A mutation in a novel ATP-dependent Lon protease gene in a kindred with mild mental retardation - PubMed (original) (raw)

Comparative Study

A mutation in a novel ATP-dependent Lon protease gene in a kindred with mild mental retardation

Joseph J Higgins et al. Neurology. 2004.

Abstract

Background: Identifying the genetic factors that contribute to memory and learning is limited by the complexity of brain development and the lack of suitable human models for mild disorders of cognition.

Methods: Previously, a disease locus was mapped for a mild type of nonsyndromic mental retardation (IQ between 50 and 70) to a 4.2-MB interval on chromosome 3p25-pter in a large kindred. The genes and transcripts within the candidate region were systematically analyzed for mutations by single-strand polymorphism analysis and DNA sequencing.

Results: A nonsense mutation causing a premature stop codon in a novel gene (cereblon; CRBN) was identified that encodes for an ATP-dependent Lon protease. The predicted protein sequence is highly conserved across species, and it belongs to a family of proteins that selectively degrade short-lived polypeptides and regulate mitochondrial replication and transcription. One member of the Lon-containing protein family is regionally expressed in the human hippocampus, an important neuroanatomic region that is involved in long-term potentiation and learning. The mutation in the CRBN gene described interrupts an N-myristoylation site and eliminates a casein kinase II phosphorylation site at the C terminus.

Conclusions: A gene on chromosome 3p that is associated with mild mental retardation in a large kindred is reported. This finding implicates a role for the ATP-dependent degradation of proteins in memory and learning.

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Figures

Figure 1

Figure 1

Identification of the 1,274C_→_T mutation in CRBN _exon 11. (A) Representative sample of the single-strand conformational polymorphism (SSCP) mobility shifts in one family branch with mild autosomal recessive nonsyndromic mental retardation. SSCP was performed on the 331-bp PCR products of exon 11, including its 5_′ _and 3_′ splice sites. The mobility shift patterns co-segregated with the affection status and correlated with the results of linkage analysis reported previously. Affected individuals (filled symbols) have two upper bands and one lower band. Carriers (partially filled symbols) have six bands including two upper bands, two middle bands, and two lower bands. Noncarriers (open symbols) have one upper band, two middle bands, and two lower bands. The SSCP pattern of the noncarrier was the same as the patterns in 200 normal individuals (400 chromosomes). (B) Direct sequencing of PCR products. (Top) The CRBN sequence between nucleotide (nt) 1,263 and 1,284 from a normal individual (noncarrier). (Bottom) A 1,274C_→_T substitution in an affected individual in the family. Below the nucleotide sequence is the amino acid sequence. The mutation (arrow) results in a stop codon at the arginine (R) residue at codon 419 and truncates the CRBN protein (R419X). (C) A 2% agarose/0.1% ethidium bromide gel showing the HpyCH4IV restriction enzyme analysis of CRBN exon 11. Lane 1 is a 100-bp ladder. The PCR product of a normal individual (lane 2) is not cleaved by HpyCH4IV and shows a single band representing the 331-bp wild-type alleles. Carriers show two bands representing the 331-bp wild-type allele and the cleaved mutant allele (164-bp, 167-bp) (lanes 3 to 6). The mutation creates a restriction site so that the amplified 331-bp fragment is cleaved in half (164-bp, 167-bp) (lanes 7 to 10).

Figure 2

Figure 2

Segregation of the CRBN mutation using HpyCH4IV restriction enzyme digestion of exon 11 in an abridged family pedigree. As in figure 1C, a single 331-bp band represents the wild-type allele and is homozygous in Individuals 2, 8, 10, 19, 24, 26, 30, and 31. Carriers (Individuals 1, 3, 4, 5, 6, 7, 9, 12, 15, 16, 20, 22, 27, 28, and 32) show two bands representing the 331-bp wild-type allele and the cleaved mutant allele (164-bp, 167-bp). The mutation creates a restriction site so that the amplified 331-bp fragment is cleaved in half (164-bp, 167-bp) and is shown as a single lower band in affected Individuals 11, 13, 14, 17, 18, 21, 23, 25, and 29. Filled circles (females) and squares (males) represent affected individuals with mental retardation. Unaffected individuals are indicated by open symbols. Consanguineous marriages are indicated by double bars joining the individuals. For confidentiality reasons, the order and the sex of at-risk individuals are changed.

Figure 3

Figure 3

Alignment of the amino acid sequences from different species with similarities to the CRBN protein. Low consensus alignments are in gray and neutral consensus alignments are in black. The underlined sequences show the Lon domain from amino acid residues 81 to 317 of the predicted protein for human CRBN and the homologous proteins for mice and rats. Prosite predictions for the casein kinase II phosphorylation site (filled circles), myristoylation sites (open circles), protein kinase C phosphorylation site (filled squares), and a glycosylation site (open squares) are shown. The arrow shows the position of the stop codon (R419X) that interrupts an N_-myristoylation site and eliminates a casein kinase II phosphorylation site._

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