Is Chlamydia pneumoniae the causative agent of microscopic polyangiitis?

Sir, Regarding the letter, we thank Dr. Gambaro et al. for the interest in our recent paper published in Nephrology Dialysis Transplantation. We agree with the observation that the majority of patients with medullary sponge kidney (MSK) do not present the typical distal renal tubular acidosis (dRTA) clinical phenotype (e.g. growth failure from infancy). However, dRTA is characterized by a wide clinical heterogeneity, and only in the last few years, has this metabolic disease been studied in all its various clinical and molecular aspects. Some cases of dRTA can show low clinical expression, so the diagnosis may be delayed until the appearance of classical complications. We recently studied two sisters affected by dRTA, one with the typical clinical manifestations of the disease and the other (30years old) with a metabolic history that remained essentially silent. Both have the same mutation in one of the causative dRTA genes. Furthermore, as reported in our article, Kasap and colleagues described a 5-year-old girl with MSK associated with dRTA [1]. Despite the fact that a molecular analysis was not performed, the clinical features described by the authors are really convincing that this patient could have both conditions. Because the urographic pictures were taken several years ago, it is difficult to answer point by point the comments of Gambaro et al. However, usually, provocative manoeuvres are applied when it is necessary to increase the sensitivity of the procedure, but the Rx images of the reported patients show the typical features of MSK, as our current consultant radiologists, who reviewed the X-ray films, confirmed. We cannot conclude that MSK is ever a consequence of dRTA, but since MSK is a heterogeneous condition, as also stressed by Gambaro et al., for the first time, our study has highlighted the association of two rare renal diseases coupled with the genes responsible of dRTA, and it revealed a possible correlation between MSK and dRTA. Further studies are certainly needed to clarify not only the relationship between these two clinical conditions but also the phenotypic expression of dRTA, which is still not well known.