No evidence of locus heterogeneity in familial microcephaly with or without chorioretinopathy, lymphedema, or mental retardation syndrome

Abstract

Background: Microcephaly with or without chorioretinopathy, lymphedema, or mental retardation syndrome (MCLMR) is a rare autosomal dominant disorder with variable expressivity. It is characterized by mild-to-severe microcephaly, often associated with intellectual disability, ocular defects and lymphedema. It can be sporadic or inherited. Eighty-seven patients have been described to carry a mutation in KIF11, which encodes a homotetrameric motor kinesin, EG5.

Methods: We tested 23 unreported MCLMR index patients for KIF11. We also reviewed the clinical phenotypes of all our patients as well as of those described in previously published studies.

Results: We identified 14 mutations, 12 of which are novel. We detected mutations in 12 affected individuals, from 6 out of 6 familial cases, and in 8 out of 17 sporadic patients. Phenotypic evaluation of patients (our 26 + 61 earlier published = 87) revealed microcephaly in 91%, eye anomalies in 72%, intellectual disability in 67% and lymphedema in 47% of the patients. Unaffected carriers were rare (4 out of 87: 5%). Family history is not a requisite for diagnosis; 31% (16 out of 52) were de novo cases.

Conclusions: All inherited cases, and 50% of sporadic cases of MCLMR are due to germline KIF11 mutations. It is possible that mosaic KIF11 mutations cause the remainder of sporadic cases, which the methods employed here were not designed to detect. On the other hand, some of them might have another mimicking disorder and genetic defect, as microcephaly is highly heterogeneous. In aggregate, KIF11 mutations likely cause the majority, if not all, of MCLMR.

Figure 1

Clinical characteristics of patients. Clinical features of MCMLR (patients I-10 (A), IX-10 (B), X-10 (C) and XIV-10 (D). Note, broad nose (A, B), long philtrum (A, B), thin upper lip (A, D), prominent ears (C, D) and bilateral pedal lymphedema (A, B D).

Figure 2

Schematic representation of EG5 (1056 amino acids) with position of mutations causing MCLMR syndrome. Functional domains (colored), in vitro mutagenized residues (signs) and post-translationally modified amino acids (signs). Mutations found in this study, top; those published earlier, below (Ostergaard et al. 2012, Hazan et al. 2012, Jones et al. 2013, Mirzaa et al. 2014 and Robitaille et al. 2014). Splice-site alterations (¶: family II and family V) shown to result in r.211_308del; p.Thr71Argfs*8 and r.789_790insAG; p.Val264Argfs*26, respectively (Figure 4). N.B. positions based on amino acids in Mirzaa et al.

Figure 3

Pedigrees and phenotypes of screened patients. Upper panel, KIF11 mutation-positive; lower panel, KIF11 mutation-negative. Individuals with a bar, clinically examined; those with a number, sequenced. Patients with a star have a mutation.

Figure 4

Effect of splice site mutations. (A) RT-PCR using primers in exon 2 and 5 on cDNA from patient II-10 (c.308+1G>T) and two controls (c1 and c2). Wild-type amplicon, 250 bp; mutant, 152 bp. (B) (Sequencing of lower band unraveled skipping of exon 3 resulting in a premature stop-codon (r.211_308del: p.Thr71Argfs*8). (C) Sequencing of cDNA of patient V-12 (c.790-3A>G) for an amplicon covering exon 7/8 splice site revealed double-sequence. (D) Sequencing of cloned amplicon fragments revealed insertion of 2 nucleotides (AG) from the intronic acceptor site (r.790_791insAG) in mutant clones. This leads to a premature stop-codon (p.Val264Argfs*26).