Monosomy 1p36 - a multifaceted and still enigmatic syndrome: four clinically diverse cases with shared white matter abnormalities.

Monosomy 1p36 is the most common subtelomeric deletion syndrome seen in humans. Uniform features of the syndrome include early developmental delay and consequent intellectual disability, muscular hypotonia, and characteristic dysmorphic facial features. The gene-rich nature of the chromosomal band, inconsistent deletion sizes and overlapping clinical features have complicated relevant genotype-phenotype correlations. We describe four patients with isolated chromosome 1p36 deletions. All patients shared white matter abnormalities, allowing us to narrow the critical region for white matter involvement to the deletion size of up to 2.5 Mb from the telomere. We hypothesise that there might be a gene(s) responsible for myelin development in the 1p36 subtelomeric region. Other significant clinical findings were progressive spastic paraparesis, epileptic encephalopathy, various skeletal anomalies, Prader-Willi-like phenotype, neoplastic changes - a haemangioma and a benign skin tumour, and in one case, sleep myoclonus, a clinical entity not previously described in association with 1p36 monosomy. Combined with prior studies, our results suggest that the clinical features seen in monosomy 1p36 have more complex causes than a classical contiguous gene deletion syndrome.