The CMT2D Locus: Refined Genetic Position and Construction of a Bacterial Clone-Based Physical Map

In 1886, two independent publications described a disorder characterized by progressive muscular atrophy with weakness of the legs and feet (Charcot and Marie 1886; Tooth 1886). Ultimately, these features were found to be shared among a heterogeneous group of inherited neurodegenerative disorders that affect primarily the peripheral nervous system; collectively, these disorders are now called Charcot-Marie-Tooth (CMT) disease. With a prevalence of 1 in 2500, CMT represents the most common inherited peripheral neuropathy (Skre 1974). Generally, CMT disease includes the following characteristics: muscle weakness of the feet and legs, decreased deep tendon reflexes, atrophy of the legs, a characteristic steppage (“equine”) gait, sensory loss, and pes cavus (Murakami et al. 1996). However, the description of the disease is complicated by the fact that some patients demonstrate a slow motor nerve conduction velocity (MNCV) accompanied by demyelination of the nerves, whereas others show normal to near normal MNCVs without marked demyelination (Dyck and Lambert 1968; Dyck et al. 1993). In the adopted classification scheme for CMT, CMT2 corresponds to nondemyelinating forms of the disease. By 1996, three autosomal dominant CMT2 loci had been reported: (1) CMT2A, mapping to chromosome 1p36 (Ben Othmane et al. 1993); (2) CMT2B, mapping to chromosome 3q13–q22 (Kwon et al. 1995); and (3) CMT2C, accounting for all other CMT2 families excluded from linkage to CMT2A and CMT2B, but otherwise unmapped in the genome (Dyck et al. 1994). Ionasescu et al. (1996) then analyzed a large American family (CMT1019) with atypical CMT and identified a novel locus, CMT2D [see Online Mendelian Inheritance in Man (OMIM) accession no. 601472; http://www.ncbi.nlm.nih.gov/Omim], within a broad genetic interval on chromosome 7p14. Subsequently, additional CMT2 families were linked to this same region of chromosome 7 (Pericak-Vance et al. 1997; Sambuughin et al. 1998), including one (named HSMN M) by Sambuughin et al. (1998) with symptoms that are also suggestive of the closely related disorder distal spinal muscular atrophy (dSMA; see OMIM accession no. 600794; http://www.ncbi.nlm.nih.gov/Omim). Interestingly, this region resides within a broad (∼37 cM) interval showing linkage with another large dSMA family (Christodoulou et al. 1995). Together, these data suggest that defects in a single gene may be responsible for CMT2 and dSMA in these families. Recently, we completed a yeast artificial chromosome (YAC)-based physical map of human chromosome 7 (Bouffard et al. 1997b; see http://www.nhgri.nih.gov/DIR/GTB/CHR7) and currently are constructing bacterial clone-based contig maps suitable for systematic sequencing of the chromosome (Marra et al. 1997). Our physical mapping data provided a refined order of the available genetic markers in the 7p14 region harboring the CMT2D gene. Here we present a refined genetic analysis of the CMT1019 family. The CMT2D-containing region defined by the CMT1019 family overlaps a more narrow (∼3 cM) critical region defined by the Sambuughin et al. (1998; HSMN M) family, resulting in a combined ∼1-Mb critical region. In addition, we report the construction of a contiguous bacterial artificial chromosome (BAC) and P1-derived artificial chromosome (PAC) contig map spanning this combined critical region. Clones across these intervals are being sequenced at the Washington University Genome Sequencing Center (see http://genome.wustl.edu/gsc). Together, these efforts should accelerate the isolation of the CMT2D/dSMA gene(s).