Complex inheritance pattern resembling autosomal recessive inheritance involving a microdeletion in thrombocytopenia-absent radius syndrome

Abstract

Thrombocytopenia-absent radius (TAR) syndrome is characterized by hypomegakaryocytic thrombocytopenia and bilateral radial aplasia in the presence of both thumbs. Other frequent associations are congenital heart disease and a high incidence of cow's milk intolerance. Evidence for autosomal recessive inheritance comes from families with several affected individuals born to unaffected parents, but several other observations argue for a more complex pattern of inheritance. In this study, we describe a common interstitial microdeletion of 200 kb on chromosome 1q21.1 in all 30 investigated patients with TAR syndrome, detected by microarray-based comparative genomic hybridization. Analysis of the parents revealed that this deletion occurred de novo in 25% of affected individuals. Intriguingly, inheritance of the deletion along the maternal line as well as the paternal line was observed. The absence of this deletion in a cohort of control individuals argues for a specific role played by the microdeletion in the pathogenesis of TAR syndrome. We hypothesize that TAR syndrome is associated with a deletion on chromosome 1q21.1 but that the phenotype develops only in the presence of an additional as-yet-unknown modifier (mTAR).

Figure 1.

Pedigrees of TAR-affected families in this study. Arrow marks indicate affected index patients. A dot inside a symbol indicates presence of microdeletion (i.e., a carrier). A bar on top of a symbol indicates that the individual was tested for the presence of the deletion.

Figure 2.

A, Adult patient (patient 1) with moderate upper-limb involvement. Note the radial deviation of both hands. B, Patient 9, showing mild upper-limb involvement with slightly reduced lengths of the arms. C, Severe phenotype with phocomelia (patient 20). D, Phocomelia in a severely affected patient (patient 8), and bowing of the long bones (E).

Figure 3.

A, Array CGH profile of chromosome 1 (patient 1). Note the microdeletion in 1q21.1. B–E, Confirmation of the microdeletion by FISH. B, Deletion of RP11-698N18 (labeled with Spectrum green) and two normal signals of control probe CEP1 (labeled with Spectrum orange) (patient 4). C, Partial deletion of RP11-258G05 (labeled with Spectrum green) and two normal signals of control probe CEP1 (labeled with Spectrum orange) (patient 4). D, Deletion of RP11-258G05 (labeled with Spectrum green) and two normal signals of control probe RP11-5P4 located in chromosomal band 1p31.3 (labeled with Spectrum orange) (patient 7). E, Two normal signals of RP11-258G05 (labeled with Spectrum green) and of control probe RP11-5P4 (labeled with Spectrum orange) (patient 20).

Figure 4.

Schematic representation of the 1q21.1 genomic region. Positions of the genes, BAC clones, and qPCR amplicons are indicated. The extent of the microdeletions detected in patients with TAR syndrome are indicated by bars. The number of patients with the respective deletion is given on the right side.

Figure 5.

A, Copy-number analysis by qPCR. The mean values for relative quantification (RQ) were exported from the 7500 SDS software. For all three groups (affected with deletion, nonaffected with deletion, and nonaffected without deletion) mean values and SDs (error bars) for each target primer relative to Albumin as a two-copy reference gene were calculated. Results were calibrated to the mean value determined for a healthy female control. A, C, and D refer to primers shown in table 2 (hChr1-A, hChr1-C, and hChr1-D, respectively). B, For determination of the distal breakpoint, the two carriers of the small deletion (patient 20 and nonaffected mother) were compared with the same control as in panel A and one nonaffected male deletion carrier. The breakpoint of the smaller deletion maps to the region between introns 11–12 and introns 13–14 of POLR3C (∼2.5 kb). Two target sequences located proximal to the deletion (Chr1-No3 and Chr1-No5) displayed normal copy numbers in all samples. The error bars indicate the 95% CI as calculated by the 7500 SDS software. F8 = factor VIII gene.