mutant (lane 2) fibulin-5 constructs resulted in 60- and 80-kDa proteins, respectively, whereas translation of an expression construct of normal fibulin-5 in the opposite orientation yielded no product (lane 3). B, Immunoprecipitation. Fibroblasts were metabolically labeled with [³⁵S]cysteine (ICN Biochemicals) overnight in serum-free medium, and culture medium (1 ml) from patient CL-5 (lane 1) and the control subjects (lane 2) was immunoprecipitated with an affinity-purified antibody against fibulin-5, following the procedure described elsewhere (Grässel et al. 1996). The rabbit antiserum was raised from recombinant fibulin-5 protein expressed in 293-EBNA cells, using the pCEP-Pu vector as described elsewhere (Giltay et al. 1999). The antibody was affinity purified on the antigen-coupled Sepharose 4B (Timpl 1982). The immunoprecipitated products were separated on a 7.5% polyacrylamide gel and were detected with a PhosphorImager.