X-linked hypomyelination with spondylometaphyseal dysplasia (H-SMD) associated with mutations in AIFM1

Abstract

An X-linked condition characterized by the combination of hypomyelinating leukodystrophy and spondylometaphyseal dysplasia (H-SMD) has been observed in only four families, with linkage to Xq25-27, and recent genetic characterization in two families with a common AIFM1 mutation. In our study, 12 patients (6 families) with H-SMD were identified and underwent comprehensive assessment accompanied by whole-exome sequencing (WES). Pedigree analysis in all families was consistent with X-linked recessive inheritance. Presentation typically occurred between 12 and 36 months. In addition to the two disease-defining features of spondylometaphyseal dysplasia and hypomyelination on MRI, common clinical signs and symptoms included motor deterioration, spasticity, tremor, ataxia, dysarthria, cognitive defects, pulmonary hypertension, nystagmus, and vision loss due to retinopathy. The course of the disease was slowly progressive. All patients had maternally inherited or de novo mutations in or near exon 7 of AIFM1, within a region of 70 bp, including synonymous and intronic changes. AIFM1 mutations have previously been associated with neurologic presentations as varied as intellectual disability, hearing loss, neuropathy, and striatal necrosis, while AIFM1 mutations in this small region present with a distinct phenotype implicating bone. Analysis of cell lines derived from four patients identified significant reductions in AIFM1 mRNA and protein levels in osteoblasts. We hypothesize that AIFM1 functions in bone metabolism and myelination and is responsible for the unique phenotype in this condition.

Keywords: AIFM1 gene; Hypomyelination; Mitochondrial leukodystrophy; Myelin; Spondylometaphyseal dysplasia; Whole exome sequencing (WES).

Fig. 1

MRI findings in H-SMD. Axial T2-weighted images (a–c, e–g, i–k) and sagittal T1-weighted images (d, h, l) of patient 7 at age 4 years (first row), patient 8 at age 5 years (second row), and patient 1 at age 4 years (third row). The supratentorial white matter shows a diffusely elevated T2 signal (b, c, f, g, j, k), with unremarkable T1 signal (d, h, l) indicating hypomyelination. Signal of the basal ganglia is normal (b, f, j). The cerebellum has a normal volume (a, d, e, h, i, l). Signal of the superior cerebellar peduncles is hyperintense in all patients (a, e, i). Patient 7 has also signal elevation of the hilus of the dentate nucleus (a). Patient 8 of the external part of the middle cerebellar peduncle (e). Patient 1 has relatively better myelination of the brainstem, corpus callosum and deep gray nuclei

Fig. 2

Spondylometaphyseal dyplasia in H-SMD. a Common findings in patients included changes in the metaphyses and epiphyses of long bones, including mild flaring, pelvic abnormalities including flat acetabula, squared iliac wings, and thick pubic rami, as well as vertebral abnormalities including end plate irregularity and kyphosis. b In some patients over time, vertebral anomalies evolved to include anterior central beaking with vertebra plana, posterior scalloping of lumbar vertebrae, and evolution of severe kyphoscoliosis. c In the hands, brachydactyly and clinodactyly with flat, shortened metacarpals and phalanges were universally seen, with variable amounts of coned epiphyses. In older individuals, irregular sclerosis was seen over time. d In metaphyses and epiphyses of long bones, severe irregular sclerosis was seen over time, present here at the knees

Fig. 3

H-SMD-associated mutations in AIFM1. a Pedigrees of each H-SMD families with subjects of this study indicated. Where the mother is not noted to be a carrier, the mother has been sequenced and the variant is de novo. b AIFM1 refseq transcripts showing known alternative splicing events. c H-SMD-associated mutations in AIFM1 are all near exon 7 splice acceptor site. The predicted intron branch point motif is indicated in red text. The Human Splicing Finder tool predicts the c.697-44 T > G mutation will disrupt the function of this branch point. d Motifs of primary sequence conservation surrounding each H-SMD-associated mutation in AIFM1 based on alignment of 15 mammalian species using WebLogo. The size of the letters indicates degree of conservation and where more than one nucleotide is visible suggests variation across species

Fig. 4

AIFM1 in fibroblasts and transdifferentiated osteoblasts. a Alizarin red, alkaline phosphatase (ALP) activity, and von Kossa staining of fibroblasts and transdifferentiated osteoblasts, confirming successful transdifferentiation. These stains provide a qualitative indication of the capability of the cells to mineralize; plates were not normalized for cell number. b Western blot of AIFM1 and actin in transdifferentiated osteoblasts (day 21). kDa kiloDalton. c AIFM1 relative mRNA expression in transdifferentiated osteoblasts (day 21), in relation to the housekeeping gene YWHAZ. d Depicts AIFM1 protein expression normalized to Actin in osteoblasts. Error bars indicate standard error of the mean (SEM, n = 3)