Dr Brennan's main areas of expertise are cancer, cardiac and endocrine genetics.
His interest in genetics started as a medical student in the 1980s when the medical speciality of clinical genetics was in its infancy. After a research degree studying genes in the developing heart, he completed his medical training and undertook postgraduate training in adult medicine and paediatrics before specialist training in Clinical Genetics.
Dr Brennan was appointed as an NHS Consultant in Clinical Genetics in 2002 with the Northern Genetics Service based in Newcastle upon Tyne. After opening a new satellite clinical genetics department in Middlesbrough, he was appointed to the role of Clinical Director of the Northern Genetics Service in 2008, managing a clinical and laboratory team providing genetics diagnostics for the population of North East England and North Cumbria.
Dr Brennan has been Clinical Lead for Cardiac and Endocrine Genetics for the Northern Genetics Service since 2003; he was Clinical Lead for Cancer Genetics 2002 to 2020.
A particular interest is the use of new technology and novel clinical pathways to improve diagnosis and management for people living with rare inherited disease. Between 2014 and 2018 he was Clinical Programme Director for the North East & North Cumbria NHS Genomic Medicine Centre, overseeing the introduction of whole genome sequencing into routine care as part of the 100,000 Genomes Project. He is engaged in a number of collaborative research projects, all of which are focused on improving diagnosis and treatment.
Dr Brennan is a keen and popular teacher and lectures to a wide range of undergraduate and postgraduate audiences in the UK and overseas. He teaches genetics for a week each year at Newcastle University's Medical School in Malaysia.