Selective pressure as an essential force in molecular evolution of myeloid leukemic clones: a view from the window of Fanconi anemia.

Specific chromosomal deletions are commonly found in bone marrow cells of children with Fanconi anemia (FA) whose disease has evolved to myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Identical deletions are found in adults with MDS/AML with a history of exposure to alkylating agents (secondary MDS/AML). While deleted chromosomal regions likely harbor genes encoding proteins with tumor suppressor (TS) function, such genes have not been identified and the environmental forces by which these mutant clones are selected remain unclear. A consistent signaling abnormality in cells bearing mutations of the Fanconi anemia complementation group C (FA-C) gene (FANCC) has revealed a potential selective force. Hematopoietic progenitor cells from patients and mice with FANCC mutations are hypersensitive to the inhibitory effects of IFNgamma and TNFalpha. Consequently, clonal outgrowths in FA likely result from strong selective pressure for stem and/or progenitor cells resistant to these inhibitory cytokines. Additional mutations that inactivate signaling pathways for these inhibitors would create a cell with a profound proliferative advantage over its apoptosis-prone counterparts. Here, we present preliminary evidence supporting a selection-based model of leukemic evolution and argue that MDS in FA patients is a de facto model of secondary MDS in non-FA adults.

Clinical and molecular analysis in Joubert syndrome.

Joubert syndrome is an autosomal recessive disorder comprising cerebellar hypoplasia, hypotonia, developmental delay, abnormal respiratory patterns, and abnormal eye movements. The biochemical basis of the Joubert syndrome is unknown. We ascertained a cohort of 50 patients with the Joubert syndrome to evaluate the presence of associated malformations, and to initiate studies leading to the identification of the Joubert syndrome gene. Only 8% of patients had polydactyly, 4% colobomas, 2% renal cysts, and 2% had soft tissue tumors of the tongue. In addition, we evaluated the WNT1 gene as a candidate gene for the Joubert syndrome based on its expression in the developing cerebellum and an associated mutation in the swaying mouse. We searched for mutations in WNT1 in a series of Joubert syndrome patients and no mutations were detected. Our analysis suggests that mutations in WNT1 do not cause the Joubert syndrome.

Severe vincristine neuropathy in Charcot-Marie-Tooth disease type 1A.

BACKGROUND: A general predisposition for vincristine-related neuropathy has been observed in persons with a family history of hereditary neuropathies. METHODS: In a retrospective case series, we investigated the possible association between the DNA rearrangement found in patients with Charcot-Marie-Tooth Disease Type 1A (CMT1A) and susceptibility to the neurotoxicity of vincristine. In selected patients and family members, we performed electrodiagnostic studies and analyzed DNA samples for 17p11.1-12 duplication associated with CMT1A. RESULTS: We describe three families with autosomal dominant CMT1, among whom a family member with a neoplastic disease suffered rapid onset, severe neuropathy after receiving initial doses of vincristine as a part of a routine chemotherapy protocol. All three families had at least one affected family member with 17p11.2-12 duplication. CONCLUSIONS: These cases show that 17p11.2-12 duplication predisposes patients to severe neurotoxicity from vincristine and that this drug should be avoided with patients with CMT1A. It is therefore essential to obtain a detailed family history for all oncology patients to screen for possible hereditary neuropathies. In patients with unexplained or preexisting familial neuropathy, testing for 17p11.2-12 duplication should be carried out prior to initiating vincristine therapy. Patients with other hereditary neuropathies may also be at risk for severe neurotoxic reactions.

Analysis of the CMT1A-REP repeat: mapping crossover breakpoints in CMT1A and HNPP.

