Bardet-Biedl Syndrome clinical outcomes

Bardet-Biedl Syndrome (BBS) is a rare genetic condition that affects many parts of the body. Its effects can include:

• obesity
• vision loss
• additional fingers and/or toes (polydactyly)
• undeveloped genitals
• learning disabilities
• kidney failure

BBS is seen in about 1 in 100,000 of the general population of the UK, but is more common in children born from marriages between family members.

This national service was established in 2010. Children are referred to their nearest specialist centre – either Great Ormond Street Hospital (GOSH) or Birmingham Children’s Hospital. A key service aim is to provide both earlier diagnosis and intervention to promote optimal health. The service at GOSH offers dedicated multi-disciplinary (MDT) care of children and young people with BBS, and their families, including specialist nursing, Dietetics, Ophthalmology, Nephrology, Endocrinology, Clinical Genetics, Speech and Language Therapy and Psychology.

Each year, GOSH sees around 60 children and young people (of which approximately 10 are new referrals), who attend our MDT clinic for expert assessment by a team that specialises in each organ system commonly affected by this syndrome. Our service is delivered in partnership with BBS UK to ensure our children, young people and their families have access to ongoing support. Additional MDT telephone clinics started in 2019 to help manage the increasing demand for our service.

In collaboration with the UCL Great Ormond Street Institute of Child Health, the team is committed to translating excellence in research into effective treatments of BBS in children and young people.

Early obesity is a major clinical feature of BBS. Managing weight gain in BBS can be challenging due to both lower energy requirements and hyperphagia (constant drive to eat). Specialist dietetic support is an integral part of this service; dietetic advice focuses on portion control strategies and promotion of activity to address underlying energy imbalance.

Body Mass Index (BMI) is a clinically useful measure of obesity for patients over 2 years of age as it’s easy to measure and it relates an individual’s weight to their height. As ideal BMI varies throughout childhood, we use a BMI z-score (standard deviation score) as our outcome measure to allow comparison to children and young people in the general population by age.

BMI z-score data for patients over 2 years of age has been compared with UK-1990 BMI z-score criteria for overweight and obesity. We compare our results for children and young people returning for review with data collected from those who attended the first year of our service.

Table 1.1 Patient BMI classifications (molecular diagnosis only) in first year of the BBS service, 2010/2011¹

¹ Total number seen: 24; Total with confirmed molecular diagnosis: 17; Exclusions: under 2 years of age at appointment: 1

Figure 1.1 Patient BMI classifications (molecular diagnosis only) in first year of the BBS service, 2010/2011¹

Table 1.2 Patient BMI classifications (molecular diagnosis only), 2017/18 to 2019/20²

² Exclusions: missing data / patients under 2 years of age at follow up: 1 (2017/18)

Figure 1.2 Patient BMI classifications (molecular diagnosis only) 2017/182

Figure 1.3 Patient BMI classifications (molecular diagnosis only) 2018/19

Figure 1.4 Patient BMI classifications (molecular diagnosis only) 2019/20

Limited data is available in the literature showing weight-related outcomes in children and young people. However, BBS is characterised by increasing weight with age. These graphs highlight the difference in our clinic population since this specialised service has been established. At the start of our service 88% of our patients were classified as obese, 12% overweight and no one seen had a healthy BMI. Data since 2017/18 shows around 9% of our population are able to maintain a healthy BMI which challenges the reported increasing weight with age.

We report results for children over 2 years of age with a molecular confirmation of diagnosis, seen in clinic between April 2017 and March 2020. Reduced, maintained or increased BMI z-score categories apply to overweight or obese patients, where the aim is to avoid weight gain and reduce BMI over time. BMI z-score reduction in overweight/obese children has been linked with positive health changes including changes in body composition, blood pressure and insulin sensitivity.

Table 2.1 Patient BMI change since last BBS MDT appointment²

3 Exclusions: missing data/patients under 2 years of age at either appointment: 4 (2017/18); 2 (2019/20)

Figure 2.1 Patient BMI change since last BBS MDT appointment, 2017/18 (2)

Figure 2.2 Patient BMI change since last BBS MDT appointment, 2018/19

Figure 2.3 Patient BMI change since last BBS MDT appointment, 2019/203

The data shows in 2019/20, 57% (16/28) of overweight or obese patients reduced their BMI z-score and 7% (2/28) maintained their BMI z-score since their last BBS multi-disciplinary team outpatient appointment. 36% (10/28) had an increase in BMI z-score.

