Craniosynostosis and hemizygosity for D7S135 caused by a de novo and apparently balanced t(6;7) translocation

Ocular phenotype correlations in patients with TWIST versus FGFR3 genetic mutations

BACKGROUND/PURPOSE

Despite the similar clinical phenotype of the Saethre-Chotzen and Muenke craniosynostoses, the 2 syndromes are now genotypically distinct. Patients with Saethre-Chotzen and Muenke syndromes carry mutations in the TWIST and fibroblast growth factor receptor (FGFR) 3 genes, respectively. We sought to assess possible ocular phenotypic differences in patients with mutations of either gene previously grouped according to phenotype only.

METHODS

A retrospective chart review was performed for 21 children with known mutations of the TWIST (n=10) or the FGFR3 (n=11) genes. Data gathered included patient sex, age, family craniofacial history, craniofacial and ophthalmic surgeries, type of strabismus, ptosis, cycloplegic refraction, visual acuity, the presence of amblyopia, nasolacrimal duct obstruction (NLDO), nystagmus, hypertelorism, epicanthal fold anomalies, and any ocular structural abnormalities.

RESULTS

In the TWIST group, ptosis was present in 90%, amblyopia in 70%, horizontal strabismus in 70%, vertical strabismus in 60%, NLDO in 60%, astigmatism in 50%, inferior oblique overaction (IOOA) in 40%, hyperopia in 40%, myopia in 30%, nystagmus in 30%, and optic nerve findings in 30%. In the FGFR3 group, ptosis was present in 36%, amblyopia in 18%, horizontal strabismus in 55%, vertical strabismus in 36%, NLDO in 0%, astigmatism in 9%, IOOA in 45%, hyperopia in 27%, myopia in 18%, nystagmus in 18%, and optic nerve findings in 27%.

CONCLUSIONS

Patients with TWIST gene mutations may have more ophthalmic abnormalities, including more strabismus, ptosis, NLDO, astigmatism, vertical deviations, and amblyopia compared with patients with FGFR3 gene mutations.

The histone deacetylase 9 gene encodes multiple protein isoforms

Histone deacetylases (HDACs) perform an important function in transcriptional regulation by modifying the core histones of the nucleosome. We have now fully characterized a new member of the Class II HDAC family, HDAC9. The enzyme contains a conserved deacetylase domain, represses reporter activity when recruited to a promoter, and utilizes histones H3 and H4 as substrates in vitro and in vivo. HDAC9 is expressed in a tissue-specific pattern that partially overlaps that of HDAC4. Within the human hematopoietic system, expression of HDAC9 is biased toward cells of monocytic and lymphoid lineages. The HDAC9 gene encodes multiple protein isoforms, some of which display distinct cellular localization patterns. For example, full-length HDAC9 is localized in the nucleus, but the isoform lacking the region encoded by exon 7 is in the cytoplasm. HDAC9 interacts and co-localizes in vivo with a number of transcriptional repressors and co-repressors, including TEL and N-CoR, whose functions have been implicated in the pathogenesis of hematological malignancies. These results suggest that HDAC9 plays a role in hematopoiesis; its deregulated expression may be associated with some human cancers.

Facial dysgenesis: a novel facial syndrome with chromosome 7 deletion p15.1-21.1

We describe a female neonate with a unique constellation of features including anophthalmia and cryptophthalmos, temporal remnant "eye tags," bilateral cleft lip, unilateral cleft palate, a proboscis with absent nasal septum, choanal atresia, micrognathia, square stoma, and bilateral external auditory canal atresia. Gross brain structure, pituitary function, limbs, trunk, and genitalia were normal. Skeletal survey, echocardiogram and abdominal viscera were unremarkable except for a split central sinus of the right kidney. BAER exam indicated she could hear and temporal CT confirmed the presence of cochlea and possible ossicles. Cytogenetic evaluation revealed an interstitial deletion at chromosome 7p15.1-21.1. TWIST, a gene encoding a transcription factor involved in craniofacial development, is deleted by FISH analysis. The absence of a mutation on the non-deleted allele of TWIST as determined by sequencing virtually eliminates complete loss of the TWIST gene as the cause of this patient's severe phenotype. The HOXA gene cluster also encodes transcription factors that are crucial for directing cephalad to caudad somatic fetal development. HOXA1, the most telomeric of the 13 members of the HOXA gene cluster, is located at the centromeric boundary of the patient's chromosome 7 deletion. By FISH analysis, neither allele of HOXA1 is deleted and sequencing reveals no mutations. Haploinsufficiency or complete loss of the HOXA1 gene also does not appear to cause this patient's severe phenotype. Previous reports of chromosome 7p15-21 deletions do not have phenotypes similar to this patient.

