Modifying behavioral phenotypes in Fmr1KO mice: genetic background differences reveal autistic-like responses

Abstract

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability in humans. In addition to cognitive impairment, patients may exhibit hyperactivity, attention deficits, social difficulties and anxiety, and autistic-like behaviors. The degree to which patients display these behaviors varies considerably and is influenced by family history, suggesting that genetic modifiers play a role in the expression of behaviors in FXS. Several studies have examined behavior in a mouse model of FXS in which the Fmr1 gene has been ablated. Most of those studies were done in Fmr1 knockout mice on a pure C57BL/6 or FVB strain background. To gain a better understanding of the effects of genetic background on behaviors resulting from the loss of Fmr1 gene expression, we generated F1 hybrid lines from female Fmr1 heterozygous mice on a pure C57BL/6J background bred with male Fmr1 wild-type (WT) mice of various background strains (A/J, DBA/2J, FVB/NJ, 129S1/SvImJ and CD-1). Male Fmr1 knockout and WT littermates from each line were examined in an extensive behavioral test battery. Results clearly indicate that multiple behavioral responses are dependent on genetic background, including autistic-like traits that are present on limited genetic backgrounds. This approach has allowed us to identify improved models for different behavioral symptoms present in FXS including autistic-like traits.

Figure 1

Exploratory activity in the open field for Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds. Bars represent means ± SEM for total distance travelled in centimeters (cm) (A), and for vertical activity (B). Data are averages of two 30 min tests on consecutive days. Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 2

Repetitive behavior in Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds was assessed by stereotypy counts in the open field (A) and number of marbles buried in the marble bury test (B). Bars represent means ± SEM. Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 3

Anxiety-related behaviors in Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds. Bars represent means ± SEM for center distance: total distance ratio in the open field test (A), number of transitions in the light-dark test (B), and amount of novel blueberry bar consumed in 20 min (C). Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 4

Sensory responses and sensorimotor gating in Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds. Bars represent means ± SEM for maximum response to 120 dB acoustic startle (A), average percent prepulse inhibition to low level (74–82 dB) prepulse sound levels (B), and latency to exhibit a hind limb response in the hotplate test (C). Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 5

Fear conditioning in Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds. Bars represent means ± SEM for percent freezing in (A) contextual fear test and (B) cued fear test. Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 6

Social behavior in the partition test for Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds. Bars represent means ± SEM for time in sec at the partition during the baseline test with familiar overnight partner (A) and percent time at the partition with the unfamiliar partner relative to the baseline time (B). Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 7

Social behavior in a social interaction test. Fmr1KO (black bars) and WT littermates (white bars) in six genetic backgrounds spent 10 min interacting with C57BL/6J partner mice after 48 hr in partitioned housing. Bars represent means ± SEM for percentage of time spent in active social behavior (A), passive/receptive social behavior (B) and nonsocial behavior (C). Asterisks indicate a significant difference (p≤ 0.05) between Fmr1KO and WT littermates within a specific line.

Figure 8

Isolation-induced ultrasonic vocalizations in male neonatal Fmr1KO mice (closed circles) and WT littermates (open circles) in the B6D2 genetic background. Circles represent means ± SEM for the number of calls recorded each day from postnatal day 3 to 14 during a 2 min isolation period from dam and littermates.