ABSTRACT: The pathophysiology of the osteomalacia in X-linked hypophosphatemia is uncertain. In this project, genomic DNA microarrays were used to identify novel genes with abnormal mRNA expression levels in mice with the dominant Hyp mutation of the Phex gene. Femoral shafts from five-week-old C57BL/6J mice, male and female, normal and Hyp (hemizygous male and heterozygous female), were flushed with saline to remove the marrow. RNA was extracted from each bone, pooled between two mice for each array, processed to cRNA, and hybridized to Affymetrix Mouse 430 2.0 GeneChip microarrays with probe sets for 45,101 genes. Twenty microarrays (40 mice) were done with 5 arrays for each treatment group (normal male, Hyp male, normal female and Hyp female). For each gene, factorial analysis of variance was performed for the main effects of genotype (normal vs. Hyp), sex (male vs. female), and genotype-by-sex interaction. The mRNA levels for 54 % of the genes on each array were scored as present. At P < 0.01, 2,635 genes were significant for genotype, 1,488 for sex, and 509 for genotype-by-sex interaction. There were two probes sets for the Phex gene. Probe 1450445, at the 3â end of the coding sequence, was low in normal samples (246 ± 37 (10), mean ± SEM (n)) and absent in Hyp samples. Probe 1421979, at the far 3â end of the untranslated region of the cDNA, 3,000 base pairs from the coding sequence, was high in normal mice (3,915 ± 315 (10); 8x brighter than the average gene), undetectable in Hyp males, and 725 ± 93 (5) in Hyp females. Both probe sets were scored as absent in kidney tissue. In Hyp bone, male and female, there was significant down-regulation of markers of osteoblasts and bone matrix synthesis with significant up-regulation of markers of blood vessel formation and cytoskeleton. No prominent skeletal gene was up-regulated in Hyp to attempt to compensate for the low skeletal mineralization. The genes with significant genotype-by-sex interaction did not show a marked fold difference between male and female Hyp mice. In conclusion, male and female Hyp mice showed similar depression of mRNA levels of genes related to bone synthesis in the femoral shaft. There was a high signal level from probes for a sequence in the 3â untranslated region of the Phex gene of normal, but not Hyp, mice, suggesting the need for further study of the molecular organization of this gene. Experiment Overall Design: Equal amounts of RNA from two mice, matched for genotype and sex were pooled to create each sample for microarray analysis. Four treatment groups were done: (1) Normal male mice, (2) Hyp male mice, (3) normal female mice, and (4) Hyp female mice. Five replicates were done with each replicate containing one sample from each of the four treatment groups for a total of 20 independent samples (40 mice total). Each replicate was matched for littermates and parallel processing.
