[Fetal achondroplasia--antenatal diagnosis of two cases]

Achondroplasia is a rare autosomal dominant skeletal dysplasia associated with rhizomelic micromelia and facial dysmorphism. Prenatal diagnosis of achondroplasia is possible after 20 weeks of gestation when its main characteristic features can be easily recognized on ultrasound. We present two cases of achondroplasia diagnosed in the second trimester of pregnancy.

PTH has the potential to rescue disturbed bone growth in achondroplasia

INTRODUCTION

Achondroplasia (Ach), the most common form of short-limb short stature, and related disorders are caused by constitutively active point mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. Recent studies have provided a large body of evidence for the role of the proliferation and differentiation of chondrocytes in these disorders. Furthermore, a G380R mutation in FGFR3 (FGFR3(Ach)), which results in achondroplasia, induces apoptosis in the chondrogenic cell line ATDC5. This is associated with a decrease in the expression of PTHrP, which shares the same receptor with PTH, and it is significant that PTHrP rescues these cells from apoptosis.

METHODS

Fetuses derived from transgenic mice expressing FGFR3(Ach) under the control of the type II collagen promoter (AchTG) or from wild-type mice were obtained on the 15th day of pregnancy. The femurs were collected from these specimens and cultured for 4 days with PTH. The effects of PTH treatment were then determined by morphometric and histological analyses, in situ hybridization of type X collagen mRNA, and the TUNEL assay.

RESULTS

AchTG femurs showed suppressed growth compared with wild type (0.29+/-0.10 mm vs. 0.46+/-0.06 mm, respectively; p<0.05), particularly in cartilage. PTH treatments improved the growth velocity in the femurs of the AchTG (0.50+/-0.06 mm; p<0.01 vs. control). This was associated with the inhibition of both differentiation and apoptosis in chondrocytes.

CONCLUSIONS

Our data suggest that PTH inhibits differentiation and apoptosis in chondrocytes and improves bone growth. These effects thus counterbalance the effects of FGFR3 mutations. PTH therefore is a potential therapeutic agent for achondroplasia.

Parathyroid hormone receptor type 1/Indian hedgehog expression is preserved in the growth plate of human fetuses affected with fibroblast growth factor receptor type 3 activating mutations

The fibroblast growth factor receptor type 3 (FGFR3) and Indian hedgehog (IHH)/parathyroid hormone (PTH)/PTH-related peptide receptor type 1 (PTHR1) systems are both essential regulators of endochondral ossification. Based on mouse models, activation of the FGFR3 system is suggested to regulate the IHH/PTHR1 pathway. To challenge this possible interaction in humans, we analyzed the femoral growth plates from fetuses carrying activating FGFR3 mutations (9 achondroplasia, 21 and 8 thanatophoric dysplasia types 1 and 2, respectively) and 14 age-matched controls by histological techniques and in situ hybridization using riboprobes for human IHH, PTHR1, type 10 and type 1 collagen transcripts. We show that bone-perichondrial ring enlargement and growth plate increased vascularization in FGFR3-mutated fetuses correlate with the phenotypic severity of the disease. PTHR1 and IHH expression in growth plates, bone-perichondrial rings and vascular canals is not affected by FGFR3 mutations, irrespective of the mutant genotype and age, and is in keeping with cell phenotypes. These results indicate that in humans, FGFR3 signaling does not down-regulate the main players of the IHH/PTHR1 pathway. Furthermore, we show that cells within the bone-perichondrial ring in controls and patients express IHH, PTHR1, and type 10 and type 1 collagen transcripts, suggesting that bone-perichondrial ring formation involves cells of both chondrocytic and osteoblastic phenotypes.

[Fetal ventilation and craniomaxillary development]

Data acquired by means of color Doppler ultrasound very explicitly suggest what the role of the fetal ventilation and nasal capsules in the morphogenesis of the maxillary prognathism, turbinates, nasal valves and nasopharynx could be. Furthermore, the dysmorphologies observed in Apert or Crouzon craniosynostosis, achondroplasia or unilateral cleft lip would also testify that the influence of the fetal ventilatory dynamics goes beyond the limits of the face and extends to the cranial base and the cranium. The wealth of raised hypothesis thanks to the contribution of this imaging system could question the validity of some conceptions of the fetal craniomaxillary morphogenesis.

Stenosis of the spinal canal in achondroplasia

Stenosis of the spinal canal is a very frequent finding in achondroplastic patients. It is secondary to abnormalities of endochondrial ossification, which is responsible for formation of the vertebral bone structures, and, subsequently, to bone degeneration. Cervical stenosis (most frequently involving the first metameres and the craniospinal junction) is more common in children while thoraco-lumbar stenosis favors adults. The surgical indications for cervical stenosis depend mainly on the clinical evolution but also on neurophysiological data. In thoraco lumbar stenosis too, clinical data determine the surgical indications and the route of attack depends on the type of anatomical alteration affecting the spine.

Prenatal diagnosis of hypochondroplasia

Prenatal diagnosis of a fetus at risk for hypochondroplasia, a short limb dwarfism condition similar to achondroplasia, was performed by ultrasound at 22 weeks' gestation. The limb bones were measured and shown to be decreased in length. The pregnancy was terminated. Post abortion X-ray did not show caudal narrowing in the lumbar spine but the pelvis had the features of hypochondroplasia.

Achondrogenesis, type I

Achondrogenesis is a rare form of lethal dwarfism. Usually discovered post partum, in a few instances only it has been diagnosed in utero. One such a case is here presented together with differential diagnosis with other forms of lethal dwarfism.