In our study, we confirmed biopsy proven KIN in 2 siblings from family A and one patient from family Z. According to the family clustering of KIN previously described [2, 4, 8], we concluded that it was the same nephropathy in all patients.
KIN is a rare but probably an underdiagnosed condition. Its prevalence is below 1% in patients who had renal biopsy [9].
In our nephrology department, we found 3 cases among 8870 renal biopsy. This underdiagnosed condition is due to several reasons: clinical presentation is not specific, chronic tubulo-interstitial nephritis is usually asymptomatic and kidneys are too small to make biopsy. The biggest series is that of Bhandari et al. [8] (6 cases) followed by Spoendlin et al. [4] (4 cases). The review of the available literature reveals approximately 33 reported cases until now. In Tunisia, 3 cases were reported by Hassen et al. [3].
In this paper we report 6 additional cases of KIN with histological evidence in 3 patients. In our series of patients, the mean age at first documentation of nephropathy was 26.83 years. In the literature, the average age is 36.42 years. Indeed, most patients including ours developed a progressive renal failure beginning in the third decade of life. The evolution to ESRD is usually at an age of 37.22 years (36.5 years in our series).
Patients had a history of recurrent infections mainly involving the respiratory tract, which was not noted in our series. Transient elevations of liver enzymes were found in our patients as described in others [2, 9, 10].
KIN is a chronic tubule-interstitial disease that was first described in two siblings and in an unrelated third patient with progressive chronic renal failure [2]. The kidney showed bizarre karyomegaly and nuclear polymorphism predominantly of tubular cells and interstitial fibrosis accompanying tubular atrophy. We describe here 3 additional cases with slowly progressive renal failure and the same histological characteristic findings.
Karyomegaly is not limited to the kidneys. In patient reported by Mihatsch [2], biopsy specimens of several organs and the autopsy of one patient disclosed polymorphic enlarged nuclei in epithelial and mesenchymal cells of the kidneys and many other organs, i.e. smooth muscle cells of the intestine, alveolar epithelial cells, brain astrocytes, bile duct epithelium and Kupffer cells. In our cases, one liver biopsy was performed and did not show karyomegalic cell nuclei.
KIN has been long described as a nephropathy of unknown etiology and pathogenesis. Karyomegalic changes in renal cells have been reported in different pathologic settings, both in human and experimental animals, including heavy metals toxicity [11, 12, 13] and virus infection (CMV).
In addition and because of the similarity of KIN with Balkan nephropathy [14, 15, 16, 17] other potential etiologies including OTA have been convincingly included by other investigators. Burry's [18] and Sclare's cases [19] were reported as affected by KIN and were found to have OTA in their blood and urine, a mycotoxin produced by a number of aspergillum and penicillum species which has been shown to cause karyomegalic lesions in proximal tubuli in rats [20, 21, 22]. The most important toxic effect of this mycotoxin is nephrotoxicity.
Studies in Tunisia, Algeria, Morocco and Egypt have shown similar disease symptoms with high incidence of chronic interstitial nephropathies of unknown etiology. Hassen et al. confirmed the involvement of this nephrotoxic mycotoxin present at high blood levels in Tunisian population in the outcome of tubulo-interstitial nephritis with unknown etiology [23, 24].
OTA is a possible, but not the only, etiologic factor in triggering KIN. In fact KIN seems to be rather multifactorial involving especially a genetic predisposition. Family clustering, highest frequency of the haplotypes HLA A9 B35 and different susceptibility to develop the nephropathy in spite of a high OTA contamination in all subjects support a genetic etiology. Besides, family clustering was reported more than once [2, 3, 4, 8, 9, 10, 25, 26, 27].
We first studied polymorphisms CYP3A4*18 and A313G respectively for the genes CYP3A4 and GSTP1 in the family A because these genes are involved in the metabolism of the OTA [28].
However, the frequency of these polymorphisms differs according to the ethnic group: 20.3% in the Chinese population [29] against 2% in the Tunisian population [30] for CYP3A4*18 and 30% for the Caucasian population [27] against 1% for the Tunisian population [30] for A313G. These low frequencies in the Tunisian population could explain the absence of significant association between polymorphisms CYP3A4*18, A313G and KIN in family A. The only study made on the Tunisian population [30] did not either find association between these polymorphisms and the arisen of a renal disease including the KIN, so confirming our results.
