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The quest for the gene responsible for medullary cystic kidney disease type 1.

The publication in February 2001 of the initial sequencing and analysis of the human genome (http://www. nhgri.nih.gov) changed the way we make diagnoses and practice medicine, but investigators are still searching, in 2004, to identify genes responsible for apparently simple monogenic diseases. In this issue of Kidney Inernational, Wolf et al [1] are continuing their quest to identify the gene in chromosome region 1q21 responsible for medullary cystic kidney disease type 1 (MCKD1). This is one of the last monogenic kidney diseases to conquer. Genes responsible for other inherited tubulointerstitial nephritis have been found, and nephrologists are now able to distinguish nephronophthisis from other hereditary medullary cystic kidney diseases (MCKD) (Table 1). Four genes (nephrocystin 1, inversin (formerly nephrocystin 2), nephrocystin 3, and nephrocystin 4; gene symbols NPHP1, INVS, NPHP3, and NPHP4, respectively) have been identified for autosomal-recessive juvenile nephronophthisis, and one (uromodulin; gene symbol UMOD) for the autosomal-dominant condition of MCKD2. MCKD is usually recognized later in life and leads to end-stage renal disease around the age of 50 years. The cell biology of these newly identified proteins responsible, when dysfunctional, for juvenile nephronophthisis is interesting because cilial dysfunction may underlie all forms of cystic disease: adult autosomal-dominant polycystic kidney disease type 1 and type 2 and also juvenile nephronophthisis [2, 3]. MCKD2 and some familial juvenile hyperuricemic nephropathy (FJHN) are allelic disorders [4]. FJHN is an autosomal-dominant disease with hyperuricemia, gout, and renal failure secondary to mutations in the UMOD gene coding for the Tamm-Horsfall glycoprotein, also referred to as uromodulin. Like MCKD, the renal histopathologic findings include chronic tubulointerstitial nephropathy with focal areas of interstitial fibrosis and inflammatory cell infiltration, thickening of