In 2018/19, 65% (22/34) of our overweight or obese patients had a reduction in BMI z-score since their last BBS multi-disciplinary team outpatient appointment. 35% (12/37) had an increase in BMI z-score. In 2017/18, 56% (18/32) of overweight or obese patients had a reduction in BMI z-score and 3% (1/32) had maintained their BMI z-score. 41% (13/32) had an increase in BMI z-score.

Though there is limited data nationally and internationally on children and young people with BBS, previous studies in adults have reported the obesity incidence as 72% to 86%. Our data challenges the reported increasing weight with age as about 9% of our patients have maintained a healthy BMI and about 60% have been able to reduce their BMI z-score.

Here, we report results for children over 2 years of age with a molecular confirmation of diagnosis of BBS1 or BBS10 seen in clinic between April 2017 and March 2020. These are the most common of identified genotypes and are known to have different general characteristics (eg children with BBS1 typically achieve a greater adult height than children with BBS10) so we would expect BMI outcomes to vary between these genotypes. Reduced, maintained or increased BMI z-score categories apply to overweight or obese patients, where the aim is to avoid weight gain and reduce BMI over time.

Table 3.1 Patient BMI change since last appointment, BBS1, 2017/18 to 2019/204

4 Exclusions: missing data/patients under 2 years of age at either review: 1 (2017-2018)

Figure 3.1 Patient BMI change since last appointment, BBS1, 2017/184

Figure 3.2 Patient BMI change since last appointment, BBS1, 2018/19

Figure 3.3 Patient BMI change since last appointment, BBS1, 2019/20

Table 3.2 Patient BMI change since last appointment, BBS10, 2017/18 to 2019/205

5 Exclusions: missing data/patients under 2 years of age at either review: 1 (2017-2018); 1 (2019/20)

Figure 3.4 Patient BMI change since last appointment, BBS10, 2017/185

Figure 3.5 Patient BMI change since last appointment, BBS10, 2018/19

Figure 3.6 Patient BMI change since last appointment, BBS10, 2019/205

Data is provided for children with BBS1 and BBS10 as these are the most common genotypes seen in our clinic. During 2019/20 55% (6/11) of our overweight or obese patients with BBS 1 reduced their BMI z-score and 60% (3/5) of our overweight or obese patients with BBS10 reduced their BMI z-score showing that this outcome is possible for both genotypes.

Notes:

• BBS is a highly variable syndrome so direct comparison between children and young people attending each year is not possible.
• Molecular confirmation of diagnosis is available for about 80% of our patients. Data for children and young people with a presumed clinical diagnosis has been excluded as aspects of this syndrome (eg vision changes) evolve with time, affecting the timing a clinical diagnosis can be confirmed.
• Less data was available for the 2019-2020 period due to the impact of the coronavirus pandemic on running face to face clinical services.

The establishment of the national BBS service has also allowed significant data to be collected across the service leading to several joint publications and improvements in patient care. We will continue to collect and review the data about the service we provide, and to seek to add more measures that tell us about our effectiveness.

This information will be updated in October 2023.

Mujahid, S. et al., The Endocrine and Metabolic Characteristics of a Large Bardet-Biedl Syndrome Clinic Population, Journal of Clinical Endocrinology Metabolism. 2018, May 1:193(5):1834-1841

Forsythe, E. et al., Managing Bardet-Biedl Syndrome – Now and in the Future, Frontiers Pediatric, 2018 Feb 13;6:23, doi: 10.3389/fped.2018.00023, eCollection 2018

Kenny, J. et al., Toward personalized medicine in Bardet-Biedl Syndrome, Personal Medicine, 2017 Sep; 14:447-456

Forsythe, E. et al., Risk Factors for Severe Renal Disease in Bardet-Biedl Syndrome, Journal of American  Society of Nephrology,. 2017 Mar;28(3):963-970, doi: 10.1681/ASN.2015091029, Epub 2016 Sep 22

Forsythe, E. et al., Bardet-Biedl Syndrome, European Journal of Human Genetics, 2013 Jan;21(1):8-13, doi: 10.1038/ejhg.2012.115, Epub 2012 Jun 20.

Poster Presentation: S. Flack, C. Peters, K. Sparks, P. Beales, Does Attendance at a Paediatric Multidisciplinary Specialist Bardet-Biedl Syndrome Annual Review Clinic Lead to a Reduction in Body Mass Index? 2017 International Conference on Nutrition and Growth