This variant is classified as a variant of unknown significance because its contribution to microcephaly, vocal cord paralysis, and atrial septal defect has not been confirmed.
In a 14-year-old Japanese boy with microcephaly, vocal cord paralysis, and atrial septal defect, Uehara et al. (2021) identified heterozygosity for a c.371T-G transversion (c.371T-G, NM_145697.3) in exon 6 of the NUF2 gene, resulting in an ile124-to-ser (I124S) substitution at a residue in the alpha-G helix within the calponin homology domain. The mutation occurred de novo and was not found in the 1000 Genomes Project, HGVD, or gnomAD databases. Western blot of whole-cell lysates from proband lymphoblastoid cell lines (LCLs) revealed markedly reduced NUF2 and NDC80 (607272) levels, by approximately 92% and 78%, respectively, compared to his parents. Immunoprecipitation assays in transfected HeLa or HEK293 cells showed marked inhibition of the interaction between NUF2 and NDC80. In addition, reduced protein levels of NUF2 and NDC80 were shown to result from degradation via the ubiquitin-proteasome system. Chromosome counting in metaphase spreads using patient LCLs revealed that only 15% of patient metaphases had a diploid number of chromosomes, compared to 91% from a control sample, consistent with a malfunctioning kinetochore. There was significant delay in proliferation of proband LCLs compared to control. Patient cells also showed increased levels of micronuclei and an increased rate of formation of multipolar spindles, compared to control cells. Dysmorphic features in the patient included micrognathia, low-set cup-shaped ears, and webbed neck. He experienced transient short stature before the age of 10 years, but growth parameters began resolving at age 11, and at last assessment his height and weight were at -1.0 SD, with occipitofrontal circumference at -3.5 SD. His cognitive development was 'borderline,' but he was able to attend normal school.
