Mutations in the human SC4MOL gene encoding a methyl sterol oxidase cause psoriasiform dermatitis, microcephaly, and developmental delay

Abstract

Defects in cholesterol synthesis result in a wide variety of symptoms, from neonatal lethality to the relatively mild dysmorphic features and developmental delay found in individuals with Smith-Lemli-Opitz syndrome. We report here the identification of mutations in sterol-C4-methyl oxidase–like gene (SC4MOL) as the cause of an autosomal recessive syndrome in a human patient with psoriasiform dermatitis, arthralgias, congenital cataracts, microcephaly, and developmental delay. This gene encodes a sterol-C4-methyl oxidase (SMO), which catalyzes demethylation of C4-methylsterols in the cholesterol synthesis pathway. C4-Methylsterols are meiosis-activating sterols (MASs). They exist at high concentrations in the testis and ovary and play roles in meiosis activation. In this study, we found that an accumulation of MASs in the patient led to cell overproliferation in both skin and blood. SMO deficiency also substantially altered immunocyte phenotype and in vitro function. MASs serve as ligands for liver X receptors α and β(LXRα and LXRβ), which are important in regulating not only lipid transport in the epidermis, but also innate and adaptive immunity. Deficiency of SMO represents a biochemical defect in the cholesterol synthesis pathway, the clinical spectrum of which remains to be defined.

Figure 1. Severe scaling and psoriasiform dermatitis in the patient.

(A) Note mild microcephaly, lusterless, fine fair hair, and blepharitis. (B and C) The ichthyosiform erythroderma covers all of the patient’s body except for the palms and soles. (D) H&E-stained section of affected skin shows hyperkeratosis (original magnification, ×10), loss of granular layer, psoriasiform hyperplasia, thinning of suprapapillary plate, and neutrophilic epidermal infiltration; these features are characteristic of psoriasis. (E) Oil red O staining of affected skin biopsy (original magnification, ×20) The arrow shows the intracellular lipid accumulation in the foamy cells in the dermis. (F) Neutrophil elastase staining (shown in red) of neutrophils in the stratum corneum of affected skin (original magnification, ×20). Photographs reproduced with signed informed consent/assent provided by the patient and her family.

Figure 2. Sterol profiles of patient samples and mutation analysis of SC4MOL in proband and parental samples.

GC-MS total ion current profiles of sterol extracts of patient skin (A), control skin (B), patient plasma (C), and control plasma (D). The ordinates are detector response, and the abscissas are elution time. The numbered compounds are: 1, cholesterol; 2, cholestanol; 3, 8(9)-cholestenol; 4, desmosterol plus 7-dehydrocholesterol; 5, lathosterol; 6, unidentified monomethylsterol; 7, campesterol; 8, 4α-methyl-5α–cholest-8(9)-en-3β-ol; 9, dihydrolanosterol; 10, 4α-methyl-5a-cholest-7(8)-en-3β-ol; 11, unidentified isomer of 4,4′-dimethyl-5α-cholesta-8(9)-en-3β-ol; 12, 4,4′-dimethyl-5α-cholesta-8(9)-en-3β-ol; 13, 4,4′-dimethyl-5α-cholesta-8(9),24-dien-3β-ol, 14, sitosterol. The ion fragment patterns of each methyl sterol peak are shown in the Supplemental Note. Note that the column run for plasma elution time is slightly shifted compared with that of the skin samples. The levels of dimethylsterols and monomethylsterols were markedly increased in patient skin. Dimethylsterol (peak 6) is most elevated in skin, suggesting the preferential accumulation of 4,4′-dimethylsterols in the patient’s skin. The absence of a 4-carboxysterol in the skin excludes a possible defect in NSDHL. (E) Two mutations were identified in SC4MOL from both gDNA and cDNA extracted from peripheral blood leukocytes: 519T→A (top) and 731A→G (bottom). 519T→A was also identified in gDNA from the patient’s father, and 731A→G was identified in the mother.

Figure 3. Cell proliferation and cycle abnormalities in SMO deficiency.

(A and B) Fibroblasts were labeled with CFSE and DAPI and analyzed by flow cytometry. The number of cell divisions was determined by the number of CFSE fluorescence peaks. The number of cells in the G₀-G₁ and S-G₂-M phases of the cell cycle was analyzed by the amount of linear DAPI fluorescence. (C) Histograms show typical DAPI fluorescence profiles of cells in G₀-G₁ and G₂-S-M phases of the cell cycle in normal human EBV–transformed lymphoblasts cultured in standard medium with 10% FBS; in cholesterol-depleted medium (CD); or in CD supplemented with either simvastatin (ST), ATZ, or fluconazole (FA). The control data represent 5 different normal cell lines with different passages. The degree of decrease in cell divisions was similar in all the controls when growing in cholesterol-depleted medium compared with regular medium. Fluorescence is expressed as GMFI. All GMFI values shown have robust coefficients of variation less than 1%.

Figure 4. Immunocyte abnormalities in SMO deficiency.

(A and B) Leukocyte populations were analyzed by multicolor flow cytometry; subsets identified by forward/scatter profiles, with mature granulocytes, monocytes, and lymphocytes falling in gates A, B, and C, respectively. The control data shown here represent blood samples donated from 20 normal individuals. (A) Cytometric profiles for CD25, CD69, CD86, HLA-DR, TLR-2, and TLR-4 (gate i, and fluorescence histograms of CD16b) and (B) for mature granulocytes/neutrophils. Also shown in A are the CD4, CD8 profiles of CD3⁺ T cells; and CD28^nullCD56⁺ in CD8^dim T cells (gate iv). No significant differences were observed in the monocyte population (data not shown). (C) Overlay of treated and untreated cytometric profiles. Note that 50% of control granulocytes migrated to TLR-2⁺TLR-4^– after the ATZ treatment. (D) IL-6 production by skin fibroblasts from healthy control and patient upon treatment of TNF-α, and TNF-α plus simvastatin, or in medium alone. *P < 0.01. Data are presented as mean ± SD.