Sebastián A. Esperante

Genetics and phenomics of hypothyroidism and goiter due to thyroglobulin mutations

Thyroglobulin (TG) defects due to TG gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. This dyshormonogenesis displays a wide phenotype variation and is characterized usually by: the presence of congenital goiter or goiter appearing shortly after birth, high (131)I uptake, negative perchlorate discharge test, low serum TG and elevated serum TSH with simultaneous low serum T(4) and low, normal or high serum T(3). Mutations in TG gene have been also reported associated with endemic and euthyroid nonendemic simple goiter. TG gene defects are inherited in an autosomal recessive manner and affected individuals are either homozygous or compound heterozygous for mutations. Up to now, 50 mutations have been identified and characterized in the human TG: 23 missense mutations, 10 nonsense mutations, 5 single and 1 large nucleotide deletions, 1 single nucleotide insertion and 10 splice site mutations. The functional consequences of this mutations could be structural changes in the protein molecule that alter the normal protein folding, assembly and biosynthesis of thyroid hormones, leading to a marked reduction in the ability to export the protein from the endoplasmic reticulum.

Congenital hypothyroidism with goitre caused by new mutations in the thyroglobulin gene

Context  Thyroid dyshormonogenesis is associated with mutations in the thyroglobulin (TG) gene and characterized by normal organification of iodide and low serum TG. These mutations give rise to congenital goitrous hypothyroidism, transmitted in an autosomal recessive mode.

Identification and characterization of four PAX8 rare sequence variants (p.T225M, p.L233L, p.G336S and p.A439A) in patients with congenital hypothyroidism and dysgenetic thyroid glands

Context  Thyroid dysgenesis may be associated with mutations in the paired box transcription factor 8 (PAX8) gene and is characterized by congenital hypothyroidism transmitted in an autosomal dominant mode.

Supernumerary marker 15 chromosome in a patient with Prader–Willi syndrome

To the Editor: The supernumerary marker 15 chromosome (SMC 15) represents 50% of the marker chromosomes. While some include euchromatin (bands q11–q13) and are linked to severe mental retardation, others without euchromatin associate with a normal phenotype in most cases but rarely with Prader–Willi syndrome (PWS) (1). This is a neurobehavioral disorder due to developmental impairment. PWS results from the loss of expression of imprinted genes in the paternal chromosome 15q11-q13, through several mechanisms such as deletions or uniparental disomy (UPD) (2). Herein, we studied a 13-year-old girl with a characteristic PWS phenotype and a small SMC(15). She was one of two dizygotic twins, underwent a severe neonatal hypotonia followed by hyperphagia, obesity, hypogonadism, and mental retardation, and hypopigmentation was absent. One distinctive feature was an early-onset type 1 diabetes. Cytogenetic results showed 47,XX,þmar, the karyotypes of her parents and the healthy twin being normal thus indicating a de novo origin of the SMC(15). It was monocentric and monosatellited by GTG (Trypsin-Giemsa staining) and Ag/NOR stainings (Fig. 1a). Fluorescence in situ hybridization (FISH) analysis revealed the 15 alpha satellite (D15Z1) signal; neither the probes for SNRPN or D15S10 nor that for PML polymorphisms (15q22) showed any signal on this SMC(15) (Fig. 1b). Thus, the karyotype was 47,XX,þdel(15)(q11.2).ish del(15)(D15Z1þ, SNRPN–, D15S10–, PML–). The methylation test for PWS showed an abnormal SNRPN pattern with no paternal contribution. A family study using DNA polymorphism find a heterozygosity at the PWS/AS locus (A55AC-1) (3) in the patient with two different maternal and no paternal allele (Fig. 1c). These results are consistent with a maternal heterodisomy. The heterozygosity for maternal alleles at a locus close to the centromere suggests that the non-disjunction is a meiosis I error with a 90% likelihood (4). The de novo SMC(15) carried by this patient should therefore be of paternal origin and could arise through a chromosomal rearrangement (deletion of most of the q arm), secondary to maternal chromosome 15 nondisjunction. The breakpoint location may be similar to the proximal common deletion breakpoints in PWS/AS patients (5). The occurrence of SMC(15) in PWS/AS patients was compiled by Cotter et al. (6). Most of them were de novo, inv dup(15), and lacked the PWS/AS region. Coexistence of UPD with SMC can be explained by the formation of a trisomic zygote and the subsequent loss of the single parental chromosome through a breakage event (7). The rearrangements in chromosome 15 may occur through unequal crossover between inverted repeats within proximal 15q. Deletions may occur when the recombination event is intrachromosomal and inv dup(15) chromosomes may result from interchromosomal recombination (8). The genetic alteration resulting in PWS in our patient is associated with advanced maternal age (39 years) like in other UPD reported cases (9). The presence of SMC(15) can be explained as a rescue event from a trisomic conception. The association of early-onset type 1 diabetes (confirmed by genotyping and immunotyping) and PWS is uncommon because these patients usually develop type 2 diabetes of late onset as a consequence of obesity. Both syndromes are probably due to independent causes. To sum up, FISH and microsatellite analyses of a patient with PWS revealed an SMC(15) and amaternal UPD. Data obtained enabled us to uncover one of the possible mechanisms of PWS origin.

