Somatic and germline mosaic mutations in the doublecortin gene are associated with variable phenotypes

Abstract

Mutations in the X-linked gene doublecortin lead to "double cortex" syndrome (DC) in females and to X-linked lissencephaly (XLIS) in males. Because most patients with DC and XLIS are sporadic, representing de novo doublecortin mutations, we considered that some of these patients could be somatic or germline mosaics. Among a population of 20 patients and their families, we found evidence for mosaic doublecortin mutations in 6 individuals. Germline mosaicism was identified in two unaffected women, each with two affected children. Additionally, one affected male with DC was found to be a somatic mosaic, which presumably spared him from the more severe phenotype of lissencephaly. The high rate of mosaicism indicates that there may be a significant recurrence risk for DC/XLIS in families at risk, even when the mother is unaffected.

Figure 1

Germline DCX mutation mosaicism in unaffected parents of two affected children. In both families, an unaffected mother has a girl with DC (half-blackened circle) and a boy with XLIS (blackened square), and the maternal genotype appears to be normal. SSCP gel analysis of both families demonstrates two normal bands in each parent (black arrows), two mutant bands from males with lissencephaly (gray arrows), and four bands (reflecting a normal allele and a mutant allele) from females with DC. In family I, other faint bands are visible in individuals II-1 and I-2, likely representing either false priming or minor conformers of the PCR product.

Figure 2

Somatic DCX–mutation mosaicism in the unaffected parents of an affected child. An unaffected mother has a daughter with DC (half-blackened circle). As is shown in panel A, this mother (I-1) was found to carry the same DCX mutation as was seen in her daughter, but the mutant band (rightward-pointing gray arrowheads) is of lower intensity than the normal band (rightward-pointing black arrowheads) (i.e., 22% of the intensity of the wild-type band), suggesting that the mother is a somatic mosaic for the DCX mutation. The mother has normal intelligence and the results of a brain MRI were normal, suggesting that she is clinically and subclinically unaffected. As is shown in panel B, the somatic DCX mutation is also visible on the basis of direct sequencing of genomic DNA in I-1 compared with that in a normal control. In I-1 sequence, the mutant sequence is seen as lower-height peaks that underlie the normal peaks. Because the mutation in I-1 is a CT deletion, the lower-height peaks in this figure are the bases AC, which are indicated by two downward-pointing gray arrows.

Figure 3

Somatic DCX–mutation mosaicism in affected females with sporadic DC (half-blackened circles). These two females with DC—Sporadic 64 (II-1) and Sporadic 17 (II-1)—have de novo DCX mutations (rightward-pointing gray arrowheads) that are, respectively, 31% and 43% of the intensity of the wild-type band (rightward-pointing black arrowheads), suggesting that they are somatic mosaics for the DCX mutation. The somatic DCX mutations are also visible on the basis of direct sequencing of genomic DNA in both patients, compared with that in a normal control. For Sporadic 64, the mutant sequence is seen as lower-height peaks that underlie the normal peaks. Because the mutation in Sporadic 64 is a CT deletion, the lower-height peaks in this figure are the bases GGA, which are indicated by three downward-pointing gray arrows. For Sporadic 17, the lower-height peaks are the bases CA, which are indicated by two downward-pointing gray arrows. The sequence for Sporadic 17 was obtained with the reverse primer, and thus the A deletion is seen as a T deletion, with a resultant leftward shift of the bases of the mutant allele. Because of an artifact of the ABI 310 sequencer, the mutant A peak, which is indicated by the second downward-pointing gray arrow, is shifted slightly leftward relative to the wild-type C peak.

Figure 4

Somatic DCX–mutation mosaicism in an affected male with sporadic DC (half-blackened square). As is shown in panel A, this male was found to carry a de novo DCX mutation allele (rightward-pointing gray arrowhead) and a wild-type DCX allele (rightward-pointing black arrowheads). Males should have just one DCX allele, suggesting that this male is a somatic mosaic for the DCX mutation. As is shown in panel B, the somatic DCX mutation is also visible on the basis of direct sequencing of genomic DNA in male 1 with sporadic DC (i.e., II-1), compared with that in a normal control. The mutant sequence is shown as a lower-height T peak that underlies the normal G peak, which is indicated by the downward-pointing gray arrow.