L.A. Pérez Jurado

TBL2, a novel transducin family member in the WBS deletion: characterization of the complete sequence, genomic structure, transcriptional variants and the mouse ortholog

Williams-Beuren syndrome (WBS) is a developmental disorder with multi-system manifestations caused by haploinsufficiency for contiguous genes deleted in chromosome region 7q11.23. The size of the deletion is similar in most patients due to a genomic duplication that predisposes to unequal meiotic crossover events. While hemizygosity at the elastin locus is responsible for the cardiovascular features, the contribution of other genes to the WBS phenotype remains to be demonstrated. We have identified a novel gene, TBL2, in the common WBS deletion. TBL2 is expressed as a 2.4-kb transcript predominantly in testis, skeletal muscle, heart and some endocrine tissues, with a larger ∼5-kb transcript detected ubiquitously at lower levels. TBL2 encodes a protein with four putative WD40-repeats. An alternatively spliced transcript in TBL2 introduces a novel second exon with an in frame stop codon. This mRNA encodes a 75 amino acid protein with 43 amino acids identical to TBL2 at the N-terminus and no known functional domain. The mouse homolog, Tbl2, shows 84% sequence identity at the nucleotide level and 92% similarity at the amino acid level. Comparison of the mouse and human sequences identifies a conserved region that extends upstream of the previously published sequence with an initiation codon common to both species that adds 21 amino acids at the N-terminus. The Tbl2 gene has been mapped to mouse chromosome 5 in a region of conserved synteny with human 7q11.23. Since haploinsufficiency has been shown for other WD-repeat containing proteins, hemizygosity of TBL2 may contribute to some of the aspects of the complex WBS phenotype.

Molecular Basis of Familial Growth Hormone Deficiency

A significant proportion of cases of GH deficiency (5-30%) may be due to genetic causes. At least four Mendelian types of isolated GH deficiency (IGHD) have been delineated based on the mode of inheritance and the degree of GH deficiency: IGHD type IA, autosomal recessive with absent endogenous GH; type IB, autosomal recessive with diminished GH; type II, autosomal dominant with diminished GH; and type III, X-linked with diminished GH. Most patients with IGHD type IA have heterogeneous deletions, ranging in size from 6.7 kb to 45 kb, that encompass the entire gene encoding for pituitary GH, GH-1. Nonsense, frameshift and splice GH-1 mutations that predict a complete lack of bioactive GH synthesis in homozygotes have also been reported in association with IGHD IA. Additionally, some cases of IGHD type II have dominant negative mutations in one allele of the GH-1 gene. Panhypopituitary Dwarfism (PD), a condition characterized by deficiency of at least other pituitary trophic hormone in addition to GH deficiency, can have autosomal and X-linked modes of inheritance. Interestingly, both recessive and dominant mutations at the gene encoding for the pituitary transcription factor Pit-1 have been found in a specific subtype of PD that combines GH, prolactin and TSH deficiencies. In contrast, the loci and mutations responsible for the other Mendelian forms of IGHD and PD remain unknown. Linkage studies using genetic markers have excluded the GH locus on chromosome 20 in all the studied families (types IB and II) in whom the mutation cannot be traced to defects in these genes.(ABSTRACT TRUNCATED AT 250 WORDS)

The human calcitonin receptor gene (CALCR) at 7q21.3 is outside the deletion associated with the Williams syndrome

The human calcitonin receptor (CTR) is a transmembrane peptide with dual action as a receptor for the hormone calcitonin and as an extracellular calcium sensor. Therefore, CTR dysfunction could lead to disorders of calcium metabolism associated with hypercalcemia, such as the Williams syndrome (WS). WS is a developmental disorder caused by a deletion at chromosome 7q11.23 that includes the elastin locus (ELN). We have mapped the CTR gene (CALCR) to chromosome band 7q21.3 by polymerase chain reaction and single-strand conformation analysis of somatic cell hybrids as well as fluorescence in situ hybridization (FISH) to metaphase chromosome spreads. Two-color FISH cohybridizing CTR and ELN probes confirmed that CALCR maps telomeric to ELN. Subsequent analysis of chromosome spreads from four WS patients revealed deletion of the ELN locus in all of them and normal hybridization of CTR probes to both chromosome 7 homologues, indicating that CALCR lies outside the deleted region.

Molecular diagnosis and endocrine evaluation of a patient with a homozygous 7.0 kb deletion of the growth hormone (GH) gene cluster: response to biosynthetic GH therapy.

A significant proportion of cases of GH deficiency (5-30%) may be due to genetic causes. At least four Mendelian types of isolated GH deficiency (IGHD) have been delineated based on the mode of inheritance and the degree of GH deficiency, with IGHD type IA being the most severe. A 2 year-old girl, the second child of consanguineous parents, with short stature was diagnosed with IGHD type IA. The analysis of the genomic DNA of this patient, performed by polymerase chain reaction (PCR) amplification of the flanking regions of the GH-1 gene, showed a homozygous deletion of 7.0 kb of sequence including the GH-1 gene. She was treated with biosynthetic GH resulting in long-lasting catch-up growth during at least three years, despite a clinically irrelevant appearance of low binding capacity GH antibodies. Growth hormone-binding protein (GHBP) levels were normal at the time of diagnosis. In addition, GHBP plasma levels did not show any significant change during the three years of therapy with GH. Diagnosis of carrier status in family relatives was done by genotyping GH gene alleles by PCR amplification from blood spots on filter paper.