Deletion of the TWIST gene in a large five-generation family

Nonsyndromic brachydactyly type D and type E mapped to 7p15 in healthy children and adults from the Jirel ethnic group in eastern Nepal

OBJECTIVES

There is phenotypic overlap between Brachydactyly Type D (BDD) and Brachydactyly Type E (BDE) that suggests a possible common underlying etiology. We seek to understand the genetic underpinnings of, and relationship between, these skeletal anomalies.

METHODS

The Jirel ethnic group of eastern Nepal participates in various genetic epidemiologic studies, including those in which hand-wrist radiographs have been taken to examine skeletal development. Nearly 2,130 individuals (969 males; 1,161 females) were phenotyped for BDD/BDE. Of these, 1,722 individuals (773 males; 949 females) were genotyped for 371 STR markers spanning the autosomal genome. Variance components-based linkage analysis was used to conduct a genome-wide linkage scan for QTL influencing the BDD/BDE phenotype.

RESULTS

BDD was present in 3.55%, and BDE was present in 0.39%, of the study sample. Because of the phenotypic overlap between two traits, affecteds of either type were considered as affected by a single combined phenotype (BDD/BDE) having a prevalence of 3.94%. The additive genetic heritability of BDD/BDE was highly significant (h(2) ± SE = 0.89 ± 0.13; P = 1.7 × 10(-11) ). Significant linkage of BDD/BDE was found to markers on chromosome 7p21-7p14 (peak LOD score = 3.74 at 7p15 between markers D7S493 and D7S516).

CONCLUSIONS

Possible positional candidate genes in the one-lod support interval of this QTL include TWIST and the HOXA1-A13 cluster. This is the first study to report significant linkage results for BDD/BDE using a large extended pedigree, and the first to suggest that mutations in TWIST and/or the HOXA1-A13 cluster may contribute to these specific skeletal anomalies.

TWIST microdeletion identified by array CGH in a patient presenting Saethre-Chotzen phenotype and a complex rearrangement involving chromosomes 2 and 7

Saethre-Chotzen syndrome (SCS), also known as acrocephalosyndactyly III, is an autosomal dominant hereditary disorder characterized by craniofacial and limb anomalies. SCS is generally caused by mutations in the TWIST gene, but several 7p21.3 microdeletions involving the entire gene have also been described. The patient reported here presented with craniosynostosis, ptosis, brachydactyly and syndactyly of toes. Standard lymphocyte karyotype showed a de novo apparently balanced but complex constitution with a translocation between the short arms of chromosomes 2 and 7 and an insertion of the 7(q21.3q22) band in the short arm of the same chromosome 7. Interestingly, array CGH displayed a unique 690 kb deletion in 7p21.3 involving the TWIST gene, consistent with the phenotype. This case illustrates the important contribution of array CGH to identification of complex chromosomal rearrangements.

Clinical and Genetic Analysis of Patients with Saethre-Chotzen Syndrome

BACKGROUND

Saethre-Chotzen syndrome is a craniosynostosis syndrome further characterized by distinctive facial and limb abnormalities. It shows complete penetrance and variable expressivity and has been linked to the TWIST gene on chromosome 7p21; more than 80 different intragenic mutations and, recently, large deletions have been detected in Saethre-Chotzen patients. The aim of this study was to genetically and phenotypically characterize patients with a clinical diagnosis of Saethre-Chotzen syndrome.

METHODS

Patients with a clinical diagnosis as well as those with a genetic diagnosis of Saethre-Chotzen syndrome (n = 34) were included in the study.

RESULTS

The study showed that the important features of Saethre-Chotzen syndrome are brachycephaly (occurring in 74 percent of patients), a broad, depressed nasal bridge (65 percent), a high forehead (56 percent), ptosis (53 percent), and prominent auricular crura (56 percent). Furthermore, using different molecular techniques, pathogenic mutations in the TWIST gene were identified in 71 percent of patients.

CONCLUSIONS

Patients with deletions of the TWIST gene did not differ from those with intragenic TWIST mutations in frequency or severity of craniofacial abnormalities. However, they did distinguish themselves by the presence of many additional anomalies and diseases and--most importantly--the high frequency of mental retardation, which was borderline significant. The authors conclude that when using stringent inclusion criteria for studies of Saethre-Chotzen syndrome, patients who have a pathogenic mutation of the TWIST gene should be excluded.