Progress in molecular diagnosis of Charcot-Marie-Tooth-disease type 1 (CMT 1, HMSN I) and hereditary neuropathy with liability to pressure palsies (HNPP) by fluorescence in situ hybridization (FISH)-detection of a potential genetic mosaicism

We tested 20 CMT 1 patients characterized according to the criteria of the European CMT consortium by Southern hybridization of MspI restricted genomic DNA with probes pVAW409R1, pVAW412Hec and pEW401HE. In 11 of the 20 CMT 1 cases (55%), we observed a duplication in 17q11.2; one patient had a dinucleotide insertion in exon 6 of the PO-gene (5%). One HNPP case had a typical 17p11.2 deletion. Analysis of CA-repeats was performed with primers RM11GT and Mfd41; SSCP-analysis of the PO, PMP22 and Cx32-genes is in progress. FISH was carried out with probe pVAW409R1. 125 interphase nuclei were analyzed for each proband by counting the signals per nucleus. Normal cells show a characteristic distribution of signals: 1 signal in 5.9% of nuclei, 2 in 86.3% and 3 in 7.8%. A duplication is indicated by a shift to 3 signals in more than approximately 60% and 2 in less than 25% of the nuclei. In contrast, the 17p11.2 deletion of the HNPP patient shifts to 82.4% of nuclei with a single hybridization signal versus 14.4% with 2 signals. We detected one case with significantly abnormal distribution of interphase nuclei hybridization signals compared to cultures of normal cells and to those with 17p11.2more » duplication or deletion: 3.2% nuclei revealed 1 signal, 48.0% two signals and 48.8% 3 signals, indicating a pathogenic but moderate dosis increase compared to the throughout duplicated cases. FISH with probe pVAW409R1 is a versatile tool to detect the HNPP deletion both in interphase nuclei and in metaphase chromosomes. In CMT 1 disease interphase nuclei are required for FISH analysis due to the small duplication of 1.5 Mbp. In contrast to Southern techniques, FISH is able to detect genetic mosaicism.« less