Valerie M. Schwitzgebel

Nephrocalcinosis is a complex disease with a multitude of triggering factors. An association with congenital hypothyroidism has been described in the literature, but the mechanisms leading to its development remain unclear. A 7‐month‐old infant presented with muscular hypotonia and signs of malnutrition was diagnosed with congenital hypothyroidism, nephrocalcinosis of unclear origin, and multiple kidney stones. Urine analysis revealed the presence of calcium oxalate crystals and slightly elevated oxaluria without hypercalciuria. Targeted exome sequencing found no variants in the four causative genes of primary oxaluria, namely AGXT , CBX5 , GRHPR , and HOGA1 . The clinical outcome was favorable with thyroid hormone and potassium citrate treatment. Ultrasound follow‐up showed progressive improvement of nephrocalcinosis. The low level of urinary oxalate and the prevalence of dihydrate calcium oxalate crystals spoke against primary or secondary oxaluria. Recent evidence from mitochondrial research shows that thyroid hormone T3 enhances mitochondrial calcium levels by stimulating calcium uptake by the mitochondrial calcium uniporter. A reduced calcium uptake and metabolism in mitochondria might represent an explanation for cytoplasmic calcium accumulation in tubular cells, subsequently precipitating nephrocalcinosis in the absence of thyroid hormone. Even if the pathophysiological mechanisms are not yet fully understood, recent evidence is supportive of a causal relationship between hypothyroidism and nephrocalcinosis, a sometimes overlooked association.

Noninvasive prenatal diagnosis relies on the presence in maternal blood of circulating cell‐free fetal DNA released by apoptotic trophoblast cells. Widely used for aneuploidy screening, it can also be applied to monogenic diseases (NIPD‐M) in case of known parental mutations. Due to the confounding effect of maternal DNA, detection of maternal or biparental mutations requires relative haplotype dosage (RHDO), a method relying on the presence of SNPs that are heterozygous in one parent and homozygous in the other. Unavoidably, there is a risk of test failure by lack of such informative SNPs, an event particularly likely for consanguineous couples who often share common haplotypes in regions of identity‐by‐descent. Here we present a novel approach, relative genotype dosage (RGDO) that bypasses this predicament by directly assessing fetal genotype with SNPs that are heterozygous in both parents (frequent in regions of identity‐by‐descent). We show that RGDO is as sensitive as RHDO and that it performs well over a large range of fetal fractions and DNA amounts, thereby opening NIPD‐M to most consanguineous couples. We also report examples of couples, consanguineous or not, where combining RGDO and RHDO allowed a diagnosis that would not have been possible with only one approach.