Genetic heterogeneity of Saethre-Chotzen syndrome, due to TWIST and FGFR mutations

Thirty-two unrelated patients with features of Saethre-Chotzen syndrome, a common autosomal dominant condition of craniosynostosis and limb anomalies, were screened for mutations in TWIST, FGFR2, and FGFR3. Nine novel and three recurrent TWIST mutations were found in 12 families. Seven families were found to have the FGFR3 P250R mutation, and one individual was found to have an FGFR2 VV269-270 deletion. To date, our detection rate for TWIST or FGFR mutations is 68% in our Saethre-Chotzen syndrome patients, including our five patients elsewhere reported with TWIST mutations. More than 35 different TWIST mutations are now known in the literature. The most common phenotypic features, present in more than a third of our patients with TWIST mutations, are coronal synostosis, brachycephaly, low frontal hairline, facial asymmetry, ptosis, hypertelorism, broad great toes, and clinodactyly. Significant intra- and interfamilial phenotypic variability is present for either TWIST mutations or FGFR mutations. The overlap in clinical features and the presence, in the same genes, of mutations for more than one craniosynostotic condition-such as Saethre-Chotzen, Crouzon, and Pfeiffer syndromes-support the hypothesis that TWIST and FGFRs are components of the same molecular pathway involved in the modulation of craniofacial and limb development in humans.

Mutations of the TWIST gene in the Saethre-Chotzen syndrome

Saethre-Chotzen syndrome (acrocephalo-syndactyly type III, ACS III) is an autosomal dominant craniosynostosis with brachydactyly, soft tissue syndactyly and facial dysmorphism including ptosis, facial asymmetry and prominent ear crura. ACS III has been mapped to chromosome 7p21-22. Of interest, TWIST, the human counterpart of the murine Twist gene, has been localized on chromosome 7p21 as well. The Twist gene product is a transcription factor containing a basic helix-loop-helix (b-HLH) domain, required in head mesenchyme for cranial neural tube morphogenesis in mice. The co-localisation of ACS III and TWIST prompted us to screen ACS III patients for TWIST gene mutations especially as mice heterozygous for Twist null mutations displayed skull defects and duplication of hind leg digits. Here, we report 21-bp insertions and nonsense mutations of the TWIST gene (S127X, E130X) in seven ACS III probands and describe impairment of head mesenchyme induction by TWIST as a novel pathophysiological mechanism in human craniosynostoses.

Craniosynostosis suggestive of Saethre-Chotzen syndrome: clinical description of a large kindred and exclusion of candidate regions on 7p

We describe the clinical manifestations of an autosomal dominant form of craniosynostosis in a large family with eight affected relatives. Unilateral or bilateral coronal synostosis, low frontal hair line, strabismus, ptosis, and partial cutaneous syndactyly of fingers and toes are findings suggestive of the diagnosis of Saethre-Chotzen syndrome. The disease locus was excluded from the two adjacent Saethre-Chotzen candidate regions on 7p by linkage analysis with markers D7S664 and D7S507. This indicates heterogeneity of Saethre-Chotzen syndrome with a locus outside the candidate regions on 7p.

Duplication of 7p: further delineation of the phenotype and restriction of the critical region to the distal part of the short arm

We report on a patient with duplication of 7p15-->pter and review the literature. Patients with partial duplication of the distal 7p, including only the distal segment 7p15-->pter, have a syndrome comparable to that of patients with a larger or complete duplication of 7p. This suggests that the critical region for the dup(7p) phenotype is restricted to 7p15-->pter. The complete clinical phenotype of dup(7)(p15-->pter) includes mental retardation, skull anomalies, large anterior fontanel, cardiovascular defects, joint dislocation and contraction, and gastrointestinal and genital defects. Recognition of the clinical spectrum in patients with a smaller duplication of 7p, and the assignment of this critical region, should prove valuable for accurate counseling, prediction of outcome, and further gene mapping.

The molecular pathology of syndromic craniosynostosis

Several monogenic disorders result in craniosynostosis, the premature fusion of skull sutures in the neonate, causing craniofacial malformation and, occasionally, neurological compromise. These malformations were initially classified on a clinical basis, but several recent reports have clarified the underlying mutations in many of these syndromes, allowing the complexity of the relationship between mutation and resultant phenotype to be viewed more clearly. This article summarizes the current situation regarding syndromic craniosynostosis, highlights the complementarity of clinical, cytogenetic and molecular approaches that have contributed to the improved understanding of the genetic basis of craniosynostosis, and considers the new challenges that have emerged.

Saethre-Chotzen syndrome associated with balanced translocations involving 7p21: three further families

We describe three families segregating different reciprocal chromosome translocations, t(7;18)(p21.2;q23), t(2;7)(q21.1;p21.2), and t(5;7)(p15.3;p21.2). A total of seven apparently balanced carriers have been identified and all manifest features of the Saethre-Chotzen syndrome, although only two have overt craniosynostosis. In one family the carriers are immediately recognisable by their unusual ears, and clefts of the hard or soft palate are present in all three families. These observations extend previous linkage and cytogenetic evidence that a locus for Saethre-Chotzen syndrome resides in band 7p21.2.

The breakpoint on 7p in a patient with t(6;7) and craniosynostosis is spanned by a YAC clone containing the D7S503 locus

We previously reported a patient with an apparently balanced t(6;7) translocation and craniosynostosis. We now demonstrate, by fluorescence in situ hybridization, that the yeast artificial chromosome clone 933-e-1 from the Centre d'Etude du Polymorphisme Humain library harbouring the D7S503 locus spans the breakpoint on distal 7p. Recent reports have defined a candidate region for a Saethre-Chotzen craniosynostosis locus between the loci D7S513 and D7S516, a region that includes the D7S503 locus. Since the translocation carrier shows only some of the symptoms characteristic for the Saethre-Chotzen syndrome, it remains unresolved whether the gene disrupted by the translocation event is the only one causing craniosynostosis in this chromosomal region.