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Clinical variability of Stickler syndrome with aCOL2A1 haploinsufficiency mutation: implications for genetic counselling
  1. J FABER*,
  2. A WINTERPACHT*,
  3. B ZABEL*,
  4. W GNOINSKI,
  5. A SCHINZEL,
  6. B STEINMANN§,
  7. A SUPERTI-FURGA§
  1. * Children's Hospital, University of Mainz, Langenbeckstrasse 1, D-55101 Mainz, Germany
  2. Clinic for Maxillary Orthopaedics and Child Dentistry, University of Zurich, CH-8028 Zurich, Switzerland
  3. Institute of Medical Genetics, University of Zurich, CH-8032 Zurich, Switzerland
  4. § Division of Metabolic and Molecular Diseases, Department of Paediatrics, University of Zurich, CH-8032 Zurich, Switzerland.

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    Editor—Snead and Yates1 have recently reviewed clinical and molecular findings in Stickler syndrome, the autosomal dominant connective tissue disorder characterised by ocular manifestations, facial abnormalities, cleft palate, sensorineural hearing loss, and degeneration of epiphyseal and articular cartilage (hereditary progressive arthro-ophthalmopathy).1-4 Mutations in the structural genes for collagen II (COL2A1) and collagen XI (COL11A1,COL11A2) have been identified in patients with a Stickler syndrome phenotype.5-13 Based on locus heterogeneity, a subclassification of COL2A1associated Stickler syndrome type I, COL11A1associated Stickler syndrome type III, andCOL11A2 associated Stickler syndrome type II was established (OMIM 108300, 120280, and 184840). A clinical subclassification based on the presence or absence of an ocular phenotype, and on the features of the ocular phenotype, correlates reasonably well with the genotype.1 9

    Clinical variability in Stickler syndrome is well known,6 14 15 but correlations with specific mutations are scarce. We report a novel COL2A1 gene mutation found in a patient with flat face, cleft palate, myopia, and hearing loss (Stickler syndrome) and unexpectedly also in her father and her paternal grandmother who were considered to be healthy. The patient is the first child of healthy, non-consanguineous Swiss parents. The pregnancy was uneventful and she was delivered at term by caesarean section because of breech position. Birth weight was 3890 g (90th centile), birth length 50 cm (50th centile), and head circumference 37 cm (>97th centile). Macrocephaly and facial dysmorphism were noted in the neonatal period, including a flat midface, deep set ears, exophthalmos, palpebral oedema with telangiectasia, micrognathia, and median clefting of the soft and part of the hard palate (fig 1). No other abnormalities were recognised at that time. At the age of 5 years, midface hypoplasia and micrognathia were still evident. In addition, she presented with mild bilateral hypoacusis, bilateral myopia (4 dioptres), slight webbing of the neck, minimal pectus carinatum, and flat feet (fig 1). Her growth was on the 90th centile. The clinical signs and symptoms as well as radiological findings of mild spondyloepiphyseal dysplasia suggested the diagnosis of Stickler syndrome.

    Figure 1

    The proband in the newborn period (note midface hypoplasia, micrognathia, apparent exophthalmos, and deep set ears) and at 5 years of age. The face is slightly flat with maxillary hypoplasia and retrognathia. Her height is on the 90th centile; note also mild neck webbing and flat feet. (All photographs reproduced with permission.)

    All 54 exons of the COL2A1 gene, including the flanking splice sites, were amplified and screened by SSCP analysis. Exon 12 gave an abnormal pattern and was subcloned and sequenced, showing a deletion of 2 bp (nt 697-698), for which the patient was heterozygous. The mutation predicts a downstream premature TGA-stop codon in exon 13 (fig 2). The father's DNA gave a similar heteroduplex pattern, and direct sequencing confirmed his heterozygous mutation carrier status. He had been regarded as unaffected, but re-evaluation showed some features consistent with Stickler syndrome including mild bilateral myopia (2 dioptres), a high arched palate, and a partially split uvula. His body habitus showed no distinctive features except for his height of 190 cm (>97th centile) (fig 3); hearing was normal and there was no history of arthropathy. Childhood photographs were reviewed and found to be unremarkable. We subsequently investigated the paternal grandparents and found that the grandmother carried the same COL2A1 gene mutation in the heterozygous state in her blood leucocytes. She considered herself healthy and declined further investigations. Her physician observed no features of Stickler syndrome. To rule out the presence of a secondCOL2A1 mutation in the proband leading to a more severe clinical phenotype, all 54 exons and the flanking intron splice site sequences were analysed by direct sequencing in the proband as well as in her father and grandmother. Except for a few already known common polymorphisms, no additional sequence abnormality was observed.

