A study led by the Kidney Pathophysiology group at Vall d'Heborn Research Institute (VHIR) has identified genetic factors that influence the progression of familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (HFHNC), an ultra-rare disease that affects the kidneys. The team has discovered some genetic variants that can worsen the disease, which may explain why some patients progress more rapidly to kidney failure. The results help to understand the biological mechanisms and set the basis for the identification of possible therapeutic targets for a disease that so far has no treatment.

The work, recently published in the journal PLOS Genetics, has been carried out in collaboration with the Paediatric Nephrology Service of Vall d'Hebron University Hospital, led by Dr. Gema Ariceta, the Statistics and Bioinformatics Unit of VHIR, and the Centro Nacional de Análisis Genómico (CNAG), and thanks to funding from the patients' association Hipofam and the Instituto de Salud Carlos III.

A minority disease with no curative treatment

HFHNC is an extremely rare kidney disease, with an incidence of one case per million people. It is caused by a mutation in the Claudin 19 (CLDN19) gene, the most frequent in Spain, or in Claudin 16 (CLDN16), both of which are involved in normal kidney function.

It is a pathology with a great impact on patients' quality of life, as in many cases it progresses to chronic renal failure during childhood or adolescence. Currently, there is no curative treatment beyond transplantation, and the available options are limited to supportive measures such as dietary restrictions.

Disease progression is not uniform among patients, even between siblings: while some need a transplant before the age of 10, others retain kidney function into adulthood. “The great variability in the evolution of patients made us think that there were genetic factors beyond the main mutation that could influence the progression of the disease”, explains Dr. Cristina Martínez, principal investigator of the Kidney Pathophysiology group at VHIR.

A study of the entire Spanish population affected

To better understand this variability, the VHIR team analysed 45 patients with HFHNC, corresponding to all affected individuals in Spain. Patients were classified according to the speed of disease progression, and genetic sequencing of each person was performed thanks to whole exome analysis (WES). Thanks to publicly available genetic databases, it was also possible to compare with the general population.

In this way, the team detected genetic variants that are only found in patients with a more rapid progression, reinforcing the hypothesis that they play a key role in the evolution of the disease. These variants, although not pathogenic on their own, can aggravate the pathology when combined with mutation in the Claudin 19 (CLDN19) or Claudin 16 (CLDN16) gene. “This is the first study that identifies modifier genes in this disease, and this allows us to better understand why some patients progress faster than others”, emphasise Dr. Mònica Vall, who carried out the doctoral thesis that led to these results at VHIR, and Julieta Torchia, predoctoral researcher of the group that has continued this project.

Among the variants identified is one in the NFU1 gene, which is involved in the functioning of mitochondria, responsible for energy production in cells. “We know that kidney cells require a lot of energy to carry out their function of filtering the blood, and any variant that alters their function can worsen the progression of the disease. In fact, pathogenic mutations in NFU1 cause another minority disease that affects several organs, including the kidney”, explains Dr. Anna Meseguer, head of the Kidney Pathophysiology group at VHIR. “In diseases as rare as HFHNC, it is difficult to obtain conclusive statistical data in this type of study and, therefore, it is key for us to find the biological meaning of the variants we identify”.

New strategies for understanding the disease

In order to confirm these results and better understand the impact of these genetic variants, the VHIR team has launched a new project, funded by the Inocente Inocente Foundation, based on induced pluripotent stem cells (iPSCs). "These cells, obtained from patients with HFHNC, allow us to generate cellular models and organoids that faithfully reproduce the conditions of the affected kidneys", explains Dr. Gerard Cantero, principal investigator of the group.

In addition, using gene editing techniques such as CRISPR/Cas9, the team will be able to introduce or correct these variants in cell cultures to analyse their effects. "With this work, we will contribute to deciphering key molecular mechanisms and finding therapeutic targets that will open up new avenues for the development of treatments to stop HFHNC that are not available yet", concludes Dr. Martínez.