Patterson-Lowry rhizomelic dysplasia: Report of two new patients

Novel form of rhizomelic skeletal dysplasia associated with a homozygous variant in GNPNAT1

BACKGROUND

Studies exploring molecular mechanisms underlying congenital skeletal disorders have revealed novel regulators of skeletal homeostasis and shown protein glycosylation to play an important role.

OBJECTIVE

To identify the genetic cause of rhizomelic skeletal dysplasia in a consanguineous Pakistani family.

METHODS

Clinical investigations were carried out for four affected individuals in the recruited family. Whole genome sequencing (WGS) was completed using DNA from two affected and two unaffected individuals from the family. Sequencing data were processed, filtered and analysed. In silico analyses were performed to predict the effects of the candidate variant on the protein structure and function. Small interfering RNAs (siRNAs) were used to study the effect of Gnpnat1 gene knockdown in primary rat chondrocytes.

RESULTS

The patients presented with short stature due to extreme shortening of the proximal segments of the limbs. Radiographs of one individual showed hip dysplasia and severe platyspondyly. WGS data analyses identified a homozygous missense variant c.226G>A; p.(Glu76Lys) in GNPNAT1, segregating with the disease. Glucosamine 6-phosphate N-acetyltransferase, encoded by the highly conserved gene GNPNAT1, is one of the enzymes required for synthesis of uridine diphosphate N-acetylglucosamine, which participates in protein glycosylation. Knockdown of Gnpnat1 by siRNAs decreased cellular proliferation and expression of chondrocyte differentiation markers collagen type 2 and alkaline phosphatase, indicating that Gnpnat1 is important for growth plate chondrocyte proliferation and differentiation.

CONCLUSIONS

This study describes a novel severe skeletal dysplasia associated with a biallelic, variant in GNPNAT1. Our data suggest that GNPNAT1 is important for growth plate chondrogenesis.

Radiologic findings of Patterson-Lowry rhizomelic dysplasia in two sisters

We report two sisters who have a rare skeletal abnormality termed Patterson-Lowry rhizomelic dysplasia. The typical findings of these cases on bone survey are isolated shortening and proximal metaphyseal enlargement and cupping of the bilateral humeri. The elder sister also has coxa vara deformity and dysplastic proximal femoral epiphyses on both sides. The younger sister has normal hip joint bones bilaterally, but her proximal femoral epiphyses are smaller than normal. All other bones of the sisters are of normal size and configuration. Our patients are two siblings, and their parents are first degree relatives, suggesting autosomal-recessive (AR) inheritance. The present patients help us to understand the genetic relationships and skeletal variabilities of this rare entity.