The unfolding clinical spectrum of holoprosencephaly due to mutations in SHH, ZIC2, SIX3 and TGIF genes

Abstract

Holoprosencephaly is a severe malformation of the brain characterized by abnormal formation and separation of the developing central nervous system. The prevalence is 1:250 during early embryogenesis, the live-born prevalence is 1:16 000. The etiology of HPE is extremely heterogeneous and can be teratogenic or genetic. We screened four known HPE genes in a Dutch cohort of 86 non-syndromic HPE index cases, including 53 family members. We detected 21 mutations (24.4%), 3 in SHH, 9 in ZIC2 and 9 in SIX3. Eight mutations involved amino-acid substitutions, 7 ins/del mutations, 1 frame-shift, 3 identical poly-alanine tract expansions and 2 gene deletions. Pathogenicity of mutations was presumed based on de novo character, predicted nonfunctionality of mutated proteins, segregation of mutations with affected family-members or combinations of these features. Two mutations were reported previously. SNP array confirmed detected deletions; one spanning the ZIC2/ZIC5 genes (approx. 100 kb) the other a 1.45 Mb deletion including SIX2/SIX3 genes. The mutation percentage (24%) is comparable with previous reports, but we detected significantly less mutations in SHH: 3.5 vs 10.7% (P¼0.043) and significantly more in SIX3: 10.5 vs 4.3% (P¼0.018). For TGIF1 and ZIC2 mutation the rate was in conformity with earlier reports. About half of the mutations were de novo, one was a germ line mosaic. The familial mutations displayed extensive heterogeneity in clinical manifestation. Of seven familial index patients only two parental carriers showed minor HPE signs, five were completely asymptomatic. Therefore, each novel mutation should be considered as a risk factor for clinically manifest HPE, with the caveat of reduced clinical penetrance.

New findings for phenotype-genotype correlations in a large European series of holoprosencephaly cases

“…These results are consistent with our previous findings for smaller series 1 9 18 34. Point mutations in SHH, SIX3 , and TGIF were highly heritable, with heritabilities of 73%, 88%, and 100%, respectively, whereas most ZIC2 mutations were de novo (70% of cases) 6 20. This may be due to a higher penetrance and the high frequency of severe HPE in individuals with ZIC2 mutations, resulting in the identification of a larger number of sporadic cases.…”

“…By contrast, facial morphology in subjects with ZIC2 mutations was generally characterised by moderate facial dysmorphia or even a normal face (category 4), associated with alobar or semilobar HPE. This feature seems to be specific to ZIC2 mutations and has also been reported by other teams 6 20. Solomon et al 6 mentioned a particular facial phenotype found in some of the cases in our series, with a high proportion of abnormal noses (nine cases), usually short, sometimes flat or large, and ear dysplasia (four cases).…”

“…Moreover, atelencephaly/aprosencephaly was associated exclusively with SIX3 mutations. Pasquier et al reported this particular association and other teams have reported a high proportion of severe cases in subjects with SIX3 mutations, including ophthalmological defects in particular 19 20 33. Thus, alobar HPE tends to be associated with SIX3 , semilobar HPE with ZIC2 , and microforms with SHH , consistent with the findings of Solomon et al 21.…”

Utilizing prospective sequence analysis of SHH, ZIC2, SIX3 and TGIF in holoprosencephaly probands to describe the parameters limiting the observed frequency of mutant gene × gene interactions

“…Descriptions of common variations (Supplementary Table 1) and NIH mutations (Supplementary Table 2 and 6) including data from Dubourg et al, 2004 [15] and Paulussen et al, 2010 [38] (Supplementary Tables 3 and 4) are based on the SHH NM_000193.2, ZIC2 NM_007129.2, SIX3 NM_005413.2 and TGIF NM_003244.2 reference sequences (Supplementary Data 7). Guidelines for the naming of the sequence variants conform to the recommendations of the human nomenclature committee (www.hgvs.org/mutnomen) and were checked using the Mutalyzer software for the predicted effects on the reference gene (http://www.mutalyzer.nl/2.0/).…”

Rare novel variants in the ZIC3 gene cause X-linked heterotaxy

“…20 The evolutionary conservation of these genes and their critical timing of expression is essential for proper embryonic development, since variants in other human zic genes also cause monogenic diseases, for instance ZIC2 causing holoprosencephaly. 21 The ZIC3 gene, however, is the only ZIC gene to be implicated in laterality disorders. Several animal studies have demonstrated the crucial role of ZIC3 expression during gastrulation and axial patterning and the laterality abnormalities created when knocked out.…”