The CMT1A-REP repeat sequence flanks a 1.5 megabase pair (Mb) segment of chromosome 17p11.2-12 which is duplicated in Charcot-Marie-Tooth neuropathy type 1A (CMT1A) and deleted in hereditary neuropathy with liability to pressure palsies (HNPP). The CMT1A-REP repeat is proposed to mediate misalignment and unequal crossover resulting in reciprocal chromosomal rearrangements in CMT1A and HNPP. We have constructed a physical map of the proximal and distal CMT1A-REP repeats. Cloned fragments from CMT1A-REP repeat regions are used to determine the size of the repeats and assess regions of homology. The crossover breakpoints were mapped in series of 30 unrelated CMT1A patients and 22 unrelated HNPP patients. The CMT1A-REP repeat spans approximately 27 kilobase pairs and appears to be continuous. Locations of restriction enzyme sites are highly conserved for the proximal and distal CMT1A-REP repeats. All crossovers mapped within the CMT1A-REP repeat sequence and heterogeneity for breakpoint location demonstrated. Seventy-seven percent (40 to 52) of CMT1A and HNPP chromosomes contained breakpoints which mapped within a 7.9 kb interval, suggesting the presence of a possible 'hotspot'for recombination in CMT1A-REP. DNA sequence analysis for 4 kb of the interval containing the majority of crossovers revealed over 98% sequence identity between proximal and distal CMT1A-REP repeat sequences. Probes useful for molecular-based diagnosis of CMT1A and HNPP are described.

New connexin32 mutations associated with X-linked Charcot-Marie-Tooth disease.

Analysis of the connexin32 gene in patients with X-linked Charcot-Marie-Tooth disease shows mutations distributed throughout the molecule, with all domains affected except the fourth transmembrane domain and the distal carboxy terminus. Sequence analysis of DNA from 19 unrelated patients detected six novel mutations and three previously reported mutations. Identification of additional mutations extends the distribution of connexin32 mutations in X-linked Charcot-Marie-Tooth disease and shows that specific mutations recur in additional families.

Hereditary neuralgic amyotrophy and hereditary neuropathy with liability to pressure palsies: two distinct genetic disorders.

Hereditary neuralgic amyotrophy with predilection for the brachial plexus (HNA) and hereditary neuropathy with liability to pressure palsies (HNPP) are autosomal dominant disorders associated with episodic, recurrent brachial neuropathies. HNPP is associated with a deletion or abnormal structure of the PMP22 gene on chromosome 17p11.2-12. The genetic locus for HNA is unknown. To address the possibility that HNPP and HNA might be identical disorders or allelic variations at the same locus, we investigated three HNA pedigrees with markers from the HNPP region. We did not find the 17p11.2-12 deletion associated with HNPP, nor an abnormality in PMP22 structure with HNA. This analysis provides genetic evidence, in addition to that suggested by the clinical, electrophysiologic, and pathologic differences, that HNA and HNPP are distinct disorders.

Two autosomal dominant neuropathies result from reciprocal DNA duplication/deletion of a region on chromosome 17.

Charcot-Marie-Tooth disease type 1A (CMT1A) is a common autosomal dominant demyelinating neuropathy that is associated with a 1.5 megabase (Mb) tandem DNA duplication in chromosome 17p11.2-p12. Hereditary neuropathy with liability to pressure palsies (HNPP, tomaculous neuropathy) is another less frequently diagnosed autosomal dominant neuropathy and is associated with a 1.5 Mb deletion in chromosome 17p11.2-12. Meiotic unequal crossover is a proposed mechanism for the generation of both the duplication in CMT1A and the deletion in HNPP. CMT1A-REP is a repeat that flanks the region which is duplicated/deleted in CMT1A/HNPP. The CMT1A-REP repeat sequence may mediate unequal crossover through misalignment of the homologous, repeated sequences. Three copies of the CMT1A-REP repeat are present on stably inherited CMT1A duplication chromosomes. In this report, molecular analysis in multiple patients detected three copies of the CMT1A-REP sequence on both inherited and de novo CMT1A duplication chromosomes, and one copy of the CMT1A-REP repeat on the deleted chromosome in both inherited and de novo HNPP. These observations support the hypothesis that a reciprocal recombination mechanism involving the CMT1A-REP is responsible for the generation of both the duplicated and deleted chromosomes, and document the first examples in humans of Mendelian syndromes resulting from the reciprocal products of unequal exchange involving large intra-chromosomal segments.