Identification and characterization of new variants of three associated SNPs and a microsatellite in the TSH receptor gene which are useful for genetic studies.

The purpose of the present work was to characterize g.IVS5-69C>T, g.IVS6+13C>T and c.561C>T SNPs and the [CT](n) microsatellite in the TSHR gene for genetic analysis. Exons 6 and 7 of the TSHR gene, including the flanking intronic sequences, were screened for the presence of g.IVS5-69C>T, g.IVS6+13C>T and c.561C>T SNPs by SSCP. We found genetic association between the three SNPs and a total of three different haplotypes were observed. Two were homozygous blocks, g.IVS5-69T/g.IVS6+13G/c.561C (Haplotype TGC, 3.3%) and g.IVS5-69C/g.IVS6+13A/c.561T (Haplotype CAT, 75%). Every individual who was heterozygous for g.IVS5-69C>T was equally heterozygous for g.IVS6+13A>G and c.561T>C (Haplotype CAT/TGC, 21.7%). The [CT](n) microsatellite, localized in intron 7 of the TSHR gene was amplified by PCR and the labeled products were separated in a polyacrylamide denaturing sequencing gel. Three variable numbers of CT motif were identified, two previously reported ([CT](6) and [CT](8)) and one previously unreported ([CT](9)). The construction and expression of the hybrid minigenes using pSPL3 and alpha-globin-fibronectin EDB (pTB) vectors showed that the [CT](n) microsatellite itself does not interfere with exon 8 definition and processing in vitro. In conclusion, g.IVS5-69C>T/g.IVS6+13C>T/c.561C>T haplotypes and [CT](n) microsatellite are informative polymorphic markers and can be used in linkage studies in families with germ line TSHR mutations or autoimmunity thyroid diseases.

Detection of mutations in argentine retinoblastoma patients by segregation of polymorphisms, exon analysis and cytogenetic test.

The aim of this study was to detect chromosomal and molecular abnormalities in 16 Argentine families with retinoblastoma (RB). Chromosomes were analyzed by G-banding, DNA from leukocytes and tumors was studied by segregation of polymorphisms within RB gene (RB1) and the DNA from chorionic villus by sequencing. The karyotype of an Rb236 bilateral patient with dismorphic signs revealed a deletion in 13q13–21. Polymorphism and exon analyses showed a deletion in the 3′ end of RB1 in an Rb72 patient. The mutant RB1 allele, detected by loss of heterozygosity (LOH) in the tumor, was identified in 14 out of 18 tumors. The analysis of chorionic villus revealed a mutation, a C-to-T transition in exon 18. Molecular and cytogenetic analyses in families with RB offer valuable information on how to assess the risk of tumor development.