    Figure 2

    (A) Exon 12 of the COL2A1 gene was amplified from genomic DNA, subcloned, and sequenced. The mutant clones show a deletion of two nucleotides in exon 12. This leads to a shift in the translational reading frame and the creation of a premature stop codon in exon 13. The corresponding truncated procollagen chains lack the carboxy-terminal domain and cannot participate in procollagen trimer assembly, without exerting other negative effects (haploinsufficiency). (B) Heteroduplex analysis of the exon 12 amplicon in the patient (Pt), her mother (Mo) and father (Fa), the paternal grandmother (Gm) and grandfather (Gf), as well as three controls (C1, C2, C3). Blood leucocyte DNA was used as template. The figure shows the presence of heteroduplex PCR products as two slower migrating bands. Slow migration is caused by the “parachute” effect of the two mismatched bases.

    Figure 3

    The patient's father at 41 years of age. He is 190 cm tall and thus on the 97th centile for the Swiss population. He has mild myopia (2 dioptres), a compatible but non-specific finding. Little in his physical appearance suggests a collagen disorder; the only telltale finding was a partially bifid uvula.

    The 2 bp deletion in exon 12 of the COL2A1gene identified in this family leads to a frameshift with a premature stop codon in exon 13. This causes the synthesis of truncated procollagen α1(II) chains, which are unable to participate in collagen II triple helix formation, without exerting other negative effects, a mechanism referred to as functional haploinsufficiency.16 17 To date, at least 10COL2A1 gene mutations have been identified in patients with Stickler syndrome type I.15 18-24Except for a 28 bp deletion of exon 12,24 all mutations generate unique premature stop codons leading to functional haploinsufficiency. Findings in our proband provide further evidence that COL2A1 haploinsufficiency is a common molecular mechanism in patients with Stickler syndrome. However, findings in this family cast some new light on the role of such mutations, as the mutation was detected in two family members who were considered healthy. Apparently, COL2A1haploinsufficiency mutations do not necessarily result in the full Stickler phenotype but may cause a mild phenotype or remain below clinical expression.

    As briefly mentioned by Snead and Yates,1 variable clinical expression of Stickler syndrome complicates the genetic counselling scenario. In a child with apparently sporadic Stickler syndrome, caution must be used before assuming a de novo mutation; accurate radiographic and clinical examination of the parents, including formal testing of hearing and vision,1 is indicated, and a mutation search should be undertaken whenever possible. On the positive side, counselling must take into account the possibility that inheritance of such a mutation may have only minimal clinical consequences or remain silent, but it can be difficult for prospective parents to make use of information with this degree of uncertainty.

    Some indications for future research may be derived. First, the question might be asked of whether variability is more frequent inCOL2A1 haploinsufficiency mutations than in the other COL11A1 andCOL11A2 mutations associated with Stickler syndrome. Second, a systematic ascertainment of families segregating such COL2A1 haploinsufficiency mutations might allow the penetrance of single clinical traits, such as cleft palate or severe myopia, to be determined. It cannot be excluded at present that among carriers of such mutations, subjects with the full blown Stickler phenotype are the exception rather than the rule; this remains to be investigated. Such insight would be helpful for counselling and parental decision making.

    Acknowledgments

    We are grateful to our young patient and her parents for cooperation and for permission to publish their photographs; to her grandparents and Dr Z Schneider for clinical information; and to Pia Hermanns, Jutta Busch, and Sabine Fleig for expert technical assistance. Our investigations were supported by a grant from the Swiss National Foundation (32-45401.95 and 32-57272.99) to Andrea Superti-Furga and from the Deutsche Forschungsgemeinschaft to Andreas Winterpacht and Bernhard Zabel.

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