Recently Zhou et al [5] identified recessive mutations of FAN1 as causing KIN, which leads to CKD with fibrotic degeneration of the kidney. They performed homozygosity mapping and whole exome sequencing (WES) in two siblings of Maori descent in order to identify additional causative genes for Nephronophthisis (NPHP)-related ciliopathies, a heterogeneous group of recessive diseases that cause CKD [31].
They obtained by homozygosity mapping 7 candidate regions of homozygosity by descent and identified by WES a homozygous nonsense mutation (Trp707*) in FAN1 in both affected siblings. This truncating mutation was the only homozygous variant detected within the mapped candidate regions. They also performed Sanger sequencing of all FAN1 exons in 5 published KIN families and 5 unpublished cases, and identified 12 different mutations in 9/10 families with KIN. Eight of the 12 mutations truncated the conceptual reading frame. They concluded that these recessive mutations of FAN1 are the cause of KIN for these families. By taking into account these results, we sequenced FAN1 for the affected members of our families. Sequencing allowed us to identify 2 different mutations for each family.
A homozygous frameshift mutation due to a one-base-pair deletion (c.2616delA) resulting in a premature STOP codon (p.Asp873ThrfsTer17) was detected in family A (Figure 4). This variant was previously reported by Zhou et al. [5]. A homozygous frameshift mutation due to a one-base-pair deletion (c.2603delT) resulting in a premature STOP codon (p.Leu868ArgfsTer22) was detected in family Z (Figure 4). Three frameshift mutations in the exon 12 of FAN1 were reported in the literature in families with KIN [5], our mutations are the newest ones.
FAN1 encodes Fanconi anemia-associated nuclease 1. FAN1 mutations cause karyomegalic interstitial nephritis, linking chronic kidney failure to defective DNA damage repair [5] , an endonuclease that interacts with FANCD2 and FANCI on Interstrand DNA crosslinks (ICL)- induced DNA damage response (DDR) and is recruited to the sites of ICL lesions, where its function is critical for resolving ICLs[ 32, 33, 34, 35].
In fact, failure to remove ICLs can induce cell cycle arrest, cell death and genome instability. However recent studies showed that deficient cells in both FAN1 and components of the FA pathway are more sensitive to ICL-inducing agents than the respective single mutants, suggesting that in ICL repair the role of FAN1 is not the same as the one in the FA pathway [36].
These findings suggest that biallelic mutations in FAN1 cause KIN [5] instead of FA. The cause of the karyomegaly in KIN is not yet understood, and it is not clear whether it or any other characteristics of KIN in patients with FAN1 mutations is caused by defective ICL repair.
Furthermore, it is not clear whether the nuclease activity of FAN1 is required to prevent KIN. The basis of a functional difference between FAN1 and the FA pathway in ICL repair will be an important area of investigation. It is possible that FAN1 may respond to a subset of ICLs that is somehow different in nature from the ICLs dealt with by the FA pathway. This might reflect a difference in the chemical nature of the ICLs or a difference in the genomic context of the ICLs recognized by the two pathways [37].
Like ours, other studies reported homozygous mutations in FAN1 associated with detectable OTA in urine and blood samples [25]. FAN1 mutations could result in more sensitive tubular cells to environmental genotoxin like OTA inducing renal DNA damage. Another study [38] on the identification of mutations in 2 other genes ZNF423 and CEP164 in patients with renal ciliopathies, confirmed the presence of a link between the pathogenesis of chronic kidney diseases and defective DNA damage repair.
More recently, Law et al [39] report the first case of concurrent LECT2 amyloidosis and KIN combined with a novel deletion in the FAN1 gene. These two rare causes of chronic kidney disease have been described in the same patient for the first time, which highlights the importance of renal biopsy in chronic kidney disease of unclear aetiology.
In conclusion, our study is the first Tunisian report of familial cases of KIN with mutations involving the FAN1 gene, which plays a key role in DNA repair. These mutations are not sufficient to develop KIN, in fact additional environmental factors, such as OTA, with inadequate DNA repair are hypothesized to cause the disease.