Showing papers by "John W. Belmont published in 2002"

A complex syndrome of left-right axis, central nervous system and axial skeleton defects in Zic3 mutant mice.

Abstract: X-linked heterotaxy (HTX1) is a rare developmental disorder characterized by disturbances in embryonic laterality and other midline developmental field defects. HTX1 results from mutations in ZIC3, a member of the GLI transcription factor superfamily. A targeted deletion of the murine Zic3 locus has been created to investigate its function and interactions with other molecular components of the left-right axis pathway. Embryonic lethality is seen in approximately 50% of null mice with an additional 30% lethality in the perinatal period. Null embryos have defects in turning, cardiac development and neural tube closure. Malformations in live born null mice include complex congenital heart defects, pulmonary reversal or isomerism, CNS defects and vertebral/rib anomalies. Investigation of nodal expression in Zic3-deficient mice indicates that, although nodal is initially expressed symmetrically in the node, there is failure to maintain expression and to shift to asymmetric expression. Subsequent nodal and Pitx2 expression in the lateral plate mesoderm in these mice is randomized, indicating that Zic3 acts upstream of these genes in the determination of left-right asymmetry. The phenotype of these mice correctly models the defects found in human HTX1 and indicates an important role for Zic3 in both left-right and axial patterning.

PTPN11 mutations in Noonan syndrome type I: detection of recurrent mutations in exons 3 and 13.

Abstract: We surveyed 16 subjects with the clinical diagnosis of Noonan Syndrome (NS1) from 12 families and their relevant family members for mutations in PTPN11/SHP2 using direct DNA sequencing. We found three different mutations among five families. Two unrelated subjects shared the same de novo missense substitution in exon 13 (S502T); an additional two unrelated families had a mutation in exon 3 (Y63C); and one subject had the amino acid substitution Y62D, also in exon 3. None of the three mutations were present in ethnically matched controls. In the mature protein model, the exon 3 mutants and the exon 13 mutant amino acids cluster at the interface between the N' SH2 domain and the phosphatase catalytic domain. Six of eight subjects with PTPN11/SHP2 mutations had pulmonary valve stenosis while no mutations were identified in those subjects (N = 4) with hypertrophic cardiomyopathy. An additional four subjects with possible Noonan syndrome were evaluated, but no mutations in PTPN11/SHP2 were identified. These results confirm that mutations in PTPN11/SHP2 underlie a common form of Noonan syndrome, and that the disease exhibits both allelic and locus heterogeneity. The observation of recurrent mutations supports the hypothesis that a special class of gain-of-function mutations in SHP2 give rise to Noonan syndrome.

Congenital sucrase-isomaltase deficiency presenting with failure to thrive, hypercalcemia, and nephrocalcinosis

Abstract: Disaccharide Intolerance Type I (Mendelian Interance in Man database: *222900) is a rare inborn error of metabolism resulting from mutation in sucrase-isomaltase (Enzyme Catalyzed 3.2.1.48). Usually, infants with SI deficiency come to attention because of chronic diarrhea and nutritional evidence of malabsorption. We describe an atypical presentation of this disorder in a 10-month-old infant. In addition to chronic diarrhea, the child displayed severe and chronic hypercalcemia, the evaluation of which was negative. An apparently coincidental right orbital hemangioma was detected. Following identification of the SI deficiency, an appropriately sucrose-restricted, but normal calcium diet regimen was instituted which led to cessation of diarrhea, substantial weight gain, and resolution of hypercalcemia. This case illustrates that, similar to congenital lactase deficiency (Mendelian Interance in Man database: *223000, Alactasia, Hereditary Disaccharide Intolerance Type II), hypercalcemia may complicate neonatal Sucrase-Isomaltase deficiency. Hypercalcemia in the presence of chronic diarrhea should suggest disaccharide intolerance in young infants.

Exclusion of PITX2 mutations as a major cause of CHARGE association.

Abstract: CHARGE is a nonrandom association of ocular coloboma, congenital heart defects, atresia of the choanae, retarded growth and development, genital hypoplasia, and ear anomalies including deafness. The cause of CHARGE remains unknown; however, there is considerable evidence of an underlying genetic basis, as discussed by Tellier et al. [1996: Clin Genet 50:548-550; 1998: Am J Med Genet 76:402-409] and by Martin et al. [2001: Am J Med Genet 99:115-119]. Based on the ocular, cardiac, and craniofacial expression pattern of Pitx2, a homeodomain transcription factor, and the pleiotropic effects of loss of PITX2 function in both mouse and human, we hypothesized that PITX2 mutations may contribute to the multiple phenotypic anomalies present in CHARGE individuals. By direct sequencing of DNA from 29 individuals with CHARGE, we did not identify any mutations in PITX2. We did, however, identify two PITX2 sequence polymorphisms. Large deletions of PITX2 were excluded in most patients by heterozygosity in at least one of several polymorphic markers near the PITX2 locus. Together, these data indicate that PITX2 mutations are unlikely to be a major contributing cause of the multiple anomalies present in individuals with CHARGE.