Molecular characterization of a de novo 6q24.2q25.3 duplication interrupting UTRN in a patient with arthrogryposis

Abstract

Chromosome 6q duplications have been documented repeatedly, allowing the delineation of a "6q duplication syndrome," characterized by hypertelorism, downslanting palpebral fissures, tented upper lip, short neck, severe mental and growth retardation, and joint contractures. Most reported cases result from malsegregation of a reciprocal translocation leading to a terminal 6q duplication and partial monosomy of another chromosome. Only 11 cases of de novo pure duplication have been reported so far. The breakpoints do not appear to be recurrent, but in most cases they have not been characterized molecularly, precluding genotype-phenotype correlation. We report on an 8-year-old girl with a phenotype consistent with mild 6q duplication syndrome, including characteristic physical findings, mild mental retardation, and joint contractures. She carries a 13 Mb de novo 6q24.2q25.3 duplication, diagnosed by high-resolution karyotype and confirmed by array-CGH. Molecular characterization of the duplicated segment with quantitative PCR showed that the proximal breakpoint is localized within the UTRN gene, encoding utrophin, the autosomal homologue of dystrophin. We discuss the possible implication of UTRN in arthrogryposis associated with duplications spanning the 6q23q26 region.

Figure 1

A,B. Photos of the patient at 4 years 11 months and 7 years 10 months showing hypertelorism, horizontal eyebrows, midface hypoplasia with relative exophthalmia, small tented upper lip, and microretrognathia. C,D. Bilateral syndactyly of the 2nd and 3rd toes, long palms of feet and hands compared to length of toes/fingers. E. Backside dimples.

Figure 2

Hand X-ray showing elongated and tubular-shaped metacarpal and phalanges, and disproportionately long first metacarpal and proximal phalanges compared to the second and third. Note joint stiffness preventing normal extension of fingers.

Figure 3

A. Partial high resolution karyotype, GTG banding showing the interstitial duplication: dup(6)(?q24.3q25.3). B. Tandem duplication of the 6q24.2q25.3 region (two arrows) compared to the normal chromosome 6 (single arrow). RP11-320C12 (green) maps to 6q24.3 and RP11-321K11 (red) maps to 6q25.2, demonstrating that the two copies are adjacent and oriented in the same sense.

Figure 4

A. Global analysis of the genome with array-CGH showing an excess of material on chromosome 6. B. Partial array-GCH results for chromosome 6 showing the duplication of eight adjacent clones in the 6q24.3q25.3 region.

Figure 5

Quantitative PCR probes were designed to detect copy number changes in the 6q24q25 region in the patient (violet) and three controls (blue). The names of the genes are followed by the number of the UPL probe used. A gene dosage ratio of 1 indicates the presence of 2 alleles and is considered normal; values above 1.5 correspond to a duplication (3 alleles). Data represent mean ± SD. The centromeric breakpoint of the duplication lies within the gene UTRN, between exon 42 and exon 54; the distal breakpoint is located between ZDHHC14 and SNX9.

Figure 6

Map of chromosome 6 showing the 6q24.2q25.3 duplication in our patient (red) and 11 other pure 6q duplications (blue). The approximate location of the previously described duplications characterized only by cytogenetic banding is represented by arrows, those mapped with FISH are represented by full lines with approximate location of breakpoints depicted as dotted lines, and one duplication mapped with SNP array is represented as a full line. The 13 Mb duplication in the present case contains 57 RefSeq genes, with the proximal breakpoint in the UTRN gene and the distal breakpoint between ZDHHC14 and SNX9 (dotted line). The position of qPCR probes is shown by verticals bars; black bars represent a normal copy number while red bars indicate a duplication.