Extreme phenotypic variability of thyroid dysgenesis in six new cases of congenital hypothyroidism due to PAX8 gene loss-of-function mutations

CONTEXT

Within the last two decades, heterozygous loss-of-function PAX8 mutations have been reported in patients with a wide degree of thyroid gland dysfunction and growth despite the presence of identical mutations.

OBJECTIVES

To search for PAX8 mutations in a cohort of patients with congenital hypothyroidism (CH) and various types of thyroid gland defects.

DESIGN

A cross-sectional study was conducted in a cohort of patients.

SETTING

The French neonatal screening program was used for recruiting patients.

PATIENTS

A total of 118 patients with CH, including 45 with familial and 73 with sporadic diseases, were included in this study. The thyroid gland was normal in 23 patients had hypoplasia, 25 had hemithyroid agenesis, 21 had athyreosis, and 21 had ectopy.

RESULTS

We found four different PAX8 mutations (p.R31C, p.R31H, p.R108X, and p.I47T) in ten patients (six patients with CH and four family members), two with sporadic and eight with familial diseases. Imaging studies performed in the index cases showed ectopic thyroid gland (n=2), hypoplasia (n=2), eutopic lobar asymmetry (n=1), and eutopic gland compatible with dyshormonogenesis (n=1). The previously reported p.R31C and the novel p.I47T PAX8 mutations are devoid of activity.

CONCLUSION

Four different PAX8 mutations were detected in six index patients with CH (ten total subjects). The p.R31C, p.R31H, and p.R108X mutations have been reported. The novel p.I47T PAX8 mutation presented loss of function leading to CH. Thyroid ectopy was observed in two cases of PAX8 (p.R31H) mutation, a finding that has not been reported previously. We observed a high inter-individual and intra-familial variability of the phenotype in PAX8 mutations, underlining that population genetic studies for CH should include patients with various clinical presentations.

The p.Val66Met polymorphism in the BDNF gene protects against early seizures in Rett syndrome

OBJECTIVE

X chromosome inactivation and the MECP2 genotype do not provide the full explanations for the clinical differences between patients with Rett syndrome (RTT), suggesting the involvement of other factors. One MeCP2 target is the brain-derived neurotrophic factor (BDNF) gene. We investigated, according to the MECP2 genotype, the role of the BDNF functional polymorphism (Val66Met) on the severity of RTT.

METHODS

This polymorphism in BDNF was analyzed by PCR and dHPLC in 81 patients with RTT. We studied the association between the MECP2 and BDNF genotypes and the clinical features in each patient.

RESULTS

We found that some RTT features can be correlated with MECP2 genotypes. Missense mutations are associated with a more severe epileptic phenotype (early onset and drug resistance) than other mutations. Non-sense and late truncating mutations lead to a less severe phenotype regarding walking. The distribution of the Val66Met polymorphism was not significantly different between the different groups in regard to the severity of all tested symptoms. However, girls with RTT bearing the Val66Val genotype tend to present earlier seizures than girls with RTT bearing the Met66 allele. No girls with RTT with the Met66 allele presented early seizures.

CONCLUSIONS

Early onset of seizures is linked to the combined MECP2 and BDNF genotypes. The BDNF Met66 allele may protect against seizures, whereas missense mutations in the MBD of MECP2 are more frequently associated with early seizures.

Spectrum and distribution of MECP2 mutations in 424 Rett syndrome patients: a molecular update

Mutations in the MECP2 (Methyl-CpG-binding protein) gene have been reported to cause Rett syndrome (RTT), an X-linked progressive encephalopathy. Recent studies have identified large gene rearrangements that escape the common PCR-based mutation screening strategy and mutations in a novel MeCP2 isoform (named MECP2B). We have collected the results of MECP2 mutational analysis concerning 424 RTT patients conducted in eight laboratories in France. In total, 121 different MECP2 mutations were identified. R168X (11.5%) is the most common of MECP2 mutations, followed by R270X (9%), R255X (8.7%), T158 M (8.3%) and R306C (6.8%). Only eight mutations had relative frequency>3%. Large and complex rearrangements not previously detected using only a PCR-based strategy represent 5.8% of MECP2 mutations. On the contrary, mutation in exon 1 appears to be rare (less than 0.5%). These data demonstrate the high allelic heterogeneity of RTT in France and suggest that routine mutation screening in MECP2 should include quantitative analysis of the MECP2 gene. This study represents an important instrument for molecular diagnosis strategy and genetic counseling in RTT families.

Gonadotropin receptors

Gonadotropin receptors belong to a subgroup of G-protein coupled receptors characterized by a large extracellular domain responsible for the binding of the hormone. Soluble, hormone-binding, alternative splicing variants of the LH receptor, are present in high concentration. A mannose rich precursor form of LH and FSH receptor is accumulated inside target cells. FSH receptors are addressed to the basolateral domain of cells through specific signaling mechanisms. Gonadotropin receptors are also present in endothelial cells of target organ vessels and are involved in hormone transcytosis. Various genetic abnormalities of these receptors (and of the GnRH receptor) are discussed.

Absence of activating mutations in the GnRH receptor gene in human pituitary gonadotroph adenomas

The monoclonal origin of gonadotropin-secreting pituitary adenomas has been well demonstrated but only few molecular abnormalities have so far been recognized in these tumors. For many years, several authors have suggested a role for GnRH and/or GnRH receptors (GnRH-R) in the development of these pituitary adenomas. To test the hypothesis that mutant genes encoding a constitutively activated GnRH-R might be involved in the pathogenesis of these tumors, the sequence of the GnRH-R gene was analyzed in tumoral pituitary tissue obtained from ten patients (six female, four male). The pituitary gonadotropin-secreting adenoma was associated with in vivo hypersecretion of FSH, LH and/or free alpha-subunit (n = 7) or was clinically silent (normal plasma levels of gonadotropins or free alpha-subunit, n = 3). In all cases, immunocytochemical studies of the removed adenoma confirmed their gonadotroph nature by revealing positivity for FSH, LH and/or alpha-subunit. Genomic DNA was extracted from the pathological tissue obtained at neurosurgery. Eight sequencing primers were used to amplify the three exons of the GnRH-R gene from tumoral DNA. The entire coding sequence of the GnRH-R gene was sequenced in the ten adenomas. No mutation was found in any of the tumor specimens examined. In conclusion, mutations in the GnRH receptor coding sequence occur infrequently if at all in gonadotropin-secreting pituitary adenomas.