Schimke immunoosseous dysplasia (SIOD), which is characterized by prominent spondyloepiphyseal dysplasia, T-cell deficiency, and focal segmental glomerulosclerosis, is a panethnic autosomal recessive multisystem disorder with variable expressivity. Biallelic mutations in switch/sucrose nonfermenting (swi/snf) related, matrix-associated, actin-dependent regulator of chromatin, subfamily a-like 1 (SMARCAL1) are the only identified cause of SIOD. However, among 72 patients from different families, we identified only 38 patients with biallelic mutations in the coding exons and splice junctions of the SMARCAL1 gene. This observation, the variable expressivity, and poor genotype-phenotype correlation led us to test several hypotheses including modifying haplotypes, oligogenic inheritance, or locus heterogeneity in SIOD. Haplotypes associated with the two more common mutations, R820H and E848X, did not correlate with phenotype. Also, contrary to monoallelic SMARCAL1 coding mutations indicating oligogenic inheritance, we found that all these patients did not express RNA and/or protein from the other allele and thus have biallelic SMARCAL1 mutations. We hypothesize therefore that the variable expressivity among patients with biallelic SMARCAL1 mutations arises from environmental, genetic, or epigenetic modifiers. Among patients without detectable SMARCAL1 coding mutations, our analyses of cell lines from four of these patients showed that they expressed normal levels of SMARCAL1 mRNA and protein. This is the first evidence for nonallelic heterogeneity in SIOD. From analysis of the postmortem histopathology from two patients and the clinical data from most patients, we propose the existence of endophenotypes of SIOD.

We report the case of a 4-year-old boy, diagnosed with atypical hemolytic uremic syndrome (HUS) due to a hybrid factor H. He progressed to end-stage renal failure despite plasmatherapy and underwent bilateral nephrectomy because of uncontrolled hypertension. Three days after, he had partial complex seizures with normal blood pressure, normal blood count and normal magnetic resonance imaging (MRI), which recurred 1 month later. Eight months later, he had a third episode of seizures, with hemoglobin of 10 g/dl without schizocytes, low haptoglobin of 0.18 g/l, and moderate thrombocytopenia (platelets 98 × 10(9)/l). He remained hypertensive and deeply confused for 2 days. The third MRI showed bilateral symmetrical hyperintensities of the cerebral pedunculas, caudate nuclei, putamens, thalami, hippocampi, and insulae suggesting thrombotic microangiopathy secondary to a relapse of HUS rather than reversible posterior leukoencephalopathy syndrome (RPLS), usually occipital and asymmetrical. Plasmatherapy led to a complete neurological recovery within 2 days although hypertension had remained uncontrolled. The fourth MRI 10 weeks after, on maintenance plasmatherapy, was normal and clinical examination remained normal, except for high blood pressure. In conclusion, brain MRI allows differentiating thrombotic microangiopathy lesions from RPLS in atypical HUS, which is crucial since lesions may be reversible with plasmatherapy.

C3NeF associated C3G has a heterogeneous presentation and outcome. Anti-proteinuric agents may control the disease during follow-up, even after nephrotic syndrome at the onset. The efficiency of immunosuppressive therapy remains questionable.