Improved strategy for molecular genetic diagnostics in juvenile nephronophthisis

Juvenile or type 1 nephronophthisis (NPH1), an autosomal recessive cystic kidney disease, represents the most common genetic cause of end-stage renal disease in the first two decades of life. Because the disease is caused by large homozygous deletions of the NPHP1 gene in approximately 66% of patients with nephronophthisis, molecular genetic testing offers a method for the definite diagnosis of NPH1 and avoids the invasive procedure of renal biopsy. We recently developed an algorithm for molecular genetic diagnosis of NPH1 that efficiently detects homozygous deletions. However, a major limitation remained for the detection of heterozygous deletions that cause NPH1 in combination with point mutations at the other NPHP1 allele. Because a partial sequence from the NPHP1 region recently became available through the Human Genome Projects, we exploited this information to develop novel polymorphic markers from this genetic region for the detection of heterozygous deletions of NPHP1, thus bridging the diagnostic gap. Five novel polymorphic microsatellites positioned within the large common NPHP1 deletion were generated. Two multiplex polymerase chain reaction sets using two and three polymorphic markers from the NPHP1 deletion region together with one positive control marker allowed four different diagnostic problems to be solved in one diagnostic setup: (1) detection of the classic homozygous deletion of NPH1, (2) detection of a rare smaller homozygous deletion of NPH1, (3) testing for a heterozygous deletion, and (4) potential exclusion of linkage to NPHP1. The newly generated multiplex marker sets will greatly enhance the efficacy of molecular diagnostics in NPH through improved detection of heterozygous deletions.