Y chromosome sequences in Turner's syndrome: association with virilization and gonadoblastoma

The presence of Y chromosome fragments in patients with Turner's syndrome is known to increase the risk of gonadoblastoma and virilization. Y chromosome material is detected in up to 6% of patients with Turner's syndrome by karyotype. By DNA analysis, Y chromosome sequences have been reported in 0-60% of patients. The putative gonadoblastoma gene has been mapped to the pericentromeric region of the Y chromosome increasing the interest in studying these sequences.

AIMS

1. To determine the frequency of occult Y chromosome sequences in patients with Turner's syndrome. 2. To analyze the clinical implications of Y sequences detected by karyotype and occult Y sequences.

STUDY DESIGN

Cross-sectional study of 58 patients with Turner's syndrome (30 45,X; two with structural anomalies; 26 mosaic [two of whom were 45,X/46,XY]). SRY, TSPY and DYZ3 sequences were amplified by PCR using genomic DNA from peripheral blood.

RESULTS

All three Y chromosome sequences were found in one out of 56 patients whose karyotype was not suggestive of having Y chromosome material and in one patient with 45,X/46,Xr(X) karyotype. The patients with the ring chromosome and 45,X/46,XY karyotype underwent surgery and were found to have a gonadoblastoma and dysgerminoma. The four patients with Y chromosome material had non-virilized female genitalia.

CONCLUSIONS

Analysis by PCR was more sensitive in detecting Y chromosome sequences than conventional karyotype. The presence of Y material was not associated with virilization. We confirmed the association of Y fragments and gonadoblastoma at an early age.

Retinoic acid is a potent regulator of growth plate chondrogenesis

Vitamin A deficiency and excess both cause abnormalities in mammalian longitudinal bone growth. Because all-trans retinoic acid (RA) is synthesized from vitamin A, we hypothesized that RA regulates growth plate chondrogenesis. Consistent with this hypothesis, a single oral dose of RA reduced the height of the rat proximal tibial growth plate. To determine whether RA acts directly on growth plate, fetal rat metatarsal bones were cultured in the presence of RA. In this system, RA inhibited longitudinal bone growth by three mechanisms: 1) decreased chondrocyte proliferation, (assessed by 3H-thymidine incorporation), particularly in the proliferative zone of the growth plate; 2) decreased matrix synthesis (assessed by 35SO4 incorporation into glycosaminoglycans); and 3) decreased cell hypertrophy (determined histologically). The growth-inhibiting effects of RA were completely reversed by a retinoic acid receptor (RAR) antagonist. In the absence of exogenous RA, this antagonist accelerated bone growth, as did an RA-specific neutralizing antibody, suggesting that endogenous RA negatively regulates growth plate chondrogenesis. We conclude that RA, acting through RARs, negatively regulates longitudinal bone growth by inhibiting growth plate chondrocyte proliferation, chondrocyte hypertrophy, and matrix synthesis.

Activating mutations of the Ca2+-sensing receptor

The Ca2+-sensing receptor (CaR) is a member of the seven-transmembrane domain, G-protein-coupled receptor superfamily. It is expressed in parathyroid, kidney, and other tissues. In parathyroid, activation of the CaR by extracellular Ca2+ negatively regulates the secretion of parathyroid hormone. In the the thick ascending limb of Henle's loop, receptor activation decreases renal reabsorption of Ca2+. Heterozygous inactivating mutations of the CaR cause familial benign hypocalciuric hypercalcemia while homozygous inactivating mutations cause neonatal severe hyperparathyroidism. Conversely, activating mutations of the CaR cause autosomal dominant and sporadic hypoparathyroidism. Affected individuals have hypocalcemia which ranges from mild and asymptomatic to life-threatening. They also show a greater tendency to hypercalciuria than do other patients with hypoparathyroidism. Most, but not all, of the reported activating mutations occur in the amino-terminal, extracellular domain of the receptor. When expressed in cultured cells, mutant receptors can show both increased receptor sensitivity to Ca2+ and increased maximal signal transduction capacity.

