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A Prader-Willi syndrome patient is described who has a de novo balanced translocation, (4;15)(q27;q11.2)pat, with breakpoints lying between SNRPN exons 2 and 3. Parental-origin studies indicate that there is no uniparental disomy and no apparent deletion. This patient expresses ZNF127, SNRPN exons 1 and 2, IPW, and D15S227E (PAR1) but does not express either SNRPN exons 3 and 4 or D15S226E (PAR5), as assayed by reverse transcription-PCR, of peripheral blood cells. Methylation studies showed normal biparental patterns of inheritance of loci DN34/ZNF127, D15S63, and SNRPN exon 1. Results for this patient and that reported by Sun et al. support the contention that an intact genomic region and/or transcription of SNRPN exons 2 and 3 play a pivotal role in the manifestations of the major clinical phenotype in Prader-Willi syndrome.

Deletions of 15q11-q13 typically result in Angelman syndrome when inherited from the mother and Prader-Willi syndrome when inherited from the father. The critical deletion region for Angelman syndrome has recently been restricted by a report of an Angelman syndrome patient with a deletion spanning less than 200 kb around the D15S113 locus. We report here on a mother and son with a deletion of chromosome 15 that includes the D15S113 locus. The son has mild to moderate mental retardation and minor anomalies, while the mother has a borderline intellectual deficit and slightly downslanting palpebral fissures. Neither patient has the seizures, excessive laughter and hand clapping, ataxia or the facial anomalies which are characteristic of Angelman syndrome. The proximal boundary of the deletion in our patients lies between the D15S10 and the D15S113 loci. Our patients do not have Angelman syndrome, despite the deletion of the D15S113 marker. This suggests that the Angelman syndrome critical deletion region is now defined as the overlap between the deletion found in the previously reported Angelman syndrome patient and the region that is intact in our patients.

Comparative molecular analysis of chromosome 15, sub-band q11.2 of patients with the Prader-Willi or Angelman syndromes demonstrates that they have a similar deletion. An hypothesis is presented that attempts to explain the tremendous degree of clinical heterogeneity in these diverse deletion-associated syndromes based on abnormal haplotypes present on the cytogenetically normal homolog. This hypothesis also addresses genetic similarities between patients who have deletion and those who have the inv dup(15) by postulating that these syndromes are caused by relative dosage ratios of normal versus abnormal alleles.

We report on a 4-year-old girl with Angelman syndrome who has an apparent de-novo del(15) (q11q13) originating from a maternally derived chromosome. Her mother had severe brachycephaly, sensorineural hearing loss, speech impediment, and mild ataxia. CT brain scans showed an enlarged foramen magnum in the mother and daughter but magnetic resonance imaging (MRI) showed no brainstem abnormality in either. This family demonstrates that some Angelman syndrome cases may be dominantly transmitted with variable expression and associated with abnormal or cytogenetically apparently normal chromosome 15.