Connexin mutations in X-linked Charcot-Marie-Tooth disease.

X-linked Charcot-Marie-Tooth disease (CMTX) is a form of hereditary neuropathy with demyelination. Recently, this disorder was mapped to chromosome Xq13.1. The gene for the gap junction protein connexin32 is located in the same chromosomal segment, which led to its consideration as a candidate gene for CMTX. With the use of Northern (RNA) blot and immunohistochemistry technique, it was found that connexin32 is normally expressed in myelinated peripheral nerve. Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families. These findings, a demonstration of inherited defects in a gap junction protein, suggest that connexin32 plays an important role in peripheral nerve.

DNA deletion associated with hereditary neuropathy with liability to pressure palsies.

Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder that causes episodes of focal demyelinating neuropathy following minor trauma to peripheral nerves. We assign the HNPP locus to chromosome 17p11.2 and demonstrate the presence of a large interstitial deletion associated with this disorder in three unrelated pedigrees. De novo deletion is documented in one pedigree. The deleted region appears uniform in all pedigrees and includes the gene for peripheral myelin protein 22 (PMP-22), suggesting that underexpression of PMP-22 may cause HNPP. The deletion in HNPP spans approximately 1.5 Mb and includes all markers that are known to map within the Charcot-Marie-Tooth neuropathy type 1A (CMT1A) duplication. Furthermore, the breakpoints in HNPP and CMT1A map to the same intervals in 17p11.2, suggesting that these genetic disorders may be the result of reciprocal products of unequal crossover.

Trisomy 17p associated with Charcot-Marie-Tooth neuropathy type 1A phenotype: evidence for gene dosage as a mechanism in CMT1A.

Charcot-Marie-Tooth neuropathy type 1A (CMT1A) is associated with a DNA duplication on chromosome 17, band p11.2, resulting in partial trisomy for this region in CMT1A patients. The 17p11.2 duplication may lead to the CMT1A phenotype either through disruption of a gene at the duplication breakpoint junction or by trisomic dosage and overexpression of a gene within the duplication. To test the latter model, we evaluated a patient with complete translocation trisomy 17p for signs of CMT1A. In addition to the dysmorphic features seen in trisomy 17p, a neurologic examination and electrophysiologic studies detected a demyelinating neuropathy, compatible with CMT1A. A karyotype on the patient's father found a balanced translocation [t(14;17)] with breakpoints on chromosome 17 in either band p11.1 or proximal p11.2. An analysis of the patient's DNA confirmed trisomy 17p and mapped the translocation breakpoint to a region in 17p11.2, proximal to the duplication breakpoint in CMT1A. Our observations in this patient with trisomy 17p are relevant to an understanding of the genetic mechanism in CMT1A and provide strong evidence that gene dosage through segmental trisomy for 17p11.2 results in the CMT1A phenotype.

Peripheral myelin protein-22 gene maps in the duplication in chromosome 17p11.2 associated with Charcot-Marie-Tooth 1A.

Charcot-Marie-Tooth disease 1A (CMT1A) is a hereditary demyelinating peripheral neuropathy, associated with a DNA duplication on chromosome 17p11.2. A related disorder in the mouse, trembler (Tr), maps to mouse chromosome 11 which has syntenic homology to human chromosome 17p. Recently, the peripheral myelin protein-22 (pmp-22) gene was identified as the likely Tr locus. We have constructed a partial yeast artificial chromosome contig spanning the CMT1A gene region and mapped the PMP-22 gene to the duplicated region. These observations further implicate PMP-22 as a candidate gene for CMT1A, and suggest that over-expression of this gene may be one mechanism that produces the CMT1A phenotype.