Molecular characteristics

Molecular characteristics
Donnai-Barrow syndrome (DBS) is caused by loss of function mutations in the Low Density Lipoprotein Receptor-Related Protein 2 (LRP2) gene, encoding the multi-ligand scavenger receptor Megalin. Megalin is a 520 kDa transmembrane protein normally expressed on the cell surface and endosomes of several absorptive epithelia, notably the renal proximal tubule, as well as in the lungs, the eyes, and the brain during organogenesis. It contains multiple extracellular LDL-receptor class A, LDL-receptor class B, EGF-like domains and a short intracellular region.
No other genes are known to cause the condition.

Type of mutations
The majority of LRP2 pathogenic variants are nonsense, frameshift or splice-site mutations. Pathogenic variants are distributed along the entire coding sequence, and no protein domain is particularly affected. Pathogenic missense variants affecting conserved residues are relatively common but pose an interpretative challenge, as many private benign missense variants are present in LRP2. A single case of uniparental isodisomy (UPD) leading to homozygosity of a recessive allele has been identified. Other mechanisms such as gene deletions are in principle possible.

Suspected pathophysiologic mechanism:
Renal function
Megalin is expressed in the proximal tubule of the kidney, and its absence causes reabsorption defects of a variety of low molecular weight proteins and other ligands, as determined in both human and animal studies. Among the most studied ligands are vitamin D-binding protein (DBP), retinol-binding protein (RBP), and transcobalamin. Hypovitaminoses have been described in animal models.
Other renal transporters, such as Cubilin, require the presence of Megalin to function properly.
Because of the central role of LRP2 in the endosomal and lysosomal systems, Megalin deficiency may partially overlap with the phenotypes of Lowe syndrome and Dent disease.
The chronic damage to the renal epithelium ultimately results in tubular necrosis in some patients.

Brain and craniofacial development
Holoprosencephaly (HPE) and agenesis of the corpus callosum were described in Lrp2-deficient mice. The HPE defect is believed to be due to the abnormal internalization and regulation of the sonic hedgehog (Shh) receptor, Patched (Ptch1), in the murine embryonic neuroepithelium. Conditional ablation of Lrp2 using a FoxG1-cre directed to the forebrain resulted in animals lacking the corpus callosum. A further ENU-screening identified a Lrp2Cys4032Ser line with abnormal midline crossing of the commissural axons and linked it with the HPE phenotype.

Ocular anomalies
In addition to its effect during forebrain morphogenesis, Lrp2 mediates Shh-controlled cellular proliferation during retinal development. Increased proliferation of the progenitor cells contained in the ciliary marginal zone likely contributes to the enlarged eye globes described in humans with DBS and in some animal models of Megalin deficiency. Bilateral elongation of the ocular globes along the anterior-posterior axis, a feature of high myopia, was recapitulated in FoxG1-cre conditional Lrp2-deficient models in the anterior neuroepithelium, providing a possible mechanism for near-sightedness observed in individuals affected with DBS.

Although Megalin is involved in the transcytosis of thyroglobulin in the thyroid, there is no reports of hyper- or hypothyroidism in patients with DBS in the medical literature.

Diagnostic testing
The presence of massive LMWP in a random urine sample, in combination with the characteristic craniofacial features and neuroimaging for the detection defects of the corpus callosum, strongly supports the diagnosis of DBS. Genetic testing is used to confirm the diagnosis. Targeted sequence analysis of the LRP2 coding region or whole exome sequencing (WES) identify biallelic pathogenic variants in the near totality of affected individuals. Chromosomal Microarray Analysis (CMA) is warranted in cases where one or both pathogenic variants are not identified after sequencing.