A Novel Mutation in LMX1B (p.Pro219Ala) Causes Focal Segmental Glomerulosclerosis with Alport Syndrome-like Phenotype

Case report: A novel R246L mutation in the LMX1B homeodomain causes isolated nephropathy in a large Chinese family

BACKGROUND

Genetic factors contribute to chronic kidney disease (CKD) and end-stage renal disease (ESRD). Advances in genetic testing have enabled the identification of hereditary kidney diseases, including those caused by LMX1B mutations. LMX1B mutations can lead to nail-patella syndrome (NPS) or nail-patella-like renal disease (NPLRD) with only renal manifestations.

CASE PRESENTATION

The proband was a 13-year-old female who was diagnosed with nephrotic syndrome at the age of 6. Then she began intermittent hormone and drug therapy. When she was 13 years old, she was admitted to our hospital due to sudden chest tightness, which progressed to end-stage kidney disease (ESRD), requiring kidney replacement therapy. Whole-Exome Sequencing (WES) results suggest the presence of LMX1B gene mutation, c.737G > T, p.Arg246Leu. Tracing her family history, we found that her father, grandmother, uncle and 2 cousins all had hematuria, or proteinuria. In addition to the grandmother, a total of 9 members of the family performed WES. The members with kidney involved all carry the mutated gene. Healthy members did not have the mutated gene. It is characterized by co-segregation of genotype and phenotype. We followed the family for 9 year, the father developed ESRD at the age of 50 and started hemodialysis treatment. The rest patients had normal renal function. No extra-renal manifestations associated with NPS were found in any member of the family.

CONCLUSIONS

This study has successfully identified missense mutation, c.737G > T (p.Arg246Leu) in the homeodomain, which appears to be responsible for isolated nephropathy in the studied family. The arginine to leucine change at codon 246 likely disrupts the DNA-binding homeodomain of LMX1B. Previous research has documented 2 types of mutations at codon R246, namely R246Q and R246P, which are known to cause NPLRD. The newly discovered mutation, R246L, is likely to be another novel mutation associated with NPLRD, thus expanding the range of mutations at the crucial renal-critical codon 246 that contribute to the development of NPLRD. Furthermore, our findings suggest that any missense mutation occurring at the 246th amino acid position within the homeodomain of the LMX1B gene has the potential to lead to NPLRD.

Biochemical composition of the glomerular extracellular matrix in patients with diabetic kidney disease

In the glomeruli, mesangial cells produce mesangial matrix while podocytes wrap glomerular capillaries with cellular extensions named foot processes and tether the glomerular basement membrane (GBM). The turnover of the mature GBM and the ability of adult podocytes to repair injured GBM are unclear. The actin cytoskeleton is a major cytoplasmic component of podocyte foot processes and links the cell to the GBM. Predominant components of the normal glomerular extracellular matrix (ECM) include glycosaminoglycans, proteoglycans, laminins, fibronectin-1, and several types of collagen. In patients with diabetes, multiorgan composition of extracellular tissues is anomalous, including the kidney, so that the constitution and arrangement of glomerular ECM is profoundly altered. In patients with diabetic kidney disease (DKD), the global quantity of glomerular ECM is increased. The level of sulfated proteoglycans is reduced while hyaluronic acid is augmented, compared to control subjects. The concentration of mesangial fibronectin-1 varies depending on the stage of DKD. Mesangial type III collagen is abundant in patients with DKD, unlike normal kidneys. The amount of type V and type VI collagens is higher in DKD and increases with the progression of the disease. The GBM contains lower amount of type IV collagen in DKD compared to normal tissue. Further, genetic variants in the α3 chain of type IV collagen may modulate susceptibility to DKD and end-stage kidney disease. Human cellular models of glomerular cells, analyses of human glomerular proteome, and improved microscopy procedures have been developed to investigate the molecular composition and organization of the human glomerular ECM.