Further defining the clinical and molecular spectrum of acromesomelic dysplasia type maroteaux: a Turkish tertiary center experience

Antenatal unilateral upper limb acromesomelic dysplasia

Acromesomelic dysplasia (AMD) is an umbrella term given to a heterogeneous group of progressive skeletal disorders characterized by short limbed dwarfism associated with disproportionate shortening of middle and distal segments of the upper as well as lower limbs. Although specific skeletal anomalies are difficult to diagnose antenatally, but because of their antenatal and postnatal implications and a possibility of reoccurrence in following pregnancies, such skeletal anomalies need to be actively addressed. A combination of radiologic, pathologic, genetic and molecular investigation prenatally as well as postnatally is required to classify a specific congenital skeletal dysplasia. Once the genetic make-up of fetal skeletal dysplasia is deciphered, a meaningful genetic counselling could be offered for future pregnancies of affected families. We describe a case of primigravida diagnosed with fetal unilateral upper limb AMD on antenatal ultrasound done at early second trimester. The radius and ulna of left upper limb were abnormally short (less than 5th centile of the mean for that gestational age). The left hand was also hypoplastic. Rest of the sonographic anomaly scan was normal. To the best of our knowledge, AMD limited to unilateral upper limb diagnosed antenatally as an isolated finding is not described in the medical literature so far.

Novel Loss-of-Function Mutations in NPR2 Cause Acromesomelic Dysplasia, Maroteaux Type

Acromesomelic dysplasia, Maroteaux type (AMDM) is a rare skeletal dysplasia characterized by severe disproportionate short stature, short hands and feet, normal intelligence, and facial dysmorphism. Homozygous or compound heterozygous mutations in the natriuretic peptide receptor 2 (NPR2) gene produce growth-restricted phenotypes. The current study was designed to identify and characterize NPR2 loss-of-function mutations in patients with AMDM and to explore therapeutic responses to recombinant growth hormone (rhGH). NPR2 was sequenced in two Chinese patients with AMDM via next generation sequencing, and in silico structural analysis or transcript analysis of two novel variants was performed to examine putative protein changes. rhGH treatment was started for patient 1. Three NPR2 mutations were identified in two unrelated cases: two compound heterozygous mutations c.1112G>A p.(Arg371Gln) and c.2887+2T>C in patient 1 and a homozygous mutation c.329G>A p.(Arg110His) in patient 2, yielding distinct phenotypes. RNA extracted from peripheral blood cells of patient 1 showed alternatively spliced transcripts not present in control cells. Homology modeling analyses suggested that the c.1112G>A p.(Arg371Gln) mutation disrupted the binding of NPR-B homodimer to its ligand (C-type natriuretic peptide) in the extracellular domain as a result of global allosteric effects on homodimer formation. Thus, c.2887+2T>C and c.1112G>A p.(Arg371Gln) in NPR2 were loss-of-function mutations. Furthermore, rhGH therapy in patient 1 increased the patient’s height by 0.6SDS over 15 months without adversely affecting the trunk-leg proportion. The short-term growth-promoting effect was equivalent to that reported for idiopathic short stature. Overall, our findings broadened the genotypic spectrum of NPR2 mutations in individuals with AMDM and provided insights into the efficacy of rhGH in these patients.

Acromesomelic dysplasia-Maroteaux type, nine patients with two novel NPR2 variants

OBJECTIVES

Acromesomelic dysplasia, type Maroteaux, is an autosomal recessive skeletal dysplasia caused by biallelic loss of function variations of NPR2, which encodes a cartilage regulator C-type natriuretic peptide receptor B. NPR2 variations impair skeletal growth. It is a rare type of dwarfism characterized by shortening of the middle and distal segments of the limbs with spondylar dysplasia.

METHODS

We performed detailed clinical and radiological evaluation and sequence analysis for NPR2.

RESULTS

Herein, we report nine patients from eight families with two novel NPR2 pathogenic variants.

CONCLUSIONS

This study describes typical clinical phenotypes of Maroteaux type acromesomelic dysplasia, and enriches the variant spectrum of NPR2 by reporting one nonsense and one missense novel variant. We emphasize the importance of detailed clinical evaluation before genetic testing in diagnosing rare skeletal disorders.