Effects of fibroblast growth factor-2 on longitudinal bone growth

In vivo, fibroblast growth factor-2 (FGF-2) inhibits longitudinal bone growth. Similarly, activating FGF receptor 3 mutations impair growth in achondroplasia and thanatophoric dysplasia. To investigate the underlying mechanisms, we chose a fetal rat metatarsal organ culture system that would maintain growth plate histological architecture. Addition of FGF-2 to the serum-free medium inhibited longitudinal growth. We next assessed each major component of longitudinal growth: proliferation, cellular hypertrophy, and cartilage matrix synthesis. Surprisingly, FGF-2 stimulated proliferation, as assessed by [3H]thymidine incorporation. However, autoradiographic studies demonstrated that this increased proliferation occurred only in the perichondrium, whereas decreased labeling was seen in the proliferative and epiphyseal chondrocytes. FGF-2 also caused a marked decrease in the number of hypertrophic chondrocytes. To assess cartilage matrix synthesis, we measured 35SO4 incorporation into newly synthesized glycosaminoglycans. Low concentrations (10 ng/ml) of FGF-2 stimulated cartilage matrix production, but high concentrations (1000 ng/ml) inhibited matrix production. We conclude that FGF-2 inhibits longitudinal bone growth by three mechanisms: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and, at high concentrations, decreased cartilage matrix production. These effects may explain the impaired growth seen in patients with achondroplasia and related skeletal dysplasias.

A Ca(2+)-sensing receptor mutation causes hypoparathyroidism by increasing receptor sensitivity to Ca2+ and maximal signal transduction

Activating mutations of the Ca(2+)-sensing receptor (CaR) gene cause autosomal dominant hypoparathyroidism. Functional expression studies have been reported for several mutations, but have produced conflicting results. Thus, the mechanism by which these mutations activate the receptor is unclear. We describe here a new family with autosomal dominant hypoparathyroidism. The mother and three daughters experienced muscle spasms and/or seizures from early childhood. They were treated with oral calcium and vitamin D analogs, and all four patients developed hypercalciuria, nephrocalcinosis, and renal insufficiency. In this family, we identified a heterozygous missense mutation (F612S) involving the extracellular region of the CaR. The mutation cosegregated with disease. It was not present in 50 normal control individuals. We used site-directed mutagenesis to introduce this mutation into the CaR cDNA, and then expressed the mutant receptor in human embryonic kidney (HEK)-293 cells. In these cells, the accumulation of inositol phosphates was measured as a function of extracellular Ca2+ concentration. Compared with the wild-type receptor, the mutant receptor showed a left-shift in the concentration-response curve and an increase in the maximal response to high Ca2+ concentration. These effects did not appear to be mediated by changes in levels of receptor expression, as judged by ELISA, or by changes in receptor glycosylation, as judged by Western analysis. We conclude that this CaR mutation causes hypoparathyroidism by a dual increase in receptor sensitivity to extracellular Ca2+ and maximal signal transduction capacity.

Sporadic hypoparathyroidism caused by de Novo gain-of-function mutations of the Ca(2+)-sensing receptor

Activating mutations of the Ca(2+)-sensing receptor (CaR) gene have been identified in families with autosomal dominant hypoparathyroidism and in one patient with sporadic hypoparathyroidism. Here, we describe two additional patients with sporadic hypoparathyroidism. One patient presented with mild symptoms at age 18 yr; the other was severely symptomatic from infancy. A heterozygous missense mutation was identified in each patient. One mutation (L773R) involved the fifth transmembrane domain of the CaR, the other (N118K) affected the amino-terminal, extracellular domain. In both cases, the probands' parents lacked the mutation, indicating that the mutations arose de novo. In expression studies the mutations shifted the concentration-response curve to the left and increased maximal activity. We conclude that 1) sporadic hypoparathyroidism can be caused by de novo gain-of-function mutations of the CaR; 2) the phenotype can vary from mild to life-threatening hypocalcemia; 3) gain-of-function mutations can involve not only extracellular regions, as previously reported, but also transmembrane domains of the CaR; and 4) the mechanism of activation can involve both increased receptor sensitivity to Ca2+ and increased maximal signal transduction.