Truncation of the Down syndrome candidate gene DYRK1A in two unrelated patients with microcephaly

Abstract

We have identified and characterized two unrelated patients with prenatal onset of microcephaly, intrauterine growth retardation, feeding problems, developmental delay, and febrile seizures/epilepsy who both carry a de novo balanced translocation that truncates the DYRK1A gene at chromosome 21q22.2. DYRK1A belongs to the dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) family, which is highly conserved throughout evolution. Given its localization in both the Down syndrome critical region and in the minimal region for partial monosomy 21, the gene has been studied intensively in animals and in humans, and DYRK1A has been proposed to be involved in the neurodevelopmental alterations associated with these syndromes. In the present study, we show that truncating mutations of DYRK1A result in a clinical phenotype including microcephaly.

Figure 1

Clinical Pictures and Laboratory Investigations of Patient 1 The pictures were taken when the patient was 3 months (A), 9 months (B), 14 months (C), and 24 months (D and E) of age. Magnetic-resonance imaging (F) at three months of age showed microcephaly and hypogenesis of corpus callosum. Cytogenetic analysis revealed a translocation with the karyotype 46,XY,t(9;21)(p12;q22) (G). For mapping of the breakpoints of the translocation, fluorescence in situ hybridization (FISH) was carried out with the use of BAC clones, according to standard procedures. The BAC clone RP11-315B15, which contains DYRK1A, spans the chromosome 21 breakpoint, as shown by signals on the normal and the derivative chromosomes (H).

Figure 2

The Genomic Region Covered by the DYRK1A Gene The locations of the chromosomal breakpoints of the translocation patients are indicated by vertical arrows. The DYRK1A gene is organized in 17 exons (boxes) and has two transcription start sites, with exon 1 and exon 2 as alternative first exons. The use of these alternative start sites yields two major transcripts, DYRK1Aa (5182 nt) and DYRK1Ab (5287 nt), which are both truncated by the translocation breakpoints in patients 1 and 2. The bracket indicates that exon 3 is alternatively spliced. The chromosome band and base-pair positions are shown above the figure.

Figure 3

Clinical Pictures and Laboratory Investigations of Patient 2 The pictures were taken when the patient was 12 years of age. She has asymmetry of the head, flat philtrum, small upper lip, abnormal whorls of hair (A); microcephaly and large ears (B); pectus excavatum (C); and shortened and broad terminal phalanges of the hands and feet (D and E). Computerized tomography (F) at seven years of age and magnetic-resonance imaging (G and H) at 12 years of age showed microcephaly with head asymmetry and mild enlargement of the ventricles. Cytogenetic analysis revealed a translocation with the karyotype 46,XX,t(2;21)(q22;q22) (I). For mapping of the breakpoints of the translocation, FISH was carried out with the use of YAC, BAC, PAC, or cosmid clones according to standard procedures. The YAC clone 662A10, which contains DYRK1A, spans the chromosome 21 breakpoint, as shown by signals on the normal and the derivative chromosomes (J). Chromosome 21 breakpoint was fine mapped with Southern-blot hybridization (K). Genomic DNA from the patient (“P”) and the control (“C”) was digested with three restriction enzymes and hybridized with the breakpoint-spanning probe (chromosome position: 37,778,809–37,780,325). Aberrant bands representing the junction fragments are indicated